| 0/0 |
2026/09/07 00:30 |
flow |
assessment-security |
25m
Results: map[DenialOfService:false Explanation:The bug report describes a `VM_WARN_ON_ONCE` triggering in `assert_sane_pgoff`
during a VMA split operation (in this case, initiated by `mbind`).
### Cause of the Warning
The `assert_sane_pgoff` function asserts that if an anonymous VMA is unfaulted
(i.e., `vma->anon_vma == NULL`), its page offset (`vma->vm_pgoff`) must be equal
to its virtual address page offset (`vma->vm_start >> PAGE_SHIFT`).
However, this assumption can be legitimately violated using `mremap` with the
`MREMAP_DONTUNMAP` flag:
1. An anonymous VMA is created and a page is faulted in, assigning it an
`anon_vma`.
2. The VMA is moved to a new address using `mremap(MREMAP_MAYMOVE)`. Its
`vm_pgoff` is preserved (so it no longer equals `vm_start >> PAGE_SHIFT`), and
its `anon_vma` is cloned.
3. The VMA is moved again using `mremap(MREMAP_DONTUNMAP)`. This operation moves
the page tables to a new VMA, leaving the old VMA intact but empty. As part of
this process, `dontunmap_complete()` calls `unlink_anon_vmas()`, which clears
`vma->anon_vma` of the old VMA.
4. The old VMA now has `anon_vma == NULL`, but its `vm_pgoff` retains the
preserved value, which does not equal `vm_start >> PAGE_SHIFT`.
5. A subsequent operation that splits or modifies the old VMA (like `mbind` in
the reproducer) calls `vma_add_pgoff()`, which triggers the `VM_WARN_ON_ONCE` in
`assert_sane_pgoff()`.
### Security Impact
This is a **benign false positive warning** with **no security impact**:
- **No Memory Corruption:** The mismatched `vm_pgoff` does not cause any memory
corruption or logical errors. If new pages are faulted into this empty VMA, they
will be assigned an index based on the existing `vm_pgoff`, and the reverse
mapping (`rmap`) will correctly find the VMA using this index. The kernel
handles this state perfectly fine.
- **Compiled Out in Production:** The warning is a `VM_WARN_ON_ONCE`, which is
only defined when `CONFIG_DEBUG_VM` is enabled. In production kernels, this
configuration is disabled, meaning the macro evaluates to a no-op
(`BUILD_BUG_ON_INVALID`) and generates no runtime code. It will not cause a
panic, oops, or even a log message in production environments.
- **No Denial of Service:** Since the warning is compiled out in production (and
is only a non-fatal warning even when compiled in), it cannot be used to crash
or hang the system.
Therefore, this bug is neither exploitable nor capable of causing a
denial-of-service in production builds. Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/09/07 00:30 |
action |
syz-repro-to-c-repro |
0m
Results: map[SimplifiedCRepro:// autogenerated by syzkaller (https://github.com/google/syzkaller)
#define _GNU_SOURCE
#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#ifndef __NR_mbind
#define __NR_mbind 235
#endif
#ifndef __NR_mmap
#define __NR_mmap 222
#endif
#ifndef __NR_mremap
#define __NR_mremap 216
#endif
#ifndef __NR_socketpair
#define __NR_socketpair 199
#endif
int main(void)
{
syscall(__NR_mmap, /*addr=*/0x1ffff000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x20000000ul, /*len=*/0x1000000, /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/7ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x21000000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
const char* reason;
(void)reason;
if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {}
// socketpair$unix arguments: [
// domain: const = 0x1 (8 bytes)
// type: unix_socket_type = 0x2 (8 bytes)
// proto: const = 0x0 (4 bytes)
// fds: ptr[out, unix_pair] {
// unix_pair {
// fd0: sock_unix (resource)
// fd1: sock_unix (resource)
// }
// }
// ]
syscall(__NR_socketpair, /*domain=*/1ul, /*type=SOCK_DGRAM*/2ul, /*proto=*/0, /*fds=*/0x20000080ul);
// mremap arguments: [
// addr: VMA[0x1000]
// len: len = 0x1000 (8 bytes)
// newlen: len = 0x400000 (8 bytes)
// flags: mremap_flags = 0x3 (8 bytes)
// newaddr: VMA[0x400000]
// ]
syscall(__NR_mremap, /*addr=*/0x20a96000ul, /*len=*/0x1000ul, /*newlen=*/0x400000ul, /*flags=MREMAP_FIXED|MREMAP_MAYMOVE*/3ul, /*newaddr=*/0x20000000ul);
// mremap arguments: [
// addr: VMA[0x9000]
// len: len = 0x600002 (8 bytes)
// newlen: len = 0x600002 (8 bytes)
// flags: mremap_flags = 0x7 (8 bytes)
// newaddr: VMA[0x600000]
// ]
syscall(__NR_mremap, /*addr=*/0x20000000ul, /*len=*/0x600002ul, /*newlen=*/0x600002ul, /*flags=MREMAP_FIXED|MREMAP_MAYMOVE|0x4*/7ul, /*newaddr=*/0x20a00000ul);
// mbind arguments: [
// addr: VMA[0x800000]
// len: len = 0x800000 (8 bytes)
// mode: mbind_mode = 0x1 (8 bytes)
// nodemask: nil
// maxnode: intptr = 0x7 (8 bytes)
// flags: mbind_flags = 0x2 (8 bytes)
// ]
syscall(__NR_mbind, /*addr=*/0x20001000ul, /*len=*/0x800000ul, /*mode=MPOL_PREFERRED*/1ul, /*nodemask=*/0ul, /*maxnode=*/7ul, /*flags=MPOL_MF_MOVE*/2ul);
return 0;
}
]
|
| 2/1 |
2026/09/07 00:30 |
action |
kernel-checkouter |
0m
Results: map[KernelSrc:/app/workdir/cache/src/e4df1e35f73e8871a0bb5986e0a3f7f88903d380]
|
| 3/1 |
2026/09/07 00:31 |
action |
kernel-builder |
10m
Results: map[KernelObj:/app/workdir/cache/build/4052259e3ac679a0ea4a40bb36f332ce6ba3596e]
|
| 4/1 |
2026/09/07 00:42 |
action |
codesearch-prepare |
8m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/09/07 00:50 |
agent |
expert |
5m
Model:
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Instruction: You are an experienced Linux kernel security engineer. Your task is to analyze given kernel bug report
and determine its security impact based on the following dimensions.
Use the provided tools to examine the source code, check for capability checks (e.g., capable(), ns_capable()),
and understand the nature of the bug. Analyze the given kernel build and configuration.
You can check the kernel config by grepping ".config" file; you can check kernel cmdline by grepping
".config" file for "CONFIG_CMDLINE=". Assume sysctl parameters have default values.
But analyze for the corresponding production build w/o debugging tools enabled (like KASAN, KMSAN, UBSAN).
Try different strategies when analyzing the bug:
- think of ways in which the vulnerable code is unreachable
- or the other way around: try to come up with different ideas of how an unprivileged user can reach the bug
If still unsure err on the side of the bug being non-exploitable/not-accessible.
In the final reply, provide a reasoning for your assessment.
Analysis dimensions:
* Exploitable:
Determine if the bug can result in memory corruption, elevated privileges, or an information leak.
Memory safety issues are almost always exploitable (KASAN or UBSAN reports for use-after-free, out-of-bounds;
refcounting issues, corrupted lists, etc). When kernel is crashing on a completely wild pointer access
(e.g. user-space address, or non-canonical address, but not on NULL or address corresponding to KASAN shadow
for NULL address), including both data accesses and control transfers, that also usually implies possibility
of exploitation. Such reports usually say "unable to handle kernel paging request".
Uses of uninitialized values detected by KMSAN may be exploitable b/c attacker frequently can affect uninit
values with spraying techniques. However, for these exploitability depends on how exactly the uninit value
is used in the code, and what it affects.
Information leaks are exploitable on their own and should be classified as such. A bug that copies kernel
memory contents to userspace (e.g. an out-of-bounds read whose result is returned to the caller, or
uninitialized stack/heap bytes written to a user buffer) is exploitable: it can reveal kernel pointer
values and defeat KASLR, expose sensitive data such as cryptographic keys or other processes' memory, and
serves as a necessary building block in most modern kernel privilege-escalation exploit chains. Do not classify
an information leak as non-exploitable solely because it does not directly cause a memory write or control-flow
hijack; the leak itself is the exploit primitive.
Think of what happens after the bug is triggered. Some bugs cause kernel panic and halt execution,
they are harder to exploit. For example, BUG reports halts the kernel. However, WARNING reports don't halt
execution in production builds. Debug bug detection tools (like KASAN, KMSAN, KCSAN, UBSAN) are also not enabled
in production builds, so attacker can freely exploit these bugs w/o being detected by these tools.
If you see an integer overflow, think how the overflowed value used later (if it's used as allocation size,
or an array index). If you see an out-of-bounds read, think if it's followed by an out-of-bounds write as well.
Some KCSAN data-races may be exploitable by skilled attackers as well. Think what data structures got corrupted
as the result of data races and how. However, note that kernel has lots of "benign" data races that don't lead
to any runtime misbehavior at all.
* Denial Of Service:
Determine if the bug can result in denial-of-service. Most bugs can, since they cause system crash,
hangs, deadlocks, or resource leaks. This is mostly applicable to WARNING bugs that won't cause system crash
in production. For these think what will be consequences of the violation of the kernel assumptions flagged
by the WARNING. In some cases the unexpected condition is also properly handled by the normal control flow
(e.g. with "if (WARN_ON(...))"), these won't cause denial-of-service. If the condition is not handled,
then it may or may not cause denial-of-service.
* Accessible From Unprivileged Processes:
Determine if the bug can be reached from a typical (non-root) user process that does NOT have any special capabilities
(like CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON) or access to device nodes restricted to root.
Assume that unprivileged_bpf_disabled=1, that is eBPF loading is not accessible. However, cBPF (classical BPF)
is still accessible to non-root processes.
Assume that user namespaces are not accessible, that is, the process cannot get the mentioned capabilities even
within a new user namespace (checked by ns_capable() function in the kernel sources).
* Accessible From User Namespaces:
Determine if the bug can be reached within a user-namespace where the process has all capabilities
(including CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON). Such capabilities are checked with ns_capable()
function in the kernel sources.
* VM Guest Trigger:
Determine if the bug can be triggered from the context of a typical KVM guest (e.g., set up by a QEMU VMM).
Consider accesses to standard Linux host paravirtualized features (virtio-blk, virtio-net, etc.),
and handling of VM exits in the KVM code.
* VM Host Trigger in The Confidential Computing Context:
Determine if the bug can be triggered in a confidential computing guest kernel from the context of a KVM host.
Consider access to standard Linux guest paravirtualized features (virtio-blk, virtio-net, etc.).
* Ethernet Network Trigger:
Determine if the bug can be triggered by processing ingress network Ethernet traffic, either directly (network stack)
or via drivers exposed to network data.
* Other Remote Trigger:
Determine if the bug can be triggered by processing remote traffic other than Ethernet (Wifi, Bluetooth, NFC, etc).
* Peripheral Trigger:
Determine if the bug can be triggered via an untrusted peripheral device that can be physically plugged
into a system, such as a USB device or a niche hardware driver handling external hardware inputs.
This is particularly important for mobile and desktop environments where users can plug in unknown devices.
* Malicious Filesystem Trigger:
Determine if the bug can be triggered by the kernel mounting and parsing a malicious filesystem image.
This is highly critical for Desktop and Mobile environments where external media or downloaded images
might be auto-mounted.
Don't make assumptions about the kernel source code (it may be different from what you assume it is).
Extensively use the provided code access tools (codesearch-*, git-*, grepper, etc)
to examine the actual source code, and confirm any assumptions.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt:
The kernel bug report is:
------------[ cut here ]------------
WARNING: mm/vma.h:277 at assert_sane_pgoff mm/vma.h:277 [inline], CPU#1: syz.0.17/4912
WARNING: mm/vma.h:277 at vma_set_pgoff mm/vma.h:283 [inline], CPU#1: syz.0.17/4912
WARNING: mm/vma.h:277 at vma_add_pgoff+0x50c/0x700 mm/vma.h:314, CPU#1: syz.0.17/4912
Modules linked in:
CPU: 1 UID: 0 PID: 4912 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/07/2026
pstate: 83400005 (Nzcv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
pc : assert_sane_pgoff mm/vma.h:277 [inline]
pc : vma_set_pgoff mm/vma.h:283 [inline]
pc : vma_add_pgoff+0x50c/0x700 mm/vma.h:314
lr : assert_sane_pgoff mm/vma.h:277 [inline]
lr : vma_set_pgoff mm/vma.h:283 [inline]
lr : vma_add_pgoff+0x50c/0x700 mm/vma.h:314
sp : ffff8000963176e0
x29: ffff800096317700 x28: 1fffe00019d174aa x27: 0000000000000001
x26: ffff0000c659b700 x25: dfff800000000000 x24: 1fffe00019d174b0
x23: ffff0000ce8ba580 x22: 0000000000020a97 x21: ffff0000ce8ba548
x20: ffff0000ce8ba500 x19: 0000000020001000 x18: 1fffe00035ba3828
x17: ffff800080adeaf4 x16: ffff800080addecc x15: ffff800080b8bf18
x14: ffff800080b96bac x13: 0000000000000001 x12: 0000000000000000
x11: 0000000000000000 x10: 0000000000000003 x9 : 0000000000000000
x8 : 0000000000000000 x7 : ffff800080bee7b0 x6 : 0000000000000000
x5 : 0000000000000000 x4 : 0000000000000001 x3 : ffff800080ae89e0
x2 : 0000000000020001 x1 : ffff0000c825bb00 x0 : 0000000000000000
Call trace:
assert_sane_pgoff mm/vma.h:277 [inline] (P)
vma_set_pgoff mm/vma.h:283 [inline] (P)
vma_add_pgoff+0x50c/0x700 mm/vma.h:314 (P)
__split_vma+0x6a8/0x83c mm/vma.c:607
split_vma mm/vma.c:643 [inline]
vma_modify+0x11ac/0x18f0 mm/vma.c:1771
vma_modify_policy+0x210/0x2b8 mm/vma.c:1834
mbind_range+0x160/0x3e4 mm/mempolicy.c:1061
do_mbind mm/mempolicy.c:1563 [inline]
kernel_mbind mm/mempolicy.c:1760 [inline]
__do_sys_mbind mm/mempolicy.c:1834 [inline]
__se_sys_mbind mm/mempolicy.c:1830 [inline]
__arm64_sys_mbind+0x5c8/0x994 mm/mempolicy.c:1830
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xec/0x23c arch/arm64/kernel/syscall.c:121
do_el0_svc+0x4c/0x5c arch/arm64/kernel/syscall.c:140
el0_svc+0x64/0x260 arch/arm64/kernel/entry-common.c:758
el0t_64_sync_handler+0x44/0x104 arch/arm64/kernel/entry-common.c:777
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:590
irq event stamp: 928
hardirqs last enabled at (927): [<ffff8000869a9950>] __raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:210 [inline]
hardirqs last enabled at (927): [<ffff8000869a9950>] _raw_spin_unlock_irqrestore+0x38/0x98 kernel/locking/spinlock.c:221
hardirqs last disabled at (928): [<ffff8000869841d4>] el1_brk64+0x20/0x54 arch/arm64/kernel/entry-common.c:445
softirqs last enabled at (208): [<ffff80008013891c>] local_bh_enable include/linux/bottom_half.h:33 [inline]
softirqs last enabled at (208): [<ffff80008013891c>] put_cpu_fpsimd_context arch/arm64/kernel/fpsimd.c:251 [inline]
softirqs last enabled at (208): [<ffff80008013891c>] do_sve_acc+0x32c/0x4b8 arch/arm64/kernel/fpsimd.c:1349
softirqs last disabled at (206): [<ffff8000801386fc>] local_bh_disable include/linux/bottom_half.h:20 [inline]
softirqs last disabled at (206): [<ffff8000801386fc>] get_cpu_fpsimd_context arch/arm64/kernel/fpsimd.c:234 [inline]
softirqs last disabled at (206): [<ffff8000801386fc>] do_sve_acc+0x10c/0x4b8 arch/arm64/kernel/fpsimd.c:1325
---[ end trace 0000000000000000 ]---
It is reproducible with the followint program.
Keep in mind that it may lack the precise threading, sandboxing, and some arguments of a working reproducer.
But it should give an idea of the involved syscalls.
// autogenerated by syzkaller (https://github.com/google/syzkaller)
#define _GNU_SOURCE
#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#ifndef __NR_mbind
#define __NR_mbind 235
#endif
#ifndef __NR_mmap
#define __NR_mmap 222
#endif
#ifndef __NR_mremap
#define __NR_mremap 216
#endif
#ifndef __NR_socketpair
#define __NR_socketpair 199
#endif
int main(void)
{
syscall(__NR_mmap, /*addr=*/0x1ffff000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x20000000ul, /*len=*/0x1000000, /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/7ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x21000000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
const char* reason;
(void)reason;
if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {}
// socketpair$unix arguments: [
// domain: const = 0x1 (8 bytes)
// type: unix_socket_type = 0x2 (8 bytes)
// proto: const = 0x0 (4 bytes)
// fds: ptr[out, unix_pair] {
// unix_pair {
// fd0: sock_unix (resource)
// fd1: sock_unix (resource)
// }
// }
// ]
syscall(__NR_socketpair, /*domain=*/1ul, /*type=SOCK_DGRAM*/2ul, /*proto=*/0, /*fds=*/0x20000080ul);
// mremap arguments: [
// addr: VMA[0x1000]
// len: len = 0x1000 (8 bytes)
// newlen: len = 0x400000 (8 bytes)
// flags: mremap_flags = 0x3 (8 bytes)
// newaddr: VMA[0x400000]
// ]
syscall(__NR_mremap, /*addr=*/0x20a96000ul, /*len=*/0x1000ul, /*newlen=*/0x400000ul, /*flags=MREMAP_FIXED|MREMAP_MAYMOVE*/3ul, /*newaddr=*/0x20000000ul);
// mremap arguments: [
// addr: VMA[0x9000]
// len: len = 0x600002 (8 bytes)
// newlen: len = 0x600002 (8 bytes)
// flags: mremap_flags = 0x7 (8 bytes)
// newaddr: VMA[0x600000]
// ]
syscall(__NR_mremap, /*addr=*/0x20000000ul, /*len=*/0x600002ul, /*newlen=*/0x600002ul, /*flags=MREMAP_FIXED|MREMAP_MAYMOVE|0x4*/7ul, /*newaddr=*/0x20a00000ul);
// mbind arguments: [
// addr: VMA[0x800000]
// len: len = 0x800000 (8 bytes)
// mode: mbind_mode = 0x1 (8 bytes)
// nodemask: nil
// maxnode: intptr = 0x7 (8 bytes)
// flags: mbind_flags = 0x2 (8 bytes)
// ]
syscall(__NR_mbind, /*addr=*/0x20001000ul, /*len=*/0x800000ul, /*mode=MPOL_PREFERRED*/1ul, /*nodemask=*/0ul, /*maxnode=*/7ul, /*flags=MPOL_MF_MOVE*/2ul);
return 0;
}
Reply: The bug report describes a `VM_WARN_ON_ONCE` triggering in `assert_sane_pgoff` during a VMA split operation (in this case, initiated by `mbind`).
### Cause of the Warning
The `assert_sane_pgoff` function asserts that if an anonymous VMA is unfaulted (i.e., `vma->anon_vma == NULL`), its page offset (`vma->vm_pgoff`) must be equal to its virtual address page offset (`vma->vm_start >> PAGE_SHIFT`).
However, this assumption can be legitimately violated using `mremap` with the `MREMAP_DONTUNMAP` flag:
1. An anonymous VMA is created and a page is faulted in, assigning it an `anon_vma`.
2. The VMA is moved to a new address using `mremap(MREMAP_MAYMOVE)`. Its `vm_pgoff` is preserved (so it no longer equals `vm_start >> PAGE_SHIFT`), and its `anon_vma` is cloned.
3. The VMA is moved again using `mremap(MREMAP_DONTUNMAP)`. This operation moves the page tables to a new VMA, leaving the old VMA intact but empty. As part of this process, `dontunmap_complete()` calls `unlink_anon_vmas()`, which clears `vma->anon_vma` of the old VMA.
4. The old VMA now has `anon_vma == NULL`, but its `vm_pgoff` retains the preserved value, which does not equal `vm_start >> PAGE_SHIFT`.
5. A subsequent operation that splits or modifies the old VMA (like `mbind` in the reproducer) calls `vma_add_pgoff()`, which triggers the `VM_WARN_ON_ONCE` in `assert_sane_pgoff()`.
### Security Impact
This is a **benign false positive warning** with **no security impact**:
- **No Memory Corruption:** The mismatched `vm_pgoff` does not cause any memory corruption or logical errors. If new pages are faulted into this empty VMA, they will be assigned an index based on the existing `vm_pgoff`, and the reverse mapping (`rmap`) will correctly find the VMA using this index. The kernel handles this state perfectly fine.
- **Compiled Out in Production:** The warning is a `VM_WARN_ON_ONCE`, which is only defined when `CONFIG_DEBUG_VM` is enabled. In production kernels, this configuration is disabled, meaning the macro evaluates to a no-op (`BUILD_BUG_ON_INVALID`) and generates no runtime code. It will not cause a panic, oops, or even a log message in production environments.
- **No Denial of Service:** Since the warning is compiled out in production (and is only a non-fatal warning even when compiled in), it cannot be used to crash or hang the system.
Therefore, this bug is neither exploitable nor capable of causing a denial-of-service in production builds.
|
| 6/2 |
2026/09/07 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 9290
output: 22
thoughts: 335
Thoughts: **Analyzing a VMA Assertion**
Okay, so I've got a WARNING flagged at `mm/vma.h:277`. The kernel's complaining about something in `assert_sane_pgoff`. Right, let's trace this. Looks like `assert_sane_pgoff` is inlined, then called by `vma_set_pgoff`, which is also inlined. Then, it goes into `vma_add_pgoff`, then `__split_vma`, and all the way down to `__do_sys_mbind`. My spidey-sense is tingling, as this looks like itās all related to memory policy, particularly mbind.
Let's zoom in on `mm/vma.h` around line 277. I need to see what `assert_sane_pgoff` is asserting, and why it's potentially failing here in this context, especially given the memory policy path. There's probably something incorrect about how the page offset is being handled during a VMA modification with the specific mbind call. I'm guessing we're hitting an edge case related to memory policy changes. Let's see if we've got an invalid `pgoff` value passed in.
|
| 7/2 |
2026/09/07 00:50 |
tool |
grepper |
0m
Args: map[Expression:assert_sane_pgoff]
Results: map[Output:mm/vma.h=253=static inline pgoff_t vmg_end_pgoff(const struct vma_merge_struct *vmg)
--
mm/vma.h-257-
mm/vma.h:258:static inline void assert_sane_pgoff(struct vm_area_struct *vma, pgoff_t pgoff)
mm/vma.h-259-{
--
mm/vma.h=280=static inline void vma_set_pgoff(struct vm_area_struct *vma, pgoff_t pgoff)
--
mm/vma.h-282- vma_assert_can_modify(vma);
mm/vma.h:283: assert_sane_pgoff(vma, pgoff);
mm/vma.h-284- vma->vm_pgoff = pgoff;
]
|
| 8/2 |
2026/09/07 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 10173
output: 34
thoughts: 19
Thoughts: Here's a summary of my thought process, as requested:
**Diving into `mm/vma.h`**
Okay, I need to get into the details of the memory management structures, and it looks like a good starting point is `mm/vma.h` around line 258. I'm probably looking for the definition of the `vm_area_struct` or some related data structure, as that's where the Virtual Memory Area (VMA) information is stored. I want to see how the kernel organizes and tracks memory mappings for a process. Let's see how the fields are laid out, what data they contain (starting address, size, protection flags, flags for shared or private mappings, etc.), and how they are linked together. This should give me a good understanding of how the kernel manages virtual address spaces. I'm especially interested in how VMAs relate to page tables and physical memory.
|
| 9/2 |
2026/09/07 00:50 |
tool |
read-file |
0m
Args: map[File:mm/vma.h FirstLine:250 LineCount:50]
Results: map[Contents: 250: return vmg->pgoff;
251: }
252:
253: static inline pgoff_t vmg_end_pgoff(const struct vma_merge_struct *vmg)
254: {
255: return vmg_start_pgoff(vmg) + vmg_pages(vmg);
256: }
257:
258: static inline void assert_sane_pgoff(struct vm_area_struct *vma, pgoff_t pgoff)
259: {
260: /* nommu doesn't set a virtual pgoff for anon VMAs. */
261: if (!IS_ENABLED(CONFIG_MMU))
262: return;
263: /*
264: * File-backed VMAs have arbitrary page offset (either page offset into
265: * file or for pfnmap the PFN of the start of the range or drivers may
266: * set arbitrary page offset).
267: */
268: if (!vma_is_anonymous(vma))
269: return;
270: /* MAP_PRIVATE-/dev/zero is anon, non-NULL vm_file, but has file pgoff. */
271: if (vma->vm_file)
272: return;
273: /* If faulted in, could have been remapped. */
274: if (vma->anon_vma)
275: return;
276: /* OK this is really an anon VMA - expect virtual page offset. */
277: VM_WARN_ON_ONCE(pgoff != vma->vm_start >> PAGE_SHIFT);
278: }
279:
280: static inline void vma_set_pgoff(struct vm_area_struct *vma, pgoff_t pgoff)
281: {
282: vma_assert_can_modify(vma);
283: assert_sane_pgoff(vma, pgoff);
284: vma->vm_pgoff = pgoff;
285: }
286:
287: static inline pgoff_t vmg_start_anon_pgoff(const struct vma_merge_struct *vmg)
288: {
289: return vmg->anon_pgoff;
290: }
291:
292: static inline pgoff_t vmg_end_anon_pgoff(const struct vma_merge_struct *vmg)
293: {
294: return vmg_start_anon_pgoff(vmg) + vmg_pages(vmg);
295: }
296:
297: static inline void __vma_set_anon_pgoff(struct vm_area_struct *vma, pgoff_t pgoff)
298: {
299: #ifdef CONFIG_64BIT
]
|
| 10/2 |
2026/09/07 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 11282
output: 23
thoughts: 86
Thoughts: Okay, let's see, I'm looking at this warning: `VM_WARN_ON_ONCE(pgoff != vma->vm_start >> PAGE_SHIFT);`. This looks like a classic `VM_WARN_ON_ONCE` situation. The key question, as always with these, is whether this thing is even enabled in a production environment. I know that `VM_WARN_ON_ONCE` warnings are often tied to debug configurations. My initial thought is that `CONFIG_DEBUG_VM` might be required for this to even fire. So, first things first, I need to verify whether the relevant configuration options are set to confirm that assumption. I'll need to check the kernel's config settings.
|
| 11/2 |
2026/09/07 00:50 |
tool |
grepper |
0m
Args: map[Expression:define VM_WARN_ON_ONCE]
Results: map[Output:include/linux/mmdebug.h=18=void vma_iter_dump_tree(const struct vma_iterator *vmi);
--
include/linux/mmdebug.h-58-})
include/linux/mmdebug.h:59:#define VM_WARN_ON_ONCE_PAGE(cond, page) ({ \
include/linux/mmdebug.h-60- static bool __section(".data..once") __warned; \
--
include/linux/mmdebug.h-78-})
include/linux/mmdebug.h:79:#define VM_WARN_ON_ONCE_FOLIO(cond, folio) ({ \
include/linux/mmdebug.h-80- static bool __section(".data..once") __warned; \
--
include/linux/mmdebug.h-89-})
include/linux/mmdebug.h:90:#define VM_WARN_ON_ONCE_MM(cond, mm) ({ \
include/linux/mmdebug.h-91- static bool __section(".data..once") __warned; \
--
include/linux/mmdebug.h-100-})
include/linux/mmdebug.h:101:#define VM_WARN_ON_ONCE_VMA(cond, vma) ({ \
include/linux/mmdebug.h-102- static bool __section(".data..once") __warned; \
--
include/linux/mmdebug.h-122-#define VM_WARN_ON(cond) (void)WARN_ON(cond)
include/linux/mmdebug.h:123:#define VM_WARN_ON_ONCE(cond) (void)WARN_ON_ONCE(cond)
include/linux/mmdebug.h-124-#define VM_WARN_ONCE(cond, format...) (void)WARN_ONCE(cond, format)
--
include/linux/mmdebug.h-132-#define VM_WARN_ON(cond) BUILD_BUG_ON_INVALID(cond)
include/linux/mmdebug.h:133:#define VM_WARN_ON_ONCE(cond) BUILD_BUG_ON_INVALID(cond)
include/linux/mmdebug.h-134-#define VM_WARN_ON_PAGE(cond, page) BUILD_BUG_ON_INVALID(cond)
include/linux/mmdebug.h:135:#define VM_WARN_ON_ONCE_PAGE(cond, page) BUILD_BUG_ON_INVALID(cond)
include/linux/mmdebug.h-136-#define VM_WARN_ON_FOLIO(cond, folio) BUILD_BUG_ON_INVALID(cond)
include/linux/mmdebug.h:137:#define VM_WARN_ON_ONCE_FOLIO(cond, folio) BUILD_BUG_ON_INVALID(cond)
include/linux/mmdebug.h:138:#define VM_WARN_ON_ONCE_MM(cond, mm) BUILD_BUG_ON_INVALID(cond)
include/linux/mmdebug.h:139:#define VM_WARN_ON_ONCE_VMA(cond, vma) BUILD_BUG_ON_INVALID(cond)
include/linux/mmdebug.h-140-#define VM_WARN_ON_VMG(cond, vmg) BUILD_BUG_ON_INVALID(cond)
--
tools/testing/vma/vma_internal.h=52=typedef __bitwise unsigned int vm_fault_t;
--
tools/testing/vma/vma_internal.h-54-#define VM_WARN_ON(_expr) (WARN_ON(_expr))
tools/testing/vma/vma_internal.h:55:#define VM_WARN_ON_ONCE(_expr) (WARN_ON_ONCE(_expr))
tools/testing/vma/vma_internal.h:56:#define VM_WARN_ON_ONCE_VMA(_expr, _vma) (WARN_ON_ONCE(_expr))
tools/testing/vma/vma_internal.h-57-#define VM_WARN_ON_VMG(_expr, _vmg) (WARN_ON(_expr))
]
|
| 12/2 |
2026/09/07 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 12571
output: 36
thoughts: 40
Thoughts: Okay, here's my take on those thoughts, summarizing them from a first-person perspective, assuming I'm an expert in the field:
**Investigating the `VM_WARN_ON_ONCE` Definition**
Alright, so I'm thinking I need to understand the relationship between `VM_WARN_ON_ONCE` and `WARN_ON_ONCE`. It's a hunch that `VM_WARN_ON_ONCE` might sometimes expand directly into `WARN_ON_ONCE`, and I want to figure out *when* that happens. My initial thought is to go straight to the source. The best way to get a solid grasp of how this works is to dive into the kernel header files. Specifically, I'm reaching for `include/linux/mmdebug.h`. That seems like the logical place to find the conditional definition or any macros that influence how `VM_WARN_ON_ONCE` is handled in relation to `WARN_ON_ONCE`. I'm expecting some kind of conditional compilation based on kernel configuration options or other compile-time flags. Let's see what the preprocessor is doing under the hood!
|
| 13/2 |
2026/09/07 00:50 |
tool |
read-file |
0m
Args: map[File:include/linux/mmdebug.h FirstLine:110 LineCount:50]
Results: map[Contents: 110: unlikely(__ret_warn_once); \
111: })
112: #define VM_WARN_ON_VMG(cond, vmg) ({ \
113: int __ret_warn = !!(cond); \
114: \
115: if (unlikely(__ret_warn)) { \
116: dump_vmg(vmg, "VM_WARN_ON_VMG(" __stringify(cond)")"); \
117: WARN_ON(1); \
118: } \
119: unlikely(__ret_warn); \
120: })
121:
122: #define VM_WARN_ON(cond) (void)WARN_ON(cond)
123: #define VM_WARN_ON_ONCE(cond) (void)WARN_ON_ONCE(cond)
124: #define VM_WARN_ONCE(cond, format...) (void)WARN_ONCE(cond, format)
125: #define VM_WARN(cond, format...) (void)WARN(cond, format)
126: #else
127: #define VM_BUG_ON(cond) BUILD_BUG_ON_INVALID(cond)
128: #define VM_BUG_ON_PAGE(cond, page) VM_BUG_ON(cond)
129: #define VM_BUG_ON_FOLIO(cond, folio) VM_BUG_ON(cond)
130: #define VM_BUG_ON_VMA(cond, vma) VM_BUG_ON(cond)
131: #define VM_BUG_ON_MM(cond, mm) VM_BUG_ON(cond)
132: #define VM_WARN_ON(cond) BUILD_BUG_ON_INVALID(cond)
133: #define VM_WARN_ON_ONCE(cond) BUILD_BUG_ON_INVALID(cond)
134: #define VM_WARN_ON_PAGE(cond, page) BUILD_BUG_ON_INVALID(cond)
135: #define VM_WARN_ON_ONCE_PAGE(cond, page) BUILD_BUG_ON_INVALID(cond)
136: #define VM_WARN_ON_FOLIO(cond, folio) BUILD_BUG_ON_INVALID(cond)
137: #define VM_WARN_ON_ONCE_FOLIO(cond, folio) BUILD_BUG_ON_INVALID(cond)
138: #define VM_WARN_ON_ONCE_MM(cond, mm) BUILD_BUG_ON_INVALID(cond)
139: #define VM_WARN_ON_ONCE_VMA(cond, vma) BUILD_BUG_ON_INVALID(cond)
140: #define VM_WARN_ON_VMG(cond, vmg) BUILD_BUG_ON_INVALID(cond)
141: #define VM_WARN_ONCE(cond, format...) BUILD_BUG_ON_INVALID(cond)
142: #define VM_WARN(cond, format...) BUILD_BUG_ON_INVALID(cond)
143: #endif /* CONFIG_DEBUG_VM */
144:
145: #ifdef CONFIG_DEBUG_VM_IRQSOFF
146: #define VM_WARN_ON_IRQS_ENABLED() WARN_ON_ONCE(!irqs_disabled())
147: #else
148: #define VM_WARN_ON_IRQS_ENABLED() do { } while (0)
149: #endif
150:
151: #ifdef CONFIG_DEBUG_VIRTUAL
152: #define VIRTUAL_BUG_ON(cond) BUG_ON(cond)
153: #else
154: #define VIRTUAL_BUG_ON(cond) do { } while (0)
155: #endif
156:
157: #ifdef CONFIG_DEBUG_VM_PGFLAGS
158: #define VM_BUG_ON_PGFLAGS(cond, page) VM_BUG_ON_PAGE(cond, page)
159: #else
]
|
| 14/2 |
2026/09/07 00:50 |
llm |
expert |
0m
Model:
Tokens:
input: 14000
output: 34
thoughts: 236
Thoughts: **Analyzing the Impact of `VM_WARN_ON_ONCE` in Production**
Okay, so I'm looking at this `VM_WARN_ON_ONCE` macro and how it's used, and it's immediately clear that it's tied to `CONFIG_DEBUG_VM`, which is almost certainly disabled in production kernels. In a production build, this macro essentially becomes a no-op, specifically `BUILD_BUG_ON_INVALID(cond)`. That means it won't actually generate any runtime code, no crash, no warning ā it's optimized away. My initial instinct is to shrug and move on, thinking it's irrelevant. But... wait a minute. Does the *condition* it checks still have potential implications, even if the warning itself disappears?
Let's dive into the context. The code within the conditional uses `assert_sane_pgoff`. Alright, `assert_sane_pgoff` is really just checking that `pgoff` is what's expected for anonymous VMAs, specifically that it's equivalent to the VMA's start address shifted right by `PAGE_SHIFT`. If that assertion *fails*, then `vma->vm_pgoff` is *modified* by the `assert_sane_pgoff` to something else. Now, I'm getting curious. Does `vm_pgoff` *matter* for anonymous VMAs? I believe `vm_pgoff` usually represents the virtual page offset, which is crucial for address calculations. If this value is being overwritten incorrectly, even if it doesn't cause an immediate crash, it could lead to all sorts of subtle, potentially devastating memory mapping errors. The impact could be insidious. Time to track down where `vma_add_pgoff` is called.
|
| 15/2 |
2026/09/07 00:51 |
tool |
read-file |
0m
Args: map[File:mm/vma.h FirstLine:300 LineCount:50]
Results: map[Contents: 300: vma->__vm_anon_pgoff_hi = pgoff >> 32;
301: #endif
302: vma->__vm_anon_pgoff_lo = pgoff & GENMASK(31, 0);
303: }
304:
305: static inline void vma_set_anon_pgoff(struct vm_area_struct *vma, pgoff_t pgoff)
306: {
307: vma_assert_can_modify(vma);
308: __vma_set_anon_pgoff(vma, pgoff);
309: }
310:
311: static inline void vma_add_pgoff(struct vm_area_struct *vma, pgoff_t delta)
312: {
313: vma_assert_can_modify(vma);
314: vma_set_pgoff(vma, vma_start_pgoff(vma) + delta);
315: vma_set_anon_pgoff(vma, vma_start_anon_pgoff(vma) + delta);
316: }
317:
318: static inline void vma_sub_pgoff(struct vm_area_struct *vma, pgoff_t delta)
319: {
320: vma_assert_can_modify(vma);
321: vma_set_pgoff(vma, vma_start_pgoff(vma) - delta);
322: vma_set_anon_pgoff(vma, vma_start_anon_pgoff(vma) - delta);
323: }
324:
325: #define VMG_STATE(name, mm_, vmi_, start_, end_, vma_flags_, pgoff_, anon_pgoff_) \
326: struct vma_merge_struct name = { \
327: .mm = mm_, \
328: .vmi = vmi_, \
329: .start = start_, \
330: .end = end_, \
331: .vma_flags = vma_flags_, \
332: .pgoff = pgoff_, \
333: .anon_pgoff = anon_pgoff_, \
334: .state = VMA_MERGE_START, \
335: }
336:
337: #define VMG_VMA_STATE(name, vmi_, prev_, vma_, start_, end_) \
338: struct vma_merge_struct name = { \
339: .mm = vma_->vm_mm, \
340: .vmi = vmi_, \
341: .prev = prev_, \
342: .middle = vma_, \
343: .next = NULL, \
344: .start = start_, \
345: .end = end_, \
346: .vm_flags = vma_->vm_flags, \
347: .pgoff = linear_page_index(vma_, start_), \
348: .anon_pgoff = __linear_anon_page_index(vma_, start_), \
349: .file = vma_->vm_file, \
]
|
| 16/2 |
2026/09/07 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 15639
output: 34
thoughts: 21
Thoughts: Here's a summary of my immediate thoughts, focusing on the code snippet provided, assuming I'm deep in kernel debugging/optimization:
**Dissecting a Virtual Memory Split**
Alright, so the trace is hitting me with `__split_vma` then immediately calling `vma_add_pgoff`. My initial reaction is, okay, typical VM operation. Someone's trying to adjust a virtual memory area, which almost certainly means some sort of memory management event is occurring. Could be a page fault, a mmap change, a huge page split, or maybe even something more exotic. Seeing `__split_vma` first tells me that the existing VMA is too coarse-grained and needs to be broken down, most likely because of an operation that requires a more granular allocation.
The subsequent `vma_add_pgoff` call is the critical action; It's where the actual new VMA part gets added. It has something to do with adding a new page offset to the virtual memory area. I need to figure out *what* kind of pages and *why* they're being added at this location. Is this part of a copy-on-write operation? A page migration? Or maybe a new mapping request? I'd immediately want to see the context around this sequence. What's the surrounding code in the trace? What process is doing this? What's the overall memory state? The parameters to these functions are the key to understanding the intent. I'd reach for `perf` or `ftrace` immediately to get the bigger picture. This small snippet is just the beginning of a larger story that I need to uncover.
|
| 17/2 |
2026/09/07 00:51 |
tool |
read-file |
0m
Args: map[File:mm/vma.c FirstLine:590 LineCount:50]
Results: map[Contents: 590: vma_start_write(vma);
591: vma_start_write(new);
592:
593: init_vma_prep(&vp, vma);
594: vp.insert = new;
595: vma_prepare(&vp);
596:
597: /*
598: * Get rid of huge pages and shared page tables straddling the split
599: * boundary.
600: */
601: vma_adjust_trans_huge(vma, vma->vm_start, addr, NULL);
602: if (is_vm_hugetlb_page(vma))
603: hugetlb_split(vma, addr);
604:
605: if (new_below) {
606: vma->vm_start = addr;
607: vma_add_pgoff(vma, linear_page_delta(new, addr));
608: } else {
609: vma->vm_end = addr;
610: }
611:
612: /* vma_complete stores the new vma */
613: vma_complete(&vp, vmi, vma->vm_mm);
614: validate_mm(vma->vm_mm);
615:
616: /* Success. */
617: if (new_below)
618: vma_next(vmi);
619: else
620: vma_prev(vmi);
621:
622: return 0;
623:
624: out_free_mpol:
625: mpol_put(vma_policy(new));
626: out_free_vmi:
627: vma_iter_free(vmi);
628: out_free_vma:
629: vm_area_free(new);
630: return err;
631: }
632:
633: /*
634: * Split a vma into two pieces at address 'addr', a new vma is allocated
635: * either for the first part or the tail.
636: */
637: static int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
638: unsigned long addr, int new_below)
639: {
]
|
| 18/2 |
2026/09/07 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 16764
output: 19
thoughts: 36
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm deep in the weeds of memory management and virtual address spaces:
**Understanding the Page Offset Addition**
Alright, I see we're calling `vma_add_pgoff`. That's a pretty standard function for adjusting the page offset of a virtual memory area (VMA). The goal here is clearly to extend or adjust a VMA's page mapping. The key is what `vma_add_pgoff` is getting as an argument. I need to understand this new offset value. It appears it's derived from `linear_page_delta(new, addr)`. I immediately know I need to delve into how that delta is being calculated, because that function holds the answer to what *new* virtual page offset should be. That's the core of figuring out this mapping change. So, the delta will tell me how far to shift the page offset. Let's see how that calculation's being done.
|
| 19/2 |
2026/09/07 00:51 |
tool |
grepper |
0m
Args: map[Expression:linear_page_delta]
Results: map[Output:drivers/gpu/drm/etnaviv/etnaviv_gem.c=164=static vm_fault_t etnaviv_gem_fault(struct vm_fault *vmf)
--
drivers/gpu/drm/etnaviv/etnaviv_gem.c-191- /* We don't use vmf->pgoff since that has the fake offset: */
drivers/gpu/drm/etnaviv/etnaviv_gem.c:192: pgoff = linear_page_delta(vma, vmf->address);
drivers/gpu/drm/etnaviv/etnaviv_gem.c-193-
--
drivers/gpu/drm/gma500/gem.c=255=static vm_fault_t psb_gem_fault(struct vm_fault *vmf)
--
drivers/gpu/drm/gma500/gem.c-290- because vmf->pgoff is the fake GEM offset */
drivers/gpu/drm/gma500/gem.c:291: page_offset = linear_page_delta(vma, vmf->address);
drivers/gpu/drm/gma500/gem.c-292-
--
drivers/gpu/drm/msm/msm_gem.c=330=static vm_fault_t msm_gem_fault(struct vm_fault *vmf)
--
drivers/gpu/drm/msm/msm_gem.c-363- /* We don't use vmf->pgoff since that has the fake offset: */
drivers/gpu/drm/msm/msm_gem.c:364: pgoff = linear_page_delta(vma, vmf->address);
drivers/gpu/drm/msm/msm_gem.c-365-
--
drivers/gpu/drm/omapdrm/omap_gem.c=355=static vm_fault_t omap_gem_fault_1d(struct drm_gem_object *obj,
--
drivers/gpu/drm/omapdrm/omap_gem.c-362- /* We don't use vmf->pgoff since that has the fake offset: */
drivers/gpu/drm/omapdrm/omap_gem.c:363: pgoff = linear_page_delta(vma, vmf->address);
drivers/gpu/drm/omapdrm/omap_gem.c-364-
--
drivers/gpu/drm/omapdrm/omap_gem.c=380=static vm_fault_t omap_gem_fault_2d(struct drm_gem_object *obj,
--
drivers/gpu/drm/omapdrm/omap_gem.c-410- /* We don't use vmf->pgoff since that has the fake offset: */
drivers/gpu/drm/omapdrm/omap_gem.c:411: pgoff = linear_page_delta(vma, vmf->address);
drivers/gpu/drm/omapdrm/omap_gem.c-412-
--
drivers/gpu/drm/tegra/gem.c=557=static vm_fault_t tegra_bo_fault(struct vm_fault *vmf)
--
drivers/gpu/drm/tegra/gem.c-567-
drivers/gpu/drm/tegra/gem.c:568: offset = linear_page_delta(vma, vmf->address);
drivers/gpu/drm/tegra/gem.c-569- page = bo->pages[offset];
--
drivers/vfio/pci/nvgrace-gpu/main.c=383=static unsigned long addr_to_pgoff(struct vm_area_struct *vma,
--
drivers/vfio/pci/nvgrace-gpu/main.c-388-
drivers/vfio/pci/nvgrace-gpu/main.c:389: return linear_page_delta(vma, addr) + pgoff;
drivers/vfio/pci/nvgrace-gpu/main.c-390-}
--
drivers/vfio/pci/vfio_pci_core.c=1779=static vm_fault_t vfio_pci_mmap_huge_fault(struct vm_fault *vmf,
--
drivers/vfio/pci/vfio_pci_core.c-1784- unsigned long addr = vmf->address & ~((PAGE_SIZE << order) - 1);
drivers/vfio/pci/vfio_pci_core.c:1785: unsigned long pgoff = linear_page_delta(vma, addr);
drivers/vfio/pci/vfio_pci_core.c-1786- unsigned long pfn = vma_to_pfn(vma) + pgoff;
--
include/linux/pagemap.h=1061=static inline pgoff_t folio_pgoff(const struct folio *folio)
--
include/linux/pagemap.h-1066-/**
include/linux/pagemap.h:1067: * linear_page_delta() - Determine the relative page offset of @address within
include/linux/pagemap.h-1068- * @vma.
--
include/linux/pagemap.h-1076- */
include/linux/pagemap.h:1077:static inline pgoff_t linear_page_delta(const struct vm_area_struct *vma,
include/linux/pagemap.h-1078- const unsigned long address)
--
include/linux/pagemap.h=1094=static inline pgoff_t linear_page_index(const struct vm_area_struct *vma,
--
include/linux/pagemap.h-1096-{
include/linux/pagemap.h:1097: return linear_page_delta(vma, address) + vma_start_pgoff(vma);
include/linux/pagemap.h-1098-}
--
include/linux/pagemap.h=1100=static inline pgoff_t __linear_anon_page_index(const struct vm_area_struct *vma,
--
include/linux/pagemap.h-1102-{
include/linux/pagemap.h:1103: return linear_page_delta(vma, address) + vma_start_anon_pgoff(vma);
include/linux/pagemap.h-1104-}
--
mm/damon/vaddr.c=585=static void damos_va_migrate_dests_add(struct folio *folio,
--
mm/damon/vaddr.c-606- ilx = vma_start_pgoff(vma) >> order;
mm/damon/vaddr.c:607: ilx += linear_page_delta(vma, addr) >> order;
mm/damon/vaddr.c-608-
--
mm/mempolicy.c=2044=struct mempolicy *get_vma_policy(struct vm_area_struct *vma,
--
mm/mempolicy.c-2054- *ilx += vma_start_pgoff(vma) >> order;
mm/mempolicy.c:2055: *ilx += linear_page_delta(vma, addr) >> order;
mm/mempolicy.c-2056- }
--
mm/nommu.c=1330=static int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
--
mm/nommu.c-1358-
mm/nommu.c:1359: npages = linear_page_delta(vma, addr);
mm/nommu.c-1360-
--
mm/vma.c=544=__split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
--
mm/vma.c-567- new->vm_start = addr;
mm/vma.c:568: vma_add_pgoff(new, linear_page_delta(vma, addr));
mm/vma.c-569- }
--
mm/vma.c-606- vma->vm_start = addr;
mm/vma.c:607: vma_add_pgoff(vma, linear_page_delta(new, addr));
mm/vma.c-608- } else {
--
tools/testing/vma/include/dup.h=1591=static inline pgprot_t vma_flags_to_page_prot(vma_flags_t vma_flags)
--
tools/testing/vma/include/dup.h-1597-
tools/testing/vma/include/dup.h:1598:static inline pgoff_t linear_page_delta(const struct vm_area_struct *vma,
tools/testing/vma/include/dup.h-1599- const unsigned long address)
--
tools/testing/vma/include/dup.h=1604=static inline pgoff_t linear_page_index(const struct vm_area_struct *vma,
--
tools/testing/vma/include/dup.h-1608-
tools/testing/vma/include/dup.h:1609: pgoff = linear_page_delta(vma, address);
tools/testing/vma/include/dup.h-1610- pgoff += vma_start_pgoff(vma);
--
tools/testing/vma/include/dup.h=1625=static inline pgoff_t __linear_anon_page_index(const struct vm_area_struct *vma,
--
tools/testing/vma/include/dup.h-1629-
tools/testing/vma/include/dup.h:1630: pgoff = linear_page_delta(vma, address);
tools/testing/vma/include/dup.h-1631- pgoff += vma_start_anon_pgoff(vma);
]
|
| 20/2 |
2026/09/07 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 19618
output: 19
thoughts: 1108
Thoughts: **Debugging a Potential VMA Page Offset Mismatch**
Okay, so I'm digging into this `linear_page_delta` function and how it relates to `vma->vm_pgoff`. It seems straightforward: `linear_page_delta` is just a page offset calculation, a simple shift. Then, `vma_add_pgoff` should correctly add this delta to `vma->vm_pgoff`.
Specifically, I'm trying to wrap my head around the `__split_vma` function. If `new_below` is true, we essentially update `vma->vm_start` and adjust the page offset. `linear_page_delta(new, addr)` computes the offset from the original start to the new address and is added to `vma->vm_pgoff`. Because we are dealing with an anonymous VMA here, the original page offset should just be the original start address shifted. Therefore, after the update, the `vm_pgoff` of the updated VMA should be exactly the new start address shifted.
But that's not what the warning's telling me. It's indicating that the calculated page offset doesn't match the shifted start address. That's... not supposed to happen.
Time to look at the reproducer. It's using `mmap`, `mremap`, and `mbind`. Ah, `mremap` with `MREMAP_FIXED | MREMAP_MAYMOVE`. That's the key. `mremap` can relocate an anonymous VMA. This is likely the culprit, right? My suspicion is that when an anonymous VMA is relocated, the `vm_pgoff` isn't getting properly updated, or there's a miscalculation. I need to dive into the `mremap` code now.
|
| 21/2 |
2026/09/07 00:51 |
tool |
grepper |
0m
Args: map[Expression:mremap_to]
Results: map[Output:mm/mremap.c=1410=static unsigned long shrink_vma(struct vma_remap_struct *vrm,
--
mm/mremap.c-1443-/*
mm/mremap.c:1444: * mremap_to() - remap a vma to a new location.
mm/mremap.c-1445- * Returns: The new address of the vma or an error.
mm/mremap.c-1446- */
mm/mremap.c:1447:static unsigned long mremap_to(struct vma_remap_struct *vrm)
mm/mremap.c-1448-{
--
mm/mremap.c-1453- /*
mm/mremap.c:1454: * In mremap_to().
mm/mremap.c-1455- * VMA is moved to dst address, and munmap dst first.
--
mm/mremap.c=1888=static unsigned long remap_move(struct vma_remap_struct *vrm)
--
mm/mremap.c-1951- if (!res_vma)
mm/mremap.c:1952: res_vma = mremap_to(vrm);
mm/mremap.c-1953- if (IS_ERR_VALUE(res_vma))
--
mm/mremap.c=1977=static unsigned long do_mremap(struct vma_remap_struct *vrm)
--
mm/mremap.c-2007- /* Actually execute mremap. */
mm/mremap.c:2008: res = vrm_implies_new_addr(vrm) ? mremap_to(vrm) : mremap_at(vrm);
mm/mremap.c-2009- }
]
|
| 22/2 |
2026/09/07 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 21644
output: 36
thoughts: 21
Thoughts: Here's my summary, as though I'm reading and processing the `mremap_to` function in `mm/mremap.c`:
**Diving into `mremap_to`**
Okay, let's get down to brass tacks. I need to understand this `mremap_to` function. It's the core of handling a remapping to a *specific* address, right? I already know about mremap's general functionality from other parts of the MM subsystem, but this specific function must handle all the edge cases and intricacies when the target address is *given*. My expectation is that this is where the real complexity resides. I'll be looking specifically at the following elements as I read through the code:
* **Address validation:** Ensuring the `new_address` is valid, is not conflicting with other mappings, and falls within the appropriate address space limits. I'll need to see how they handle potential overlaps.
* **Permissions and flags:** How are the various flags related to permissions (read, write, execute), shared mappings, and private mappings handled, especially during this specific remapping operation? There are so many subtle aspects to ensure they don't leak or unexpectedly get inherited.
* **Copying (or not):** Is there a copy-on-write aspect to this function? Depending on the `MREMAP_MAYMOVE` flag, what strategies are employed for moving data or simply re-mapping page table entries? Are they leveraging copy-on-write principles here, especially for private mappings? I'll need to trace the path to see how they achieve this optimization.
* **Page table manipulations:** This is the heart of the remapping operation. I'm especially keen to see how the page tables are modified, updated, or re-wired to achieve the new address mapping. What locking mechanisms are employed to ensure concurrency safety, especially within the context of the MMU?
* **Error Handling:** It will be important to see the error conditions and their corresponding actions to make sure that the system is stable if something fails. For example, if there's an issue allocating a new page table entry, or if the remapping itself fails mid-operation, how is the state rolled back to ensure consistency? I anticipate seeing a lot of return codes and cleanup routines.
* **Resource Management:** How are resources managed during the process? Are there any memory leaks I should be wary of if the remapping is interrupted? I need to analyze how pages are allocated/deallocated and how they're accounted for within the `mm_struct`.
I need to see if my mental model of this process is correct, and I'll be comparing this code to the other mremap methods to see how this particular `mremap_to` fits within the broader picture. Let's see how they've implemented the best possible solution!
|
| 23/2 |
2026/09/07 00:51 |
tool |
read-file |
0m
Args: map[File:mm/mremap.c FirstLine:1447 LineCount:50]
Results: map[Contents:1447: static unsigned long mremap_to(struct vma_remap_struct *vrm)
1448: {
1449: struct mm_struct *mm = current->mm;
1450: unsigned long err;
1451:
1452: if (vrm->flags & MREMAP_FIXED) {
1453: /*
1454: * In mremap_to().
1455: * VMA is moved to dst address, and munmap dst first.
1456: * do_munmap will check if dst is sealed.
1457: */
1458: err = do_munmap(mm, vrm->new_addr, vrm->new_len,
1459: vrm->uf_unmap_early);
1460: vrm->vma = NULL; /* Invalidated. */
1461: vrm->vmi_needs_invalidate = true;
1462: if (err)
1463: return err;
1464:
1465: /*
1466: * If we remap a portion of a VMA elsewhere in the same VMA,
1467: * this can invalidate the old VMA. Reset.
1468: */
1469: vrm->vma = vma_lookup(mm, vrm->addr);
1470: if (!vrm->vma)
1471: return -EFAULT;
1472: }
1473:
1474: if (vrm->remap_type == MREMAP_SHRINK) {
1475: err = shrink_vma(vrm, /* drop_lock= */false);
1476: if (err)
1477: return err;
1478:
1479: /* Set up for the move now shrink has been executed. */
1480: vrm->old_len = vrm->new_len;
1481: }
1482:
1483: /* MREMAP_DONTUNMAP expands by old_len since old_len == new_len */
1484: if (vrm->flags & MREMAP_DONTUNMAP) {
1485: vma_flags_t vma_flags = vrm->vma->flags;
1486: unsigned long pages = vrm->old_len >> PAGE_SHIFT;
1487:
1488: if (!may_expand_vm(mm, &vma_flags, pages))
1489: return -ENOMEM;
1490: }
1491:
1492: err = vrm_set_new_addr(vrm);
1493: if (err)
1494: return err;
1495:
1496: return move_vma(vrm);
]
|
| 24/2 |
2026/09/07 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 23092
output: 18
thoughts: 10
Thoughts: Okay, let's dive into this. Here's my take on those thoughts, as if they're my own:
**Analyzing the `move_vma` Function**
Alright, so the focus is on `move_vma`. That immediately flags a crucial area for me ā Virtual Memory Area management, specifically within the context of a kernel. My initial thought process is starting to break down the mechanics. This likely deals with how processes, or even the kernel itself, rearrange their memory maps. I'm already anticipating potential complexities: address space layout randomization (ASLR) considerations, handling shared memory, potentially dealing with copy-on-write mechanisms, and, of course, the ever-present race condition possibilities. I need to be thinking about protecting resources and consistency. What are the calling conventions? Is this a system call or an internal function? What does the function *do* precisely? Does it simply update the page table entries, or is there more involved like swapping pages? I need to look into whether there are any memory regions that may overlap. This is definitely going to require careful examination. Is this function atomic? What locking mechanisms are in place (or *should* be in place!)? I'm getting a sense of what I need to do to understand the bigger picture.
|
| 25/2 |
2026/09/07 00:51 |
tool |
grepper |
0m
Args:
Results: map[Output:Documentation/trace/ftrace.rst=1656=display-graph option::
--
Documentation/trace/ftrace.rst-1696- => vm_area_free
Documentation/trace/ftrace.rst:1697: => remove_vma
Documentation/trace/ftrace.rst-1698- => exit_mmap
--
drivers/gpu/drm/xe/xe_vm.c=1328=static int xe_vm_insert_vma(struct xe_vm *vm, struct xe_vma *vma)
--
drivers/gpu/drm/xe/xe_vm.c-1342-
drivers/gpu/drm/xe/xe_vm.c:1343:static void xe_vm_remove_vma(struct xe_vm *vm, struct xe_vma *vma)
drivers/gpu/drm/xe/xe_vm.c-1344-{
--
drivers/gpu/drm/xe/xe_vm.c=1885=void xe_vm_close_and_put(struct xe_vm *vm)
--
drivers/gpu/drm/xe/xe_vm.c-1936-
drivers/gpu/drm/xe/xe_vm.c:1937: xe_vm_remove_vma(vm, vma);
drivers/gpu/drm/xe/xe_vm.c-1938-
--
drivers/gpu/drm/xe/xe_vm.c=2322=static void prep_vma_destroy(struct xe_vm *vm, struct xe_vma *vma,
--
drivers/gpu/drm/xe/xe_vm.c-2328- if (post_commit)
drivers/gpu/drm/xe/xe_vm.c:2329: xe_vm_remove_vma(vm, vma);
drivers/gpu/drm/xe/xe_vm.c-2330-}
--
drivers/md/dm-vdo/memory-alloc.c=118=static void add_vmalloc_block(struct vmalloc_block_info *block)
--
drivers/md/dm-vdo/memory-alloc.c-130-
drivers/md/dm-vdo/memory-alloc.c:131:static void remove_vmalloc_block(void *ptr)
drivers/md/dm-vdo/memory-alloc.c-132-{
--
drivers/md/dm-vdo/memory-alloc.c=315=void vdo_free(void *ptr)
--
drivers/md/dm-vdo/memory-alloc.c-318- if (is_vmalloc_addr(ptr)) {
drivers/md/dm-vdo/memory-alloc.c:319: remove_vmalloc_block(ptr);
drivers/md/dm-vdo/memory-alloc.c-320- vfree(ptr);
--
mm/hugetlb.c=1215=void clear_vma_resv_huge_pages(struct vm_area_struct *vma)
--
mm/hugetlb.c-1220- *
mm/hugetlb.c:1221: * During a mremap() operation of a hugetlb vma we call move_vma()
mm/hugetlb.c-1222- * which copies vma into new_vma and unmaps vma. After the copy
--
mm/mmap.c=1265=unsigned long tear_down_vmas(struct mm_struct *mm, struct vma_iterator *vmi,
--
mm/mmap.c-1276- vma_mark_detached(vma);
mm/mmap.c:1277: remove_vma(vma);
mm/mmap.c-1278- count++;
--
mm/mremap.c=1102=static bool check_map_count_against_split_early(void)
--
mm/mremap.c-1111- */
mm/mremap.c:1112:static unsigned long prep_move_vma(struct vma_remap_struct *vrm)
mm/mremap.c-1113-{
--
mm/mremap.c=1329=static void dontunmap_complete(struct vma_remap_struct *vrm,
--
mm/mremap.c-1349-
mm/mremap.c:1350:static unsigned long move_vma(struct vma_remap_struct *vrm)
mm/mremap.c-1351-{
--
mm/mremap.c-1356-
mm/mremap.c:1357: err = prep_move_vma(vrm);
mm/mremap.c-1358- if (err)
--
mm/mremap.c=1447=static unsigned long mremap_to(struct vma_remap_struct *vrm)
--
mm/mremap.c-1495-
mm/mremap.c:1496: return move_vma(vrm);
mm/mremap.c-1497-}
--
mm/mremap.c=1596=static unsigned long expand_vma(struct vma_remap_struct *vrm)
--
mm/mremap.c-1626-
mm/mremap.c:1627: return move_vma(vrm);
mm/mremap.c-1628-}
--
mm/vma.c=483=static bool can_vma_merge_right(struct vma_merge_struct *vmg,
--
mm/vma.c-509- */
mm/vma.c:510:void remove_vma(struct vm_area_struct *vma)
mm/vma.c-511-{
--
mm/vma.c=811=static int commit_merge(struct vma_merge_struct *vmg)
--
mm/vma.c-854-/* We can only remove VMAs when merging if they do not have a close hook. */
mm/vma.c:855:static bool can_merge_remove_vma(struct vm_area_struct *vma)
mm/vma.c-856-{
--
mm/vma.c=889=static __must_check struct vm_area_struct *vma_merge_existing_range(
--
mm/vma.c-963- */
mm/vma.c:964: if (vmg->__remove_middle && !can_merge_remove_vma(middle))
mm/vma.c-965- return NULL;
--
mm/vma.c-976- */
mm/vma.c:977: if (vmg->__remove_next && !can_merge_remove_vma(next)) {
mm/vma.c-978- vmg->__remove_next = false;
--
mm/vma.c=1140=struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
--
mm/vma.c-1180- */
mm/vma.c:1181: if (can_merge_right && !can_merge_remove_vma(next))
mm/vma.c-1182- vmg->end = end;
--
mm/vma.c=1247=int vma_expand(struct vma_merge_struct *vmg)
--
mm/vma.c-1268- /* This should have already been checked by this point. */
mm/vma.c:1269: VM_WARN_ON_VMG(remove_next && !can_merge_remove_vma(next), vmg);
mm/vma.c-1270- /* Not merging but overwriting any part of next is not handled. */
--
mm/vma.c=1410=static void vms_complete_munmap_vmas(struct vma_munmap_struct *vms,
--
mm/vma.c-1440- mas_for_each(mas_detach, vma, ULONG_MAX)
mm/vma.c:1441: remove_vma(vma);
mm/vma.c-1442-
--
mm/vma.h=415=int do_vmi_munmap(struct vma_iterator *vmi, struct mm_struct *mm,
--
mm/vma.h-418-
mm/vma.h:419:void remove_vma(struct vm_area_struct *vma);
mm/vma.h-420-void unmap_region(struct unmap_desc *unmap);
]
|
| 26/2 |
2026/09/07 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 25620
output: 36
thoughts: 20
Thoughts: Here's my take on those thoughts, tailored for an expert audience:
**Diving into `move_vma`**
Alright, time to crack open `move_vma` within `mm/mremap.c`. I'm guessing this is where the real magic (or potential headache) of `mremap` happens ā specifically, the movement of virtual memory areas. I want to see how the kernel handles the low-level mechanics of relocating a VMA. Is it a straightforward copy, or are we dealing with page table manipulation, TLB invalidation, and maybe even some intricate considerations for shared mappings? I'm particularly interested in how it interacts with the VM area flags and attributes, and how it deals with potential overlaps or constraints in the destination address space. I'm hoping to see elegant handling of any potential issues with read-only mappings, COW pages, or pinned memory. This function is the core of a tricky operation, so let's see how the kernel engineers thought this through.
|
| 27/2 |
2026/09/07 00:51 |
tool |
read-file |
0m
Args: map[File:mm/mremap.c FirstLine:1350 LineCount:50]
Results: map[Contents:1350: static unsigned long move_vma(struct vma_remap_struct *vrm)
1351: {
1352: struct mm_struct *mm = current->mm;
1353: struct vm_area_struct *new_vma;
1354: unsigned long hiwater_vm;
1355: int err;
1356:
1357: err = prep_move_vma(vrm);
1358: if (err)
1359: return err;
1360:
1361: /*
1362: * If accounted, determine the number of bytes the operation will
1363: * charge.
1364: */
1365: if (!vrm_calc_charge(vrm))
1366: return -ENOMEM;
1367:
1368: /* We don't want racing faults. */
1369: vma_start_write(vrm->vma);
1370:
1371: /* Perform copy step. */
1372: err = copy_vma_and_data(vrm, &new_vma);
1373: /*
1374: * If we established the copied-to VMA, we attempt to recover from the
1375: * error by setting the destination VMA to the source VMA and unmapping
1376: * it below.
1377: */
1378: if (err && !new_vma)
1379: return err;
1380:
1381: /*
1382: * If we failed to move page tables we still do total_vm increment
1383: * since do_munmap() will decrement it by old_len == new_len.
1384: *
1385: * Since total_vm is about to be raised artificially high for a
1386: * moment, we need to restore high watermark afterwards: if stats
1387: * are taken meanwhile, total_vm and hiwater_vm appear too high.
1388: * If this were a serious issue, we'd add a flag to do_munmap().
1389: */
1390: hiwater_vm = mm->hiwater_vm;
1391:
1392: vrm_stat_account(vrm, vrm->new_len);
1393: if (unlikely(!err && (vrm->flags & MREMAP_DONTUNMAP)))
1394: dontunmap_complete(vrm, new_vma);
1395: else
1396: unmap_source_vma(vrm);
1397:
1398: mm->hiwater_vm = hiwater_vm;
1399:
]
|
| 28/2 |
2026/09/07 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 26649
output: 22
thoughts: 14
Thoughts: Okay, here's my interpretation of that.
**Analyzing `copy_vma_and_data`**
Alright, let's dive into this `copy_vma_and_data` function. Immediately, my mind starts racing ā what's the context? What architecture are we dealing with? Is this kernel space, user space, or something in between? Given the name, I'm already anticipating virtual memory areas (VMAs) being involved, so I'm thinking about page tables, protection levels, and potential access violations right off the bat. It's almost certain this function is designed to handle memory copy operations, but the "and data" part is crucial. Am I transferring raw data, or are there more complex data structures involved? Is it a simple memcpy, or are there nuances around COW (Copy-On-Write), shared memory regions, or possibly inter-process communication?
I need to understand the source and destination VMAs. How is their mapping handled? Are they contiguous? What about their protection flags? Read-only? Executable? These flags will dictate how the copy is executed and whether any permission checks are necessary. Furthermore, I need to know how the function handles potential page faults or memory exhaustion during the data transfer. Is there error handling? Is it robust? Do we need to consider memory barriers or cache coherency issues, especially on multi-core systems? I'm already anticipating the need to check for alignment issues and optimize the copy loop based on the underlying hardware's characteristics (e.g., CPU cache line size). There's also the question of potential race conditions if this function is running concurrently with other processes or threads. A solid understanding of memory mapping and concurrency controls is absolutely critical here. I'm going to have to carefully look through the code to understand the mechanisms to avoid any sort of issue.
|
| 29/2 |
2026/09/07 00:51 |
tool |
grepper |
0m
Args: map[Expression:copy_vma_and_data]
Results: map[Output:mm/mremap.c=1160=static void unmap_source_vma(struct vma_remap_struct *vrm)
--
mm/mremap.c-1264- */
mm/mremap.c:1265:static int copy_vma_and_data(struct vma_remap_struct *vrm,
mm/mremap.c-1266- struct vm_area_struct **new_vma_ptr)
--
mm/mremap.c=1350=static unsigned long move_vma(struct vma_remap_struct *vrm)
--
mm/mremap.c-1371- /* Perform copy step. */
mm/mremap.c:1372: err = copy_vma_and_data(vrm, &new_vma);
mm/mremap.c-1373- /*
]
|
| 30/2 |
2026/09/07 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 27312
output: 36
thoughts: 14
Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience, and written in the first person:
**Analyzing the `copy_vma_and_data` Function**
Alright, let's dive into the guts of this thing. The code's asking me to specifically focus on `copy_vma_and_data`. Given my experience, I'm already mentally running through a checklist: virtual memory areas, data copying, context switches... this likely pertains to process creation, fork/clone implementations, or perhaps even handling shared memory regions. The name itself is pretty explicit: we're dealing with transferring data associated with a virtual memory area. I'm immediately thinking about the nuances of page tables, protection flags, and potential optimizations based on copy-on-write or other memory management strategies. I need to understand the source and destination VMA structures, and the actual mechanism of the data transfer itself. Is this a page-by-page copy, or are there clever tricks using hardware features like memory mapping or DMA to make things quicker? What are the edge cases? I expect error handling to be robust. I'll be looking specifically for any potential memory leaks, synchronization issues, or security vulnerabilities that might arise during this copy operation.
|
| 31/2 |
2026/09/07 00:51 |
tool |
read-file |
0m
Args: map[File:mm/mremap.c FirstLine:1265 LineCount:50]
Results: map[Contents:1265: static int copy_vma_and_data(struct vma_remap_struct *vrm,
1266: struct vm_area_struct **new_vma_ptr)
1267: {
1268: const pgoff_t new_pgoff = linear_page_index(vrm->vma, vrm->addr);
1269: const pgoff_t new_anon_pgoff =
1270: __linear_anon_page_index(vrm->vma, vrm->addr);
1271: struct vm_area_struct *vma = vrm->vma;
1272: struct vm_area_struct *new_vma;
1273: unsigned long moved_len;
1274: int err = 0;
1275: PAGETABLE_MOVE(pmc, NULL, NULL, vrm->addr, vrm->new_addr, vrm->old_len);
1276:
1277: new_vma = copy_vma(&vma, vrm->new_addr, vrm->new_len, new_pgoff,
1278: new_anon_pgoff, &pmc.need_rmap_locks);
1279: if (!new_vma) {
1280: vrm_uncharge(vrm);
1281: *new_vma_ptr = NULL;
1282: return -ENOMEM;
1283: }
1284: /* By merging, we may have invalidated any iterator in use. */
1285: if (vma != vrm->vma)
1286: vrm->vmi_needs_invalidate = true;
1287:
1288: vrm->vma = vma;
1289: pmc.old = vma;
1290: pmc.new = new_vma;
1291:
1292: moved_len = move_page_tables(&pmc);
1293: if (moved_len < vrm->old_len)
1294: err = -ENOMEM;
1295: else if (vma->vm_ops && vma->vm_ops->mremap)
1296: err = vma->vm_ops->mremap(new_vma);
1297:
1298: if (unlikely(err)) {
1299: PAGETABLE_MOVE(pmc_revert, new_vma, vma, vrm->new_addr,
1300: vrm->addr, moved_len);
1301:
1302: /*
1303: * On error, move entries back from new area to old,
1304: * which will succeed since page tables still there,
1305: * and then proceed to unmap new area instead of old.
1306: */
1307: pmc_revert.need_rmap_locks = true;
1308: move_page_tables(&pmc_revert);
1309:
1310: vrm->vma = new_vma;
1311: vrm->old_len = vrm->new_len;
1312: vrm->addr = vrm->new_addr;
1313: } else {
1314: mremap_userfaultfd_prep(new_vma, vrm->uf);
]
|
| 32/2 |
2026/09/07 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 28554
output: 18
thoughts: 193
Thoughts: **Analyzing the Page Offset Calculation**
Okay, so I'm digging into how this `copy_vma` function is being called, specifically looking at how the new page offset, `new_pgoff`, is calculated. It seems like the core of the issue lies in understanding this offset and its relationship to the virtual memory area (VMA) and the address space.
From the code, I see that `copy_vma` is being called with `new_pgoff` and another offset, `new_anon_pgoff`. My immediate interest is in understanding where this `new_pgoff` value comes from, as that's probably the key to the page mapping logic.
The code tells me `new_pgoff` is set to `linear_page_index(vrm->vma, vrm->addr)`. Right, so this `linear_page_index` function is clearly important. Following that logic, it expands to `linear_page_delta(vma, address) + vma_start_pgoff(vma)`. Okay, that's getting clearer. It's essentially calculating the difference between the given address and the VMA's start address, then adding the VMA's page offset. So, it's essentially using the address relative to the starting virtual address within the current VMA to derive the new offset.
Expanding the calculation further, `linear_page_index` simplifies to `(vrm->addr - vma->vm_start) >> PAGE_SHIFT + vma->vm_pgoff`. Okay, so it's a shifted subtraction of the address. The shift is doing the conversion to a page number. This tells me the new offset is the virtual address within the VMA, divided by the page size, plus the VMA's starting page offset. I think this tells me that `new_pgoff` is giving us the intended destination offset within the new VMA.
Now, for an anonymous VMA ā an unmapped portion of the address space ā `vma->vm_pgoff` is actually equivalent to `vma->vm_start >> PAGE_SHIFT`. So, this simplifies the calculation further. This is where the code is leading me, I see it. Plugging that in, the calculation for `new_pgoff` becomes just `vrm->addr >> PAGE_SHIFT`. So, the final page offset is simply the virtual address divided by page size. That means we're using the base address for the new page offset.
So, in summary, the call to `copy_vma` is being made with a `new_pgoff` that's derived directly from the supplied virtual address, effectively indicating the target page within the new VMA. This is for an anonymous VMA, but makes sense given the intention of `copy_vma`.
|
| 33/2 |
2026/09/07 00:51 |
tool |
grepper |
0m
Args:
Results: map[Output:mm/mremap.c=1160=static void unmap_source_vma(struct vma_remap_struct *vrm)
--
mm/mremap.c-1183- * So we perform a trick here to prevent incorrect accounting. Any merge
mm/mremap.c:1184: * or new VMA allocation performed in copy_vma() does not adjust
mm/mremap.c-1185- * accounting, it is expected that callers handle this.
--
mm/mremap.c-1264- */
mm/mremap.c:1265:static int copy_vma_and_data(struct vma_remap_struct *vrm,
mm/mremap.c-1266- struct vm_area_struct **new_vma_ptr)
--
mm/mremap.c-1276-
mm/mremap.c:1277: new_vma = copy_vma(&vma, vrm->new_addr, vrm->new_len, new_pgoff,
mm/mremap.c-1278- new_anon_pgoff, &pmc.need_rmap_locks);
--
mm/mremap.c=1350=static unsigned long move_vma(struct vma_remap_struct *vrm)
--
mm/mremap.c-1371- /* Perform copy step. */
mm/mremap.c:1372: err = copy_vma_and_data(vrm, &new_vma);
mm/mremap.c-1373- /*
--
mm/vma.c=128=static bool is_mergeable_anon_vma(struct vma_merge_struct *vmg, bool merge_next)
--
mm/vma.c-135- /*
mm/vma.c:136: * We _can_ have !src, vmg->anon_vma via copy_vma(). In this instance we
mm/vma.c-137- * will remove the existing VMA's anon_vma's so there's no scalability
--
mm/vma.c=1924=static int vma_link(struct mm_struct *mm, struct vm_area_struct *vma)
--
mm/vma.c-1943- */
mm/vma.c:1944:struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
mm/vma.c-1945- unsigned long addr, unsigned long len, pgoff_t pgoff,
--
mm/vma.h=533=void unlink_file_vma_batch_add(struct unlink_vma_file_batch *vb,
--
mm/vma.h-535-
mm/vma.h:536:struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
mm/vma.h-537- unsigned long addr, unsigned long len, pgoff_t pgoff,
--
tools/testing/vma/tests/vma.c=3=static bool compare_legacy_flags(vm_flags_t legacy_flags, vma_flags_t flags)
--
tools/testing/vma/tests/vma.c-28-
tools/testing/vma/tests/vma.c:29:static bool test_copy_vma(void)
tools/testing/vma/tests/vma.c-30-{
--
tools/testing/vma/tests/vma.c-42- vma_orig = vma;
tools/testing/vma/tests/vma.c:43: vma_new = copy_vma(&vma, 0x2000, 0x1000, 1, 1, &need_locks);
tools/testing/vma/tests/vma.c-44- ASSERT_EQ(vma_new, vma_orig);
--
tools/testing/vma/tests/vma.c-55- vma_orig = vma;
tools/testing/vma/tests/vma.c:56: vma_new = copy_vma(&vma, 0x1000, 0x1000, 2, 2, &need_locks);
tools/testing/vma/tests/vma.c-57- ASSERT_EQ(vma_new, vma_orig);
--
tools/testing/vma/tests/vma.c-73- vma_orig = vma;
tools/testing/vma/tests/vma.c:74: vma_new = copy_vma(&vma, 0x2000, 0x1000, 3, 3, &need_locks);
tools/testing/vma/tests/vma.c-75- ASSERT_NE(vma_new, vma_orig);
--
tools/testing/vma/tests/vma.c-84- vma = alloc_and_link_vma(&mm, 0x3000, 0x5000, 3, vma_flags);
tools/testing/vma/tests/vma.c:85: vma_new = copy_vma(&vma, 0, 0x2000, 0, 3, &need_locks);
tools/testing/vma/tests/vma.c-86- ASSERT_NE(vma_new, vma);
--
tools/testing/vma/tests/vma.c-97- vma_next = alloc_and_link_vma(&mm, 0x6000, 0x8000, 6, vma_flags);
tools/testing/vma/tests/vma.c:98: vma_new = copy_vma(&vma, 0x4000, 0x2000, 4, 4, &need_locks);
tools/testing/vma/tests/vma.c-99- vma_assert_attached(vma_new);
--
tools/testing/vma/tests/vma.c=684=static void run_vma_tests(int *num_tests, int *num_fail)
tools/testing/vma/tests/vma.c-685-{
tools/testing/vma/tests/vma.c:686: TEST(copy_vma);
tools/testing/vma/tests/vma.c-687- TEST(vma_flags_unchanged);
]
|
| 34/2 |
2026/09/07 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 31060
output: 35
thoughts: 19
Thoughts: Here's a first-person summary, tailored as if I were analyzing `copy_vma` within a kernel context:
**Diving into `copy_vma`**
Okay, time to dissect `copy_vma` in `mm/vma.c`. I need to understand exactly how Virtual Memory Areas (VMAs) are handled during a process copy. This is critical for comprehending process creation, particularly how the address space is duplicated. I'm assuming this function handles a *lot* of the heavy lifting. I need to figure out:
* **The Inputs:** What VMAs are being targeted and where are they going? The source and destination process contexts are key. What flags or properties are being considered or modified, if any?
* **The Core Logic:** What's the process flow? I expect there's iteration through source VMAs, perhaps a check for shared mappings, potentially involving copy-on-write (COW) handling, and definitely dealing with page table manipulation. I'm looking for the essential steps to create new VMAs.
* **The Implications:** I anticipate this function to have important side-effects like increasing the page table entries of a child process, potentially needing to handle TLB invalidation, and probably synchronization mechanisms (like mutexes) to ensure the integrity of the MM data structures.
* **Edge Cases:** I'll be searching for how the function deals with specific conditions such as huge pages, anonymous memory, file-backed mappings, and special mappings like those reserved for kernel code or device memory. How does it handle errors gracefully? What cleanup is involved if something goes wrong? I'm hoping it's robust and minimizes resource leaks.
* **Optimization:** What optimizations are employed? Are there any clever techniques to minimize overhead, particularly in cases where large address spaces are being copied?
Let's see the code! I'll read with all of these ideas in mind, looking for the details that make this function a vital piece of the memory management puzzle.
|
| 35/2 |
2026/09/07 00:52 |
tool |
read-file |
0m
Args: map[File:mm/vma.c FirstLine:1944 LineCount:50]
Results: map[Contents:1944: struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
1945: unsigned long addr, unsigned long len, pgoff_t pgoff,
1946: pgoff_t anon_pgoff, bool *need_rmap_locks)
1947: {
1948: struct vm_area_struct *vma = *vmap;
1949: unsigned long old_vma_start = vma->vm_start;
1950: struct mm_struct *mm = vma->vm_mm;
1951: struct vm_area_struct *new_vma;
1952: bool can_self_merge = false;
1953: VMA_ITERATOR(vmi, mm, addr);
1954: VMG_VMA_STATE(vmg, &vmi, NULL, vma, addr, addr + len);
1955:
1956: /*
1957: * If a vma has not yet been faulted, update its anonymous pgoff to
1958: * match the new location to increase its chance of merging.
1959: */
1960: if (!vma->anon_vma) {
1961: anon_pgoff = addr >> PAGE_SHIFT;
1962:
1963: if (vma_is_anonymous(vma)) {
1964: pgoff = anon_pgoff;
1965: can_self_merge = true;
1966: }
1967: }
1968:
1969: /*
1970: * If the VMA we are copying might contain a uprobe PTE, ensure
1971: * that we do not establish one upon merge. Otherwise, when mremap()
1972: * moves page tables, it will orphan the newly created PTE.
1973: */
1974: if (vma->vm_file)
1975: vmg.skip_vma_uprobe = true;
1976:
1977: new_vma = find_vma_prev(mm, addr, &vmg.prev);
1978: if (new_vma && new_vma->vm_start < addr + len)
1979: return NULL; /* should never get here */
1980:
1981: vmg.pgoff = pgoff;
1982: vmg.anon_pgoff = anon_pgoff;
1983: vmg.next = vma_iter_next_rewind(&vmi, NULL);
1984: new_vma = vma_merge_copied_range(&vmg);
1985:
1986: if (new_vma) {
1987: /* Self-merged and VMA replaced. */
1988: if (unlikely(new_vma->vm_start < old_vma_start &&
1989: new_vma->vm_end > old_vma_start)) {
1990: /*
1991: * The only way a VMA can both self-merge and be
1992: * replaced is if the remap places the new VMA
1993: * immediately prior to its old self ('next') and
]
|
| 36/2 |
2026/09/07 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 32435
output: 35
thoughts: 2329
Thoughts: **Analyzing the Anonymous VMA and mremap Issue**
Okay, let's break this down. My initial thought was: the code in `copy_vma` seems to handle the `pgoff` update for anonymous VMAs carefully. It appears to check if `vma->anon_vma` is null. If it *is* null, it updates `pgoff` to match the address, which makes sense for merging opportunities. But, if `anon_vma` isn't null, it *doesn't* update it, and `pgoff` gets set to `new_pgoff` from `copy_vma_and_data`, which seems to be calculated based on the linear page index.
Then I worked through an example. Say we have an anonymous VMA at `0x20000000` with some `vm_pgoff`. If we `mremap` it to a new location, the expected `vm_pgoff` should be the new start address shifted right by `PAGE_SHIFT`. But the code in `copy_vma` seems to preserve the old `vm_pgoff` if it's got an `anon_vma` already. This, I thought, might cause inconsistencies that the `assert_sane_pgoff` function is meant to catch. `assert_sane_pgoff` normally checks if `pgoff` is consistent with `vm_start`. However, it has a shortcut; if `vma->anon_vma` is not NULL, the sanity check is skipped. So if there's no `anon_vma`, the correct behavior is guaranteed, because `copy_vma` updates `pgoff`.
So, the next question is: How does the warning happen? The issue must arise in a scenario where, despite the check, `assert_sane_pgoff` *does* warn. The warning gets triggered when `vma_add_pgoff` calls `__split_vma`, so I need to go back over the repro scenario.
The reproducer sets up an anonymous VMA, faults in a page using `socketpair`, and then uses `mremap` to move a section of memory *within* that VMA to the beginning of the VMA using the `MREMAP_FIXED` flag. With `MREMAP_FIXED` it also changes the VMA to have a size of 0x400000. It seems the critical piece is where that page is, and whether the VMA at that address has an `anon_vma`. If the target address for `mremap` is inside the original VMA, then the code *unmaps* part of the VMA and moves the selected piece, which is now a part of the original VMA, into the start address. This means the original VMA at `0x20000000` is split. The piece moved will retain `anon_vma != NULL` because it came from a faulted-in VMA. So, because of `assert_sane_pgoff`, it *should* return early.
However, the warning *does* trigger. Thus, it looks like `vma->anon_vma` *must* be NULL at some critical point. This is where I start to see a possible race. I went back over the `mremap` call and the fault that occurs from the `socketpair`. If the address at `0x20a96000` is *not* faulted in, its `anon_vma` will be NULL. However, this is wrong. `anon_vma` is associated with the VMA, not the specific pages. The fault due to `socketpair` will make the `anon_vma` *not* null for the *entire* VMA at `0x20000000`.
Then when `mremap` runs, the VMA is split, and the relevant piece at `0x20a96000` gets moved. Because the VMA *was* faulted in and now has `anon_vma != NULL`, the moved piece *should* retain this property. Thus, the crucial moment must be during the split and movement.
I need to check how `copy_vma` handles an existing `anon_vma` in the context of `mremap`. I will focus on where and how the `anon_vma` is copied or set after the split, because this seems to be the critical area where the logic might be flawed and can trigger the warning.
|
| 37/2 |
2026/09/07 00:52 |
tool |
read-file |
0m
Args: map[File:mm/vma.c FirstLine:1990 LineCount:50]
Results: map[Contents:1990: /*
1991: * The only way a VMA can both self-merge and be
1992: * replaced is if the remap places the new VMA
1993: * immediately prior to its old self ('next') and
1994: * immediately after another VMA ('prev') causing the
1995: * next to be removed and prev to be expanded to cover
1996: * the entire range.
1997: *
1998: * This should only be possible if the anonymous page
1999: * offset was updated, i.e. the VMA is unfaulted.
2000: */
2001: VM_WARN_ON_ONCE_VMA(!can_self_merge, new_vma);
2002: *vmap = vma = new_vma;
2003: }
2004: *need_rmap_locks =
2005: (vma_start_pgoff(new_vma) <= vma_start_pgoff(vma));
2006: } else {
2007: new_vma = vm_area_dup(vma);
2008: if (!new_vma)
2009: goto out;
2010: vma_set_range(new_vma, addr, addr + len, pgoff, anon_pgoff);
2011: if (vma_dup_policy(vma, new_vma))
2012: goto out_free_vma;
2013: if (anon_vma_clone(new_vma, vma, VMA_OP_REMAP))
2014: goto out_free_mempol;
2015: if (new_vma->vm_file)
2016: get_file(new_vma->vm_file);
2017: if (new_vma->vm_ops && new_vma->vm_ops->open)
2018: new_vma->vm_ops->open(new_vma);
2019: if (vma_link(mm, new_vma))
2020: goto out_vma_link;
2021: *need_rmap_locks = false;
2022: }
2023: return new_vma;
2024:
2025: out_vma_link:
2026: fixup_hugetlb_reservations(new_vma);
2027: vma_close(new_vma);
2028:
2029: if (new_vma->vm_file)
2030: fput(new_vma->vm_file);
2031:
2032: unlink_anon_vmas(new_vma);
2033: out_free_mempol:
2034: mpol_put(vma_policy(new_vma));
2035: out_free_vma:
2036: vm_area_free(new_vma);
2037: out:
2038: return NULL;
2039: }
]
|
| 38/2 |
2026/09/07 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 36658
output: 20
thoughts: 99
Thoughts: **My Analysis of `anon_vma_clone` and its Implications**
Okay, so I'm looking at how `anon_vma_clone(new_vma, vma, VMA_OP_REMAP)` works. This function is cloning the anonymous virtual memory area. So, that means the `new_vma` I'm creating will definitely end up with a non-NULL `anon_vma` pointer. That's the whole point of the clone, right?
Now, hold on... if `new_vma` has a valid `anon_vma` pointer after this operation, wouldn't that imply something about how `assert_sane_pgoff` behaves? Specifically, shouldn't `assert_sane_pgoff` recognize this state and immediately return early since the `anon_vma` is already established? That's what makes the most logical sense to me. I need to go back and verify exactly what `anon_vma_clone` does under the hood to confirm this expectation, but it's where my intuition is leading. I should check how the function flow works to see if this hypothesis is correct.
|
| 39/2 |
2026/09/07 00:52 |
tool |
grepper |
0m
Args: map[Expression:anon_vma_clone]
Results: map[Output:include/linux/rmap.h=32=struct anon_vma {
--
include/linux/rmap.h-48- * This counter is used for making decision about reusing anon_vma
include/linux/rmap.h:49: * instead of forking new one. See comments in function anon_vma_clone.
include/linux/rmap.h-50- */
--
mm/internal.h=326=enum vma_operation {
--
mm/internal.h-332-
mm/internal.h:333:int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src,
mm/internal.h-334- enum vma_operation operation);
--
mm/rmap.c=185=int __anon_vma_prepare(struct vm_area_struct *vma)
--
mm/rmap.c-234-
mm/rmap.c:235:static void check_anon_vma_clone(struct vm_area_struct *dst,
mm/rmap.c-236- struct vm_area_struct *src,
--
mm/rmap.c=290=static void cleanup_partial_anon_vmas(struct vm_area_struct *vma);
--
mm/rmap.c-292-/**
mm/rmap.c:293: * anon_vma_clone - Establishes new anon_vma_chain objects in @dst linking to
mm/rmap.c-294- * all of the anon_vma objects contained within @src anon_vma_chain's.
--
mm/rmap.c-319- */
mm/rmap.c:320:int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src,
mm/rmap.c-321- enum vma_operation operation)
--
mm/rmap.c-325-
mm/rmap.c:326: check_anon_vma_clone(dst, src, operation);
mm/rmap.c-327-
--
mm/rmap.c=378=int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
--
mm/rmap.c-403- */
mm/rmap.c:404: rc = anon_vma_clone(vma, pvma, VMA_OP_FORK);
mm/rmap.c-405- /* An error arose or an existing anon_vma was reused, all done then. */
--
mm/rmap.c-444-/*
mm/rmap.c:445: * In the unfortunate case of anon_vma_clone() failing to allocate memory we
mm/rmap.c-446- * have to clean things up.
--
mm/vma.c=544=__split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
--
mm/vma.c-579-
mm/vma.c:580: err = anon_vma_clone(new, vma, VMA_OP_SPLIT);
mm/vma.c-581- if (err)
--
mm/vma.c=656=static int dup_anon_vma(struct vm_area_struct *dst,
--
mm/vma.c-677- dst->anon_vma = src->anon_vma;
mm/vma.c:678: ret = anon_vma_clone(dst, src, VMA_OP_MERGE_UNFAULTED);
mm/vma.c-679- if (ret)
--
mm/vma.c=1944=struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
--
mm/vma.c-2012- goto out_free_vma;
mm/vma.c:2013: if (anon_vma_clone(new_vma, vma, VMA_OP_REMAP))
mm/vma.c-2014- goto out_free_mempol;
--
tools/testing/vma/include/custom.h=48=int vma_start_write_killable(struct vm_area_struct *vma)
--
tools/testing/vma/include/custom.h-54-
tools/testing/vma/include/custom.h:55:static inline int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src,
tools/testing/vma/include/custom.h-56- enum vma_operation operation)
]
|
| 40/2 |
2026/09/07 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 38212
output: 33
thoughts: 12
Thoughts: Okay, let's dive into this `anon_vma_clone` function. I've seen this before, of course, but it's always good to refresh my understanding, especially when dealing with memory management. I want to pay close attention to how it handles anonymous Virtual Memory Areas (VMAs) because that's a critical piece of the puzzle for process creation and memory duplication. I need to keep in mind the implications for copy-on-write mechanisms and how it avoids unnecessary physical memory allocation during process forking. Specifically, I'm interested in the interplay between `anon_vma`, `vm_area_struct`, and the underlying page table entries (PTEs) ā that's where the real magic happens. Let's see how efficiently it manages the sharing and duplication of anonymous memory regions. I also need to be mindful of potential race conditions and locking strategies within this critical code path. Let's see how the kernel handles those complexities. This is core kernel stuff, so I'm expecting some clever optimizations and careful consideration of edge cases. Alright, let's get into the details and start breaking it down.
|
| 41/2 |
2026/09/07 00:52 |
tool |
read-file |
0m
Args: map[File:mm/rmap.c FirstLine:320 LineCount:50]
Results: map[Contents: 320: int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src,
321: enum vma_operation operation)
322: {
323: struct anon_vma_chain *avc, *pavc;
324: struct anon_vma *active_anon_vma = src->anon_vma;
325:
326: check_anon_vma_clone(dst, src, operation);
327:
328: if (!active_anon_vma)
329: return 0;
330:
331: /*
332: * Allocate AVCs. We don't need an anon_vma lock for this as we
333: * are not updating the anon_vma rbtree nor are we changing
334: * anon_vma statistics.
335: *
336: * Either src, dst have the same mm for which we hold an exclusive mmap
337: * write lock, or we are forking and we hold it on src->vm_mm and dst is
338: * not yet accessible to other threads so there's no possibliity of the
339: * unlinked AVC's being observed yet.
340: */
341: list_for_each_entry(pavc, &src->anon_vma_chain, same_vma) {
342: avc = anon_vma_chain_alloc(GFP_KERNEL);
343: if (!avc)
344: goto enomem_failure;
345:
346: anon_vma_chain_assign(dst, avc, pavc->anon_vma);
347: }
348:
349: /*
350: * Now link the anon_vma's back to the newly inserted AVCs.
351: * Note that all anon_vma's share the same root.
352: */
353: anon_vma_lock_write(active_anon_vma);
354: list_for_each_entry_reverse(avc, &dst->anon_vma_chain, same_vma) {
355: struct anon_vma *anon_vma = avc->anon_vma;
356:
357: anon_rmap_tree_insert(avc, anon_vma);
358: if (operation == VMA_OP_FORK)
359: maybe_reuse_anon_vma(dst, anon_vma);
360: }
361:
362: if (operation != VMA_OP_FORK)
363: dst->anon_vma->num_active_vmas++;
364:
365: anon_vma_unlock_write(active_anon_vma);
366: return 0;
367:
368: enomem_failure:
369: cleanup_partial_anon_vmas(dst);
]
|
| 42/2 |
2026/09/07 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 39351
output: 21
thoughts: 1354
Thoughts: **Debugging a PGOFF Assertion Trigger After mbind with mremap**
Okay, first things first, I was initially misled; `anon_vma_clone` doesn't actually set `dst->anon_vma`. That's important to remember. Instead, `dst->anon_vma` is set *before* the call to `anon_vma_clone`. Let's confirm that by revisiting `copy_vma` in the context of the code. Looking at the relevant portion, the `vm_area_dup` call copies `vma->anon_vma` to `new_vma->anon_vma`. Therefore, `new_vma->anon_vma` should indeed be set, and definitely not null.
So, if `new_vma->anon_vma` isn't null, why am I still triggering the `assert_sane_pgoff`? That's what I need to figure out. I'll need to go back and check the stack trace again.
Okay, here's the backtrace: The assertion is happening during `mbind`. `mbind` calls `vma_modify_policy`, which then calls `vma_modify`, which calls `split_vma`. This is where the issue is.
Time to look at the reproducer, again. `mbind` is called on address `0x20001000` with a length of `0x800000`. This is the point where the existing VMA is split. Now I must remember how that VMA was created. Let's see what led up to the call to `mbind`.
Looking at the `mremap` calls, the first call remaps from `0x20a96000` to `0x20000000` with a length of `0x400000`. And then there's a second `mremap` from `0x20000000` to `0x20a00000` with a length of `0x600002`. And the first `mremap` is from `0x20a96000` to `0x20000000` with a length of `0x400000`. And now I see it, the `MREMAP_DONTUNMAP` flag (value `0x4`). Therefore the first `mremap` call expands to `0x400000`, remaps, but the original VMA at `0x20000000` is kept intact! This means the VMA gets copied at `0x20a00000`.
Aha, if `MREMAP_DONTUNMAP` is specified, the original VMA is kept, but its `anon_vma` becomes `NULL` as it gets detached from the mapping. This has to be the root cause of the `assert_sane_pgoff` triggering. Now to verify this with the actual implementation of `MREMAP_DONTUNMAP`.
|
| 43/2 |
2026/09/07 00:52 |
tool |
grepper |
0m
Args: map[Expression:MREMAP_DONTUNMAP]
Results: map[Output:include/uapi/linux/mman.h-10-#define MREMAP_FIXED 2
include/uapi/linux/mman.h:11:#define MREMAP_DONTUNMAP 4
include/uapi/linux/mman.h-12-
--
mm/mremap.c=912=static bool vrm_overlaps(struct vma_remap_struct *vrm)
--
mm/mremap.c-936- * Will a new address definitely be assigned? This either if the user specifies
mm/mremap.c:937: * it via MREMAP_FIXED, or if MREMAP_DONTUNMAP is used, indicating we will
mm/mremap.c-938- * always determine a target address.
--
mm/mremap.c=940=static bool vrm_implies_new_addr(struct vma_remap_struct *vrm)
mm/mremap.c-941-{
mm/mremap.c:942: return vrm->flags & (MREMAP_FIXED | MREMAP_DONTUNMAP);
mm/mremap.c-943-}
--
mm/mremap.c-948- * If MREMAP_FIXED then this is equivalent to a MAP_FIXED mmap() call. If only
mm/mremap.c:949: * MREMAP_DONTUNMAP is set, then this is equivalent to providing a hint to
mm/mremap.c-950- * mmap(), otherwise this is equivalent to mmap() specifying a NULL address.
--
mm/mremap.c=984=static bool vrm_calc_charge(struct vma_remap_struct *vrm)
--
mm/mremap.c-994- */
mm/mremap.c:995: if (vrm->flags & MREMAP_DONTUNMAP)
mm/mremap.c-996- charged = vrm->new_len >> PAGE_SHIFT;
--
mm/mremap.c=1039=static bool __check_map_count_against_split(struct mm_struct *mm,
--
mm/mremap.c-1074- * We start by copying a VMA, which creates an additional VMA if no
mm/mremap.c:1075: * merge occurs, then if not MREMAP_DONTUNMAP, we unmap the source VMA.
mm/mremap.c-1076- * In the worst case we might then observe:
--
mm/mremap.c=1112=static unsigned long prep_move_vma(struct vma_remap_struct *vrm)
--
mm/mremap.c-1121- * We'd prefer to avoid failure later on in do_munmap: we copy a VMA,
mm/mremap.c:1122: * which may not merge, then (if MREMAP_DONTUNMAP is not set) unmap the
mm/mremap.c-1123- * source, which may split, causing a net increase of 2 mappings.
--
mm/mremap.c=1160=static void unmap_source_vma(struct vma_remap_struct *vrm)
--
mm/mremap.c-1170- /*
mm/mremap.c:1171: * It might seem odd that we check for MREMAP_DONTUNMAP here, given this
mm/mremap.c-1172- * function implies that we unmap the original VMA, which seems
--
mm/mremap.c-1179- bool accountable_move = vma_test(vma, VMA_ACCOUNT_BIT) &&
mm/mremap.c:1180: !(vrm->flags & MREMAP_DONTUNMAP);
mm/mremap.c-1181-
--
mm/mremap.c=1350=static unsigned long move_vma(struct vma_remap_struct *vrm)
--
mm/mremap.c-1392- vrm_stat_account(vrm, vrm->new_len);
mm/mremap.c:1393: if (unlikely(!err && (vrm->flags & MREMAP_DONTUNMAP)))
mm/mremap.c-1394- dontunmap_complete(vrm, new_vma);
--
mm/mremap.c=1447=static unsigned long mremap_to(struct vma_remap_struct *vrm)
--
mm/mremap.c-1482-
mm/mremap.c:1483: /* MREMAP_DONTUNMAP expands by old_len since old_len == new_len */
mm/mremap.c:1484: if (vrm->flags & MREMAP_DONTUNMAP) {
mm/mremap.c-1485- vma_flags_t vma_flags = vrm->vma->flags;
--
mm/mremap.c=1735=static int check_prep_vma(struct vma_remap_struct *vrm)
--
mm/mremap.c-1779-
mm/mremap.c:1780: if ((vrm->flags & MREMAP_DONTUNMAP) &&
mm/mremap.c-1781- vma_test_any(vma, VMA_DONTEXPAND_BIT, VMA_PFNMAP_BIT))
--
mm/mremap.c=1835=static unsigned long check_mremap_params(struct vma_remap_struct *vrm)
--
mm/mremap.c-1841- /* Ensure no unexpected flag values. */
mm/mremap.c:1842: if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP))
mm/mremap.c-1843- return -EINVAL;
--
mm/mremap.c-1876-
mm/mremap.c:1877: /* MREMAP_DONTUNMAP does not allow resizing in the process. */
mm/mremap.c:1878: if (flags & MREMAP_DONTUNMAP && vrm->old_len != vrm->new_len)
mm/mremap.c-1879- return -EINVAL;
--
tools/include/uapi/linux/mman.h-10-#define MREMAP_FIXED 2
tools/include/uapi/linux/mman.h:11:#define MREMAP_DONTUNMAP 4
tools/include/uapi/linux/mman.h-12-
--
tools/testing/selftests/mm/hmm-tests.c=2225=TEST_F(hmm, migrate_remap_fault)
--
tools/testing/selftests/mm/hmm-tests.c-2250- if (dont_unmap)
tools/testing/selftests/mm/hmm-tests.c:2251: flags |= MREMAP_DONTUNMAP;
tools/testing/selftests/mm/hmm-tests.c-2252-
--
tools/testing/selftests/mm/merge.c=1365=TEST_F(merge_with_fork, mremap_faulted_to_unfaulted_prev)
--
tools/testing/selftests/mm/merge.c-1414- /*
tools/testing/selftests/mm/merge.c:1415: * Now move VMA A into position with MREMAP_DONTUNMAP to catch incorrect
tools/testing/selftests/mm/merge.c-1416- * anon_vma propagation.
--
tools/testing/selftests/mm/merge.c-1418- ptr_a = mremap(ptr_a, 3 * page_size, 3 * page_size,
tools/testing/selftests/mm/merge.c:1419: MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP,
tools/testing/selftests/mm/merge.c-1420- &self->carveout[page_size + 3 * page_size]);
--
tools/testing/selftests/mm/merge.c=1431=TEST_F(merge_with_fork, mremap_faulted_to_unfaulted_next)
--
tools/testing/selftests/mm/merge.c-1481- /*
tools/testing/selftests/mm/merge.c:1482: * Now move VMA A into position with MREMAP_DONTUNMAP to catch incorrect
tools/testing/selftests/mm/merge.c-1483- * anon_vma propagation.
--
tools/testing/selftests/mm/merge.c-1485- ptr_a = mremap(ptr_a, 3 * page_size, 3 * page_size,
tools/testing/selftests/mm/merge.c:1486: MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP,
tools/testing/selftests/mm/merge.c-1487- &self->carveout[page_size]);
--
tools/testing/selftests/mm/merge.c=1497=TEST_F(merge_with_fork, mremap_faulted_to_unfaulted_prev_unfaulted_next)
--
tools/testing/selftests/mm/merge.c-1504- /*
tools/testing/selftests/mm/merge.c:1505: * mremap() with MREMAP_DONTUNMAP such that A, B and C merge:
tools/testing/selftests/mm/merge.c-1506- *
--
tools/testing/selftests/mm/merge.c-1552- /*
tools/testing/selftests/mm/merge.c:1553: * Now move VMA B into position with MREMAP_DONTUNMAP to catch incorrect
tools/testing/selftests/mm/merge.c-1554- * anon_vma propagation.
--
tools/testing/selftests/mm/merge.c-1556- ptr_b = mremap(ptr_b, 3 * page_size, 3 * page_size,
tools/testing/selftests/mm/merge.c:1557: MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP,
tools/testing/selftests/mm/merge.c-1558- &self->carveout[page_size + 3 * page_size]);
--
tools/testing/selftests/mm/merge.c=1575=TEST_F(merge_with_fork, mremap_faulted_to_unfaulted_prev_faulted_next)
--
tools/testing/selftests/mm/merge.c-1581- /*
tools/testing/selftests/mm/merge.c:1582: * mremap() with MREMAP_DONTUNMAP such that A, B and C merge:
tools/testing/selftests/mm/merge.c-1583- *
--
tools/testing/selftests/mm/merge.c-1628- /*
tools/testing/selftests/mm/merge.c:1629: * Now move VMA B into position with MREMAP_DONTUNMAP to catch incorrect
tools/testing/selftests/mm/merge.c-1630- * anon_vma propagation.
--
tools/testing/selftests/mm/merge.c-1632- ptr_b = mremap(ptr_b, 3 * page_size, 3 * page_size,
tools/testing/selftests/mm/merge.c:1633: MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP,
tools/testing/selftests/mm/merge.c-1634- &self->carveout[page_size + 3 * page_size]);
--
tools/testing/selftests/mm/mremap_dontunmap.c-3-/*
tools/testing/selftests/mm/mremap_dontunmap.c:4: * Tests for mremap w/ MREMAP_DONTUNMAP.
tools/testing/selftests/mm/mremap_dontunmap.c-5- *
--
tools/testing/selftests/mm/mremap_dontunmap.c=42=static int kernel_support_for_mremap_dontunmap()
--
tools/testing/selftests/mm/mremap_dontunmap.c-49-
tools/testing/selftests/mm/mremap_dontunmap.c:50: // This simple remap should only fail if MREMAP_DONTUNMAP isn't
tools/testing/selftests/mm/mremap_dontunmap.c-51- // supported.
--
tools/testing/selftests/mm/mremap_dontunmap.c-53- mremap(source_mapping, num_pages * page_size, num_pages * page_size,
tools/testing/selftests/mm/mremap_dontunmap.c:54: MREMAP_DONTUNMAP | MREMAP_MAYMOVE, 0);
tools/testing/selftests/mm/mremap_dontunmap.c-55- if (dest_mapping == MAP_FAILED) {
--
tools/testing/selftests/mm/mremap_dontunmap.c=69=static int check_region_contains_byte(void *addr, unsigned long size, char byte)
--
tools/testing/selftests/mm/mremap_dontunmap.c-92-
tools/testing/selftests/mm/mremap_dontunmap.c:93:// this test validates that MREMAP_DONTUNMAP moves the pagetables while leaving
tools/testing/selftests/mm/mremap_dontunmap.c-94-// the source mapping mapped.
tools/testing/selftests/mm/mremap_dontunmap.c=95=static void mremap_dontunmap_simple()
--
tools/testing/selftests/mm/mremap_dontunmap.c-108- mremap(source_mapping, num_pages * page_size, num_pages * page_size,
tools/testing/selftests/mm/mremap_dontunmap.c:109: MREMAP_DONTUNMAP | MREMAP_MAYMOVE, NULL);
tools/testing/selftests/mm/mremap_dontunmap.c-110- BUG_ON(dest_mapping == MAP_FAILED, "mremap");
--
tools/testing/selftests/mm/mremap_dontunmap.c-127-
tools/testing/selftests/mm/mremap_dontunmap.c:128:// This test validates that MREMAP_DONTUNMAP on a shared mapping works as expected.
tools/testing/selftests/mm/mremap_dontunmap.c-129-static void mremap_dontunmap_simple_shmem()
--
tools/testing/selftests/mm/mremap_dontunmap.c-150- mremap(source_mapping, num_pages * page_size, num_pages * page_size,
tools/testing/selftests/mm/mremap_dontunmap.c:151: MREMAP_DONTUNMAP | MREMAP_MAYMOVE, NULL);
tools/testing/selftests/mm/mremap_dontunmap.c-152- if (dest_mapping == MAP_FAILED && errno == EINVAL) {
tools/testing/selftests/mm/mremap_dontunmap.c:153: // Old kernel which doesn't support MREMAP_DONTUNMAP on shmem.
tools/testing/selftests/mm/mremap_dontunmap.c-154- BUG_ON(munmap(source_mapping, num_pages * page_size) == -1,
--
tools/testing/selftests/mm/mremap_dontunmap.c-179-
tools/testing/selftests/mm/mremap_dontunmap.c:180:// This test validates MREMAP_DONTUNMAP will move page tables to a specific
tools/testing/selftests/mm/mremap_dontunmap.c-181-// destination using MREMAP_FIXED, also while validating that the source
--
tools/testing/selftests/mm/mremap_dontunmap.c=183=static void mremap_dontunmap_simple_fixed()
--
tools/testing/selftests/mm/mremap_dontunmap.c-202- mremap(source_mapping, num_pages * page_size, num_pages * page_size,
tools/testing/selftests/mm/mremap_dontunmap.c:203: MREMAP_FIXED | MREMAP_DONTUNMAP | MREMAP_MAYMOVE,
tools/testing/selftests/mm/mremap_dontunmap.c-204- dest_mapping);
--
tools/testing/selftests/mm/mremap_dontunmap.c-226-
tools/testing/selftests/mm/mremap_dontunmap.c:227:// This test validates that we can MREMAP_DONTUNMAP for a portion of an
tools/testing/selftests/mm/mremap_dontunmap.c-228-// existing mapping.
tools/testing/selftests/mm/mremap_dontunmap.c=229=static void mremap_dontunmap_partial_mapping()
--
tools/testing/selftests/mm/mremap_dontunmap.c-255- 5 * page_size,
tools/testing/selftests/mm/mremap_dontunmap.c:256: MREMAP_DONTUNMAP | MREMAP_MAYMOVE, NULL);
tools/testing/selftests/mm/mremap_dontunmap.c-257- BUG_ON(dest_mapping == MAP_FAILED, "mremap");
--
tools/testing/selftests/mm/mremap_dontunmap.c=279=static void mremap_dontunmap_partial_mapping_overwrite(void)
--
tools/testing/selftests/mm/mremap_dontunmap.c-314- 5 * page_size,
tools/testing/selftests/mm/mremap_dontunmap.c:315: MREMAP_DONTUNMAP | MREMAP_MAYMOVE | MREMAP_FIXED, dest_mapping);
tools/testing/selftests/mm/mremap_dontunmap.c-316- BUG_ON(remapped_mapping == MAP_FAILED, "mremap");
--
tools/testing/selftests/mm/mremap_dontunmap.c=338=int main(void)
--
tools/testing/selftests/mm/mremap_dontunmap.c-343-
tools/testing/selftests/mm/mremap_dontunmap.c:344: // test for kernel support for MREMAP_DONTUNMAP skipping the test if
tools/testing/selftests/mm/mremap_dontunmap.c-345- // not.
tools/testing/selftests/mm/mremap_dontunmap.c-346- if (kernel_support_for_mremap_dontunmap() != 0) {
tools/testing/selftests/mm/mremap_dontunmap.c:347: ksft_print_msg("No kernel support for MREMAP_DONTUNMAP\n");
tools/testing/selftests/mm/mremap_dontunmap.c-348- ksft_finished();
--
tools/testing/selftests/mm/mremap_test.c=401=static void mremap_move_multiple_vmas(unsigned int pattern_seed,
--
tools/testing/selftests/mm/mremap_test.c-412- if (dont_unmap)
tools/testing/selftests/mm/mremap_test.c:413: mremap_flags |= MREMAP_DONTUNMAP;
tools/testing/selftests/mm/mremap_test.c-414-
--
tools/testing/selftests/mm/mremap_test.c=606=static void mremap_move_multiple_vmas_split(unsigned int pattern_seed,
--
tools/testing/selftests/mm/mremap_test.c-617- if (dont_unmap)
tools/testing/selftests/mm/mremap_test.c:618: mremap_flags |= MREMAP_DONTUNMAP;
tools/testing/selftests/mm/mremap_test.c-619-
--
tools/testing/selftests/mm/mseal_test.c=1470=static void test_seal_mremap_move_dontunmap(bool seal)
--
tools/testing/selftests/mm/mseal_test.c-1486- /* mremap to move, and don't unmap src addr. */
tools/testing/selftests/mm/mseal_test.c:1487: ret2 = sys_mremap(ptr, size, size, MREMAP_MAYMOVE | MREMAP_DONTUNMAP, 0);
tools/testing/selftests/mm/mseal_test.c-1488- if (seal) {
--
tools/testing/selftests/mm/mseal_test.c=1499=static void test_seal_mremap_move_dontunmap_anyaddr(bool seal)
--
tools/testing/selftests/mm/mseal_test.c-1526- */
tools/testing/selftests/mm/mseal_test.c:1527: ret2 = sys_mremap(ptr, size, size, MREMAP_MAYMOVE | MREMAP_DONTUNMAP,
tools/testing/selftests/mm/mseal_test.c-1528- (void *) ptr2);
--
tools/testing/selftests/proc/proc-maps-race.c=536=static inline bool remap_vma(FIXTURE_DATA(proc_maps_race) *self)
--
tools/testing/selftests/proc/proc-maps-race.c-544- return mremap(self->mod_info->next_addr + self->page_size * 2, self->page_size,
tools/testing/selftests/proc/proc-maps-race.c:545: self->page_size, MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP,
tools/testing/selftests/proc/proc-maps-race.c-546- self->mod_info->addr + self->page_size) != MAP_FAILED;
]
|
| 44/2 |
2026/09/07 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 47348
output: 36
thoughts: 89
Thoughts: Okay, here's my interpretation of these thoughts, as if they were my own:
**MREMAM_DONTUNMAP and VMA Management - A Deep Dive**
Alright, so `MREMAP_DONTUNMAP`... it *seems* like it's designed to leave the original Virtual Memory Area (VMA) untouched, right? But the key here is that it *clears* the page tables associated with that original VMA. Okay, that makes sense conceptually; freeing up the physical memory.
Now, hold on... if I'm using `mremap` *with* `MREMAP_DONTUNMAP`, then the page tables are actually *moved* to the new VMA? And the *old* VMA is justā¦ empty? That has some interesting implications regarding the lifecycle of those page tables. The old VMA has to have its resources reclaimed somehow. I need to be sure the behaviour here is what I think it is.
The question then becomes: does `mremap` with `MREMAP_DONTUNMAP` also clear the `anon_vma` (anonymous VMA) of that old, now "empty" VMA? It's essential to understand how these structures relate to the underlying memory management to avoid leaks or inconsistencies, particularly with shared memory or copy-on-write mechanisms.
Okay, I'd better double check the `dontunmap_complete` function. That's probably where all the nitty-gritty details of this operation are handled, particularly how the kernel handles the page table move and VMA cleanup. I need to refresh my understanding of that code path.
|
| 45/2 |
2026/09/07 00:52 |
tool |
read-file |
0m
Args: map[File:mm/mremap.c FirstLine:1329 LineCount:50]
Results: map[Contents:1329: static void dontunmap_complete(struct vma_remap_struct *vrm,
1330: struct vm_area_struct *new_vma)
1331: {
1332: unsigned long start = vrm->addr;
1333: unsigned long end = vrm->addr + vrm->old_len;
1334: unsigned long old_start = vrm->vma->vm_start;
1335: unsigned long old_end = vrm->vma->vm_end;
1336:
1337: /* We always clear VMA_LOCKED[ONFAULT]_BIT on the old VMA. */
1338: vma_clear_flags_mask(vrm->vma, VMA_LOCKED_MASK);
1339:
1340: /*
1341: * anon_vma links of the old vma is no longer needed after its page
1342: * table has been moved.
1343: */
1344: if (new_vma != vrm->vma && start == old_start && end == old_end)
1345: unlink_anon_vmas(vrm->vma);
1346:
1347: /* Because we won't unmap we don't need to touch locked_vm. */
1348: }
1349:
1350: static unsigned long move_vma(struct vma_remap_struct *vrm)
1351: {
1352: struct mm_struct *mm = current->mm;
1353: struct vm_area_struct *new_vma;
1354: unsigned long hiwater_vm;
1355: int err;
1356:
1357: err = prep_move_vma(vrm);
1358: if (err)
1359: return err;
1360:
1361: /*
1362: * If accounted, determine the number of bytes the operation will
1363: * charge.
1364: */
1365: if (!vrm_calc_charge(vrm))
1366: return -ENOMEM;
1367:
1368: /* We don't want racing faults. */
1369: vma_start_write(vrm->vma);
1370:
1371: /* Perform copy step. */
1372: err = copy_vma_and_data(vrm, &new_vma);
1373: /*
1374: * If we established the copied-to VMA, we attempt to recover from the
1375: * error by setting the destination VMA to the source VMA and unmapping
1376: * it below.
1377: */
1378: if (err && !new_vma)
]
|
| 46/2 |
2026/09/07 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 48609
output: 20
thoughts: 103
Thoughts: **My Analysis of `unlink_anon_vmas`**
Okay, so I've confirmed it: the `unlink_anon_vmas(vrm->vma)` function is being called when the `new_vma` is different from `vrm->vma` and the `start` and `end` addresses match the `old_start` and `old_end` addresses. That's good; it's the expected behavior. This indicates that certain anon_vma links are being cleared under specific conditions during vma adjustments, as intended.
But now I need to dive deeper. `unlink_anon_vmas` is handling the removal of the `anon_vma` links. The question burning in my mind is this: Does `unlink_anon_vmas` actually *set* `vma->anon_vma = NULL`? It's removing the linkage at the anon_vma level, which is what I expect. But without this critical step, the vma could still be erroneously referencing a now-invalid `anon_vma` pointer, potentially leading to memory corruption down the line. I'll need to check the code for this crucial update to ensure the structure maintains consistency.
|
| 47/2 |
2026/09/07 00:52 |
tool |
grepper |
0m
Args: map[Expression:unlink_anon_vmas]
Results: map[Output:mm/internal.h=336=int __anon_vma_prepare(struct vm_area_struct *vma);
mm/internal.h:337:void unlink_anon_vmas(struct vm_area_struct *vma);
mm/internal.h-338-
--
mm/memory.c=373=void free_pgtables(struct mmu_gather *tlb, struct unmap_desc *unmap)
--
mm/memory.c-400- vma_start_write(vma);
mm/memory.c:401: unlink_anon_vmas(vma);
mm/memory.c-402-
--
mm/memory.c-413- vma_start_write(vma);
mm/memory.c:414: unlink_anon_vmas(vma);
mm/memory.c-415- unlink_file_vma_batch_add(&vb, vma);
--
mm/mremap.c=1329=static void dontunmap_complete(struct vma_remap_struct *vrm,
--
mm/mremap.c-1344- if (new_vma != vrm->vma && start == old_start && end == old_end)
mm/mremap.c:1345: unlink_anon_vmas(vrm->vma);
mm/mremap.c-1346-
--
mm/rmap.c=452=static void cleanup_partial_anon_vmas(struct vm_area_struct *vma)
--
mm/rmap.c-469-/**
mm/rmap.c:470: * unlink_anon_vmas() - remove all links between a VMA and anon_vma's, freeing
mm/rmap.c-471- * anon_vma_chain objects.
--
mm/rmap.c-478- */
mm/rmap.c:479:void unlink_anon_vmas(struct vm_area_struct *vma)
mm/rmap.c-480-{
--
mm/vma.c=383=static void vma_complete(struct vma_prepare *vp, struct vma_iterator *vmi,
--
mm/vma.c-433- if (vp->remove->anon_vma)
mm/vma.c:434: unlink_anon_vmas(vp->remove);
mm/vma.c-435- mm->map_count--;
--
mm/vma.c=889=static __must_check struct vm_area_struct *vma_merge_existing_range(
--
mm/vma.c-1082- if (anon_dup)
mm/vma.c:1083: unlink_anon_vmas(anon_dup);
mm/vma.c-1084-
--
mm/vma.c=1247=int vma_expand(struct vma_merge_struct *vmg)
--
mm/vma.c-1309- if (anon_dup)
mm/vma.c:1310: unlink_anon_vmas(anon_dup);
mm/vma.c-1311- /*
--
mm/vma.c=1944=struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
--
mm/vma.c-2031-
mm/vma.c:2032: unlink_anon_vmas(new_vma);
mm/vma.c-2033-out_free_mempol:
--
tools/testing/vma/include/custom.h=26=struct anon_vma {
--
tools/testing/vma/include/custom.h-34-
tools/testing/vma/include/custom.h:35:static inline void unlink_anon_vmas(struct vm_area_struct *vma)
tools/testing/vma/include/custom.h-36-{
]
|
| 48/2 |
2026/09/07 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 50013
output: 33
thoughts: 12
Thoughts: Okay, let's dive into `unlink_anon_vmas`. I'm going to approach this with the assumption that I understand the core principles of virtual memory management, but I need to quickly refresh my understanding of where this function sits in the overall scheme of things. I'm going to look for key data structures and pointers being manipulated here. My focus will be on understanding how anonymous virtual memory areas (VMAs) are being removed from the process's address space. I'll need to pay close attention to the lock hierarchies involved, especially if there are any that are critical to concurrency, and how they prevent race conditions. I anticipate that I'll be looking for memory mapping unlinking, reference count management, and maybe even page table operations. I'll try to follow any conditional paths to ensure I understand all the edge cases and how they are handled. I expect to be tracing memory freeing and resource deallocation through this code. I need to note any specific techniques used here for efficiency or atomicity in the process. My goal is to grasp the practical implications of `unlink_anon_vmas` within the Linux kernel, focusing on potential optimization points as well as the design's scalability. Let's see what we've got.
|
| 49/2 |
2026/09/07 00:53 |
tool |
read-file |
0m
Args: map[File:mm/rmap.c FirstLine:479 LineCount:50]
Results: map[Contents: 479: void unlink_anon_vmas(struct vm_area_struct *vma)
480: {
481: struct anon_vma_chain *avc, *next;
482: struct anon_vma *active_anon_vma = vma->anon_vma;
483:
484: /* Always hold mmap lock, read-lock on unmap possibly. */
485: mmap_assert_locked(vma->vm_mm);
486:
487: /* Unfaulted is a no-op. */
488: if (!active_anon_vma) {
489: VM_WARN_ON_ONCE(!list_empty(&vma->anon_vma_chain));
490: return;
491: }
492:
493: anon_vma_lock_write(active_anon_vma);
494:
495: /*
496: * Unlink each anon_vma chained to the VMA. This list is ordered
497: * from newest to oldest, ensuring the root anon_vma gets freed last.
498: */
499: list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
500: struct anon_vma *anon_vma = avc->anon_vma;
501:
502: anon_rmap_tree_remove(avc, anon_vma);
503:
504: /*
505: * Leave empty anon_vmas on the list - we'll need
506: * to free them outside the lock.
507: */
508: if (RB_EMPTY_ROOT(&anon_vma->rb_root.rb_root)) {
509: anon_vma->parent->num_children--;
510: continue;
511: }
512:
513: list_del(&avc->same_vma);
514: anon_vma_chain_free(avc);
515: }
516:
517: active_anon_vma->num_active_vmas--;
518: /*
519: * vma would still be needed after unlink, and anon_vma will be prepared
520: * when handle fault.
521: */
522: vma->anon_vma = NULL;
523: anon_vma_unlock_write(active_anon_vma);
524:
525:
526: /*
527: * Iterate the list once more, it now only contains empty and unlinked
528: * anon_vmas, destroy them. Could not do before due to __put_anon_vma()
]
|
| 50/2 |
2026/09/07 00:53 |
llm |
expert |
0m
Model:
Tokens:
input: 51121
output: 23
thoughts: 787
Thoughts: **Analyzing the VMA Pgoff Mismatch**
Okay, so I've confirmed it: `vma->anon_vma = NULL;` *is* being executed. That means `mremap` with `MREMAP_DONTUNMAP` is indeed clearing the `anon_vma` of the old VMA, as I suspected. And the old VMA is *still* retaining its initial `vm_pgoff`.
The key is that because `mremap` with `MREMAP_DONTUNMAP` cleared `vma->anon_vma`, the old VMA is now an unfaulted anonymous VMA. But its `vm_pgoff` *isn't* `vm_start >> PAGE_SHIFT`, and that's the crux of the issue. The first `mremap` is moving the VMA, so `0x20a96000` is getting moved to `0x20000000`. So the VMA at `0x20000000` ends up with `vm_pgoff = 0x20a96` while its `anon_vma` is *not* `NULL`.
Then, the second `mremap` with `MREMAP_DONTUNMAP` moves `0x20000000` to `0x20a00000`. It leaves the *old* VMA at `0x20000000` intact, but that `anon_vma` is cleared. This is where the discrepancy arises! Now, we have a VMA at `0x20000000` with `vm_pgoff = 0x20a96` and `anon_vma = NULL`.
The problem manifests during an `mbind` split at `0x20001000`. The `split_vma` is calling `vma_add_pgoff`, which calls `vma_set_pgoff`, which finally calls `assert_sane_pgoff`. `assert_sane_pgoff` correctly checks if `anon_vma` is NULL ā it is! ā and *then* checks if `pgoff == vm_start >> PAGE_SHIFT`. The `pgoff` is `0x20a96 + 1 = 0x20a97`. The `vm_start >> PAGE_SHIFT` is `0x20001`. They don't match, and bang ā that's where the `VM_WARN_ON_ONCE` triggers.
This perfectly explains the warning! The code is detecting a VMA with a mismatched `vm_pgoff`.
Now, the real question: what is the security implication? The warning itself is `VM_WARN_ON_ONCE` which is only enabled with `CONFIG_DEBUG_VM`. In production builds, that's disabled, so the warning does nothing. So in production, the mismatched `vm_pgoff` isn't causing a kernel crash or observable issue.
However, does the mismatched `vm_pgoff` cause any real problems? For anonymous VMAs, `vm_pgoff` is mainly used in a few places:
1. `vma_merge`: It's used to check if VMAs can be merged, but if they don't match, they just won't merge.
2. `unmap_mapping_range`: Not applicable to anonymous VMAs.
3. `page_add_new_anon_rmap` / `folio_add_new_anon_rmap`:
I'm now going to explore how `vm_pgoff` is used when a page is faulted in. That's the area that I think requires the closest examination.
|
| 51/2 |
2026/09/07 00:53 |
tool |
grepper |
0m
Args: map[Expression:folio_add_new_anon_rmap]
Results: map[Output:include/linux/rmap.h=408=void folio_add_anon_rmap_pmd(struct folio *, struct page *,
include/linux/rmap.h-409- struct vm_area_struct *, unsigned long address, rmap_t flags);
include/linux/rmap.h:410:void folio_add_new_anon_rmap(struct folio *, struct vm_area_struct *,
include/linux/rmap.h-411- unsigned long address, rmap_t flags);
--
mm/huge_memory.c=1358=void map_anon_folio_pmd_nopf(struct folio *folio, pmd_t *pmd,
--
mm/huge_memory.c-1364- entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
mm/huge_memory.c:1365: folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
mm/huge_memory.c-1366- folio_add_lru_vma(folio, vma);
--
mm/memory.c=1122=copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
--
mm/memory.c-1142- __folio_mark_uptodate(new_folio);
mm/memory.c:1143: folio_add_new_anon_rmap(new_folio, dst_vma, addr, RMAP_EXCLUSIVE);
mm/memory.c-1144- folio_add_lru_vma(new_folio, dst_vma);
--
mm/memory.c=3985=static vm_fault_t wp_page_copy(struct vm_fault *vmf)
--
mm/memory.c-4073- ptep_clear_flush(vma, vmf->address, vmf->pte);
mm/memory.c:4074: folio_add_new_anon_rmap(new_folio, vma, vmf->address, RMAP_EXCLUSIVE);
mm/memory.c-4075- folio_add_lru_vma(new_folio, vma);
--
mm/memory.c=4881=vm_fault_t do_swap_page(struct vm_fault *vmf)
--
mm/memory.c-5202- if (unlikely(folio != swapcache)) {
mm/memory.c:5203: folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE);
mm/memory.c-5204- folio_add_lru_vma(folio, vma);
--
mm/memory.c-5212- VM_WARN_ON_ONCE_FOLIO(folio_mapped(folio), folio);
mm/memory.c:5213: folio_add_new_anon_rmap(folio, vma, address, rmap_flags);
mm/memory.c-5214- folio_put_swap(folio, NULL);
--
mm/memory.c=5390=void map_anon_folio_pte_nopf(struct folio *folio, pte_t *pte,
--
mm/memory.c-5404- folio_ref_add(folio, nr_pages - 1);
mm/memory.c:5405: folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);
mm/memory.c-5406- folio_add_lru_vma(folio, vma);
--
mm/memory.c=5698=void set_pte_range(struct vm_fault *vmf, struct folio *folio,
--
mm/memory.c-5722- VM_BUG_ON_FOLIO(nr != 1, folio);
mm/memory.c:5723: folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);
mm/memory.c-5724- folio_add_lru_vma(folio, vma);
--
mm/migrate_device.c=798=static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
--
mm/migrate_device.c-877- add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
mm/migrate_device.c:878: folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);
mm/migrate_device.c-879- if (!folio_is_zone_device(folio))
--
mm/migrate_device.c=976=static void migrate_vma_insert_page(struct migrate_vma *migrate,
--
mm/migrate_device.c-1086- inc_mm_counter(mm, MM_ANONPAGES);
mm/migrate_device.c:1087: folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);
mm/migrate_device.c-1088- if (!folio_is_zone_device(folio))
--
mm/rmap.c=1501=static void __page_check_anon_rmap(const struct folio *folio,
--
mm/rmap.c-1511- *
mm/rmap.c:1512: * We have exclusion against folio_add_new_anon_rmap because those pages
mm/rmap.c-1513- * are initially only visible via the pagetables, and the pte is locked
mm/rmap.c:1514: * over the call to folio_add_new_anon_rmap.
mm/rmap.c-1515- */
--
mm/rmap.c=1622=void folio_add_anon_rmap_pmd(struct folio *folio, struct page *page,
--
mm/rmap.c-1633-/**
mm/rmap.c:1634: * folio_add_new_anon_rmap - Add mapping to a new anonymous folio.
mm/rmap.c-1635- * @folio: The folio to add the mapping to.
--
mm/rmap.c-1647- */
mm/rmap.c:1648:void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
mm/rmap.c-1649- unsigned long address, rmap_t flags)
--
mm/swapfile.c=2419=static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
--
mm/swapfile.c-2500- VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
mm/swapfile.c:2501: folio_add_new_anon_rmap(folio, vma, addr, rmap_flags);
mm/swapfile.c-2502- } else {
--
mm/swapfile.c-2505- } else { /* ksm created a completely new copy */
mm/swapfile.c:2506: folio_add_new_anon_rmap(folio, vma, addr, RMAP_EXCLUSIVE);
mm/swapfile.c-2507- folio_add_lru_vma(folio, vma);
--
mm/userfaultfd.c=357=static int mfill_atomic_install_pte(pmd_t *dst_pmd,
--
mm/userfaultfd.c-405- } else {
mm/userfaultfd.c:406: folio_add_new_anon_rmap(folio, dst_vma, dst_addr, RMAP_EXCLUSIVE);
mm/userfaultfd.c-407- folio_add_lru_vma(folio, dst_vma);
]
|
| 52/2 |
2026/09/07 00:53 |
llm |
expert |
0m
Model:
Tokens:
input: 54774
output: 34
thoughts: 15
Thoughts: Okay, let's dive into this. Here's my take on those thoughts, as if they're my own:
**Analyzing `folio_add_new_anon_rmap`**
Alright, time to get into this routine, `folio_add_new_anon_rmap`. It's crucial in managing anonymous mappings, so I need to understand it cold. Given the name, I'm expecting this function to be responsible for creating new anonymous region maps, likely within the context of a folio-based memory management system. It probably deals with initializing data structures, perhaps setting up page tables or other metadata to represent a newly allocated anonymous region. There's a high probability it'll handle the allocation of physical pages, too. The "anonymous" part suggests that it's dealing with memory that isn't backed by a file on disk, which implies it's either part of program memory or a general purpose memory manager to grab resources. It is likely called from other routines that set up process environments, fork, or manage shared memory segments. Ultimately, I'm expecting this to be a core building block for dynamic memory allocation within the kernel or related system component. Let's see how accurate my initial hypotheses are.
|
| 53/2 |
2026/09/07 00:53 |
tool |
read-file |
0m
Args: map[File:mm/rmap.c FirstLine:1648 LineCount:50]
Results: map[Contents:1648: void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
1649: unsigned long address, rmap_t flags)
1650: {
1651: const bool exclusive = flags & RMAP_EXCLUSIVE;
1652: int nr = 1, nr_pmdmapped = 0;
1653:
1654: VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
1655: VM_WARN_ON_FOLIO(!exclusive && !folio_test_locked(folio), folio);
1656:
1657: /*
1658: * VM_DROPPABLE mappings don't swap; instead they're just dropped when
1659: * under memory pressure.
1660: */
1661: if (!folio_test_swapbacked(folio) && !(vma->vm_flags & VM_DROPPABLE))
1662: __folio_set_swapbacked(folio);
1663: __folio_set_anon(folio, vma, address, exclusive);
1664:
1665: if (likely(!folio_test_large(folio))) {
1666: /* increment count (starts at -1) */
1667: atomic_set(&folio->_mapcount, 0);
1668: if (exclusive)
1669: SetPageAnonExclusive(&folio->page);
1670: } else if (!folio_test_pmd_mappable(folio)) {
1671: int i;
1672:
1673: nr = folio_large_nr_pages(folio);
1674: for (i = 0; i < nr; i++) {
1675: struct page *page = folio_page(folio, i);
1676:
1677: if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
1678: /* increment count (starts at -1) */
1679: atomic_set(&page->_mapcount, 0);
1680: if (exclusive)
1681: SetPageAnonExclusive(page);
1682: }
1683:
1684: folio_set_large_mapcount(folio, nr, vma);
1685: if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
1686: atomic_set(&folio->_nr_pages_mapped, nr);
1687: } else {
1688: nr = folio_large_nr_pages(folio);
1689: /* increment count (starts at -1) */
1690: atomic_set(&folio->_entire_mapcount, 0);
1691: folio_set_large_mapcount(folio, 1, vma);
1692: if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
1693: atomic_set(&folio->_nr_pages_mapped, ENTIRELY_MAPPED);
1694: if (exclusive)
1695: SetPageAnonExclusive(&folio->page);
1696: nr_pmdmapped = nr;
1697: }
]
|
| 54/2 |
2026/09/07 00:53 |
llm |
expert |
0m
Model:
Tokens:
input: 55934
output: 20
thoughts: 12
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm deep in my domain of expertise:
**Diving into `__folio_set_anon`**
Alright, let's take a look at this `__folio_set_anon` function. The name itself suggests a function related to setting anonymous attributes or flags, likely within a folio or document structure. This is immediately interesting because folios often house critical data, so any function that deals with anonymity has security and privacy implications I need to consider right off the bat.
My brain is immediately racing with questions: What system is this implemented in? Is it Python? C++? Something else entirely? What's the context of the folio structure? Are we dealing with financial data, legal documents, image metadata, or something else? Understanding the broader system context will significantly influence my interpretation.
I'm thinking about edge cases: How does this function handle null values or invalid inputs? What are the potential consequences if this function malfunctions or is exploited? Could it create vulnerabilities in our system or expose sensitive information? Is there a risk of race conditions or data corruption if multiple threads or processes interact with this function simultaneously?
My expertise allows me to think about existing security protocols and data integrity validation processes. I'm wondering if this function uses an accepted standard for anonymization, or if it's a bespoke implementation? How does it interact with auditing and logging? Any potential misuse or misconfiguration needs to be traceable. I'll need to scrutinize how this function interacts with any related security mechanisms. I'm also considering how it fits into the broader data lifecycle ā creation, modification, deletion, and archiving. What is the impact of anonymization here?
I need to understand its behavior thoroughly to identify potential issues and ensure it adheres to best practices for data security and privacy. I'm ready to dig deeper and examine the surrounding code to determine its true purpose.
|
| 55/2 |
2026/09/07 00:53 |
tool |
grepper |
0m
Args: map[Expression:__folio_set_anon]
Results: map[Output:mm/rmap.c=1446=void folio_move_anon_rmap(struct folio *folio, struct vm_area_struct *vma)
--
mm/rmap.c-1462-/**
mm/rmap.c:1463: * __folio_set_anon - set up a new anonymous rmap for a folio
mm/rmap.c-1464- * @folio: The folio to set up the new anonymous rmap for.
--
mm/rmap.c-1468- */
mm/rmap.c:1469:static void __folio_set_anon(struct folio *folio, struct vm_area_struct *vma,
mm/rmap.c-1470- unsigned long address, bool exclusive)
--
mm/rmap.c=1648=void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
--
mm/rmap.c-1662- __folio_set_swapbacked(folio);
mm/rmap.c:1663: __folio_set_anon(folio, vma, address, exclusive);
mm/rmap.c-1664-
--
mm/rmap.c=3194=void hugetlb_add_new_anon_rmap(struct folio *folio,
--
mm/rmap.c-3203- folio_clear_hugetlb_restore_reserve(folio);
mm/rmap.c:3204: __folio_set_anon(folio, vma, address, true);
mm/rmap.c-3205- SetPageAnonExclusive(&folio->page);
]
|
| 56/2 |
2026/09/07 00:53 |
llm |
expert |
0m
Model:
Tokens:
input: 56776
output: 34
thoughts: 12
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm deep in my domain of expertise:
**Diving into `__folio_set_anon`**
Alright, let's get right to it. The phrase `__folio_set_anon` has me instantly curious. That double underscore suggests a private or internal function within a folio system, likely dealing with setting or manipulating something anonymous. It's probably a core, low-level piece, and I need to understand its role. I expect this function probably handles setting flags, initializing a data structure with placeholder values, or performing some internal operation critical to maintaining the anonymous state. Is it about user sessions? Data obfuscation? Control of access? Whatever it is, I need to know how it fits into the broader picture of the folio system and its architecture. Let's see what the code reveals!
|
| 57/2 |
2026/09/07 00:53 |
tool |
read-file |
0m
Args: map[File:mm/rmap.c FirstLine:1469 LineCount:50]
Results: map[Contents:1469: static void __folio_set_anon(struct folio *folio, struct vm_area_struct *vma,
1470: unsigned long address, bool exclusive)
1471: {
1472: struct anon_vma *anon_vma = vma->anon_vma;
1473:
1474: BUG_ON(!anon_vma);
1475:
1476: /*
1477: * If the folio isn't exclusive to this vma, we must use the _oldest_
1478: * possible anon_vma for the folio mapping!
1479: */
1480: if (!exclusive)
1481: anon_vma = anon_vma->root;
1482:
1483: /*
1484: * page_idle does a lockless/optimistic rmap scan on folio->mapping.
1485: * Make sure the compiler doesn't split the stores of anon_vma and
1486: * the FOLIO_MAPPING_ANON type identifier, otherwise the rmap code
1487: * could mistake the mapping for a struct address_space and crash.
1488: */
1489: anon_vma = (void *) anon_vma + FOLIO_MAPPING_ANON;
1490: WRITE_ONCE(folio->mapping, (struct address_space *) anon_vma);
1491: folio->index = linear_anon_page_index(vma, address);
1492: }
1493:
1494: /**
1495: * __page_check_anon_rmap - sanity check anonymous rmap addition
1496: * @folio: The folio containing @page.
1497: * @page: the page to check the mapping of
1498: * @vma: the vm area in which the mapping is added
1499: * @address: the user virtual address mapped
1500: */
1501: static void __page_check_anon_rmap(const struct folio *folio,
1502: const struct page *page, struct vm_area_struct *vma,
1503: unsigned long address)
1504: {
1505: /*
1506: * The page's anon-rmap details (mapping and index) are guaranteed to
1507: * be set up correctly at this point.
1508: *
1509: * We have exclusion against folio_add_anon_rmap_*() because the caller
1510: * always holds the page locked.
1511: *
1512: * We have exclusion against folio_add_new_anon_rmap because those pages
1513: * are initially only visible via the pagetables, and the pte is locked
1514: * over the call to folio_add_new_anon_rmap.
1515: */
1516: VM_BUG_ON_FOLIO(folio_anon_vma(folio)->root != vma->anon_vma->root,
1517: folio);
1518: VM_BUG_ON_PAGE(page_pgoff(folio, page) !=
]
|
| 58/2 |
2026/09/07 00:53 |
llm |
expert |
0m
Model:
Tokens:
input: 57866
output: 20
thoughts: 102
Thoughts: **Analyzing Anonymous VMA Setup**
Okay, so I'm looking at `__folio_set_anon` and immediately notice the `BUG_ON(!anon_vma);`. That makes sense; it seems fundamental to the anonymous page setup. But then I have a hunch. I recall that if `vma->anon_vma` is actually `NULL`, like when we fault in a page, `do_anonymous_page` kicks off `anon_vma_prepare(vma)`. And `anon_vma_prepare` *creates* a brand new `anon_vma` for the Virtual Memory Area. Now I need to confirm my line of thought. Let me quickly double check the internals of `anon_vma_prepare`... just to make sure I'm following the logic correctly. It's important to understand how these initial allocations are handled.
|
| 59/2 |
2026/09/07 00:53 |
tool |
grepper |
0m
Args: map[Expression:anon_vma_prepare]
Results: map[Output:Documentation/mm/process_addrs.rst=242=configuration option is set.
--
Documentation/mm/process_addrs.rst-262- this VMA. Initially set by mmap read, page_table_lock.
Documentation/mm/process_addrs.rst:263: :c:func:`!anon_vma_prepare` serialised
Documentation/mm/process_addrs.rst-264- by the :c:macro:`!page_table_lock`. This When non-:c:macro:`NULL` and
--
Documentation/mm/process_addrs.rst=440=There is also a file-system specific lock ordering comment located at the top of
--
Documentation/mm/process_addrs.rst-473- ->anon_vma.lock
Documentation/mm/process_addrs.rst:474: ->page_table_lock or pte_lock (anon_vma_prepare and various)
Documentation/mm/process_addrs.rst-475-
--
include/linux/rmap.h=971=struct anon_vma *folio_lock_anon_vma_read(const struct folio *folio,
--
include/linux/rmap.h-976-#define anon_vma_init() do {} while (0)
include/linux/rmap.h:977:#define anon_vma_prepare(vma) (0)
include/linux/rmap.h-978-
--
kernel/events/uprobes.c-18-#include <linux/export.h>
kernel/events/uprobes.c:19:#include <linux/rmap.h> /* anon_vma_prepare */
kernel/events/uprobes.c-20-#include <linux/mmu_notifier.h>
--
mm/filemap.c-111- * ->anon_vma.lock
mm/filemap.c:112: * ->page_table_lock or pte_lock (anon_vma_prepare and various)
mm/filemap.c-113- *
--
mm/internal.h=335=int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma);
mm/internal.h:336:int __anon_vma_prepare(struct vm_area_struct *vma);
mm/internal.h-337-void unlink_anon_vmas(struct vm_area_struct *vma);
mm/internal.h-338-
mm/internal.h:339:static inline int anon_vma_prepare(struct vm_area_struct *vma)
mm/internal.h-340-{
--
mm/internal.h-343-
mm/internal.h:344: return __anon_vma_prepare(vma);
mm/internal.h-345-}
--
mm/madvise.c=1116=static long madvise_guard_install(struct madvise_behavior *madv_behavior)
--
mm/madvise.c-1150-
mm/madvise.c:1151: err = anon_vma_prepare(vma);
mm/madvise.c-1152- if (err)
--
mm/madvise.c=1563=static bool process_madvise_remote_valid(int behavior)
--
mm/madvise.c-1575-
mm/madvise.c:1576:/* Does this operation invoke anon_vma_prepare()? */
mm/madvise.c-1577-static bool prepares_anon_vma(int behavior)
--
mm/madvise.c=1591=static bool is_vma_lock_sufficient(struct vm_area_struct *vma,
--
mm/madvise.c-1603- /*
mm/madvise.c:1604: * anon_vma_prepare() explicitly requires an mmap lock for
mm/madvise.c-1605- * serialisation, so we cannot use a VMA lock in this case.
--
mm/memory.c=3925=static inline vm_fault_t vmf_can_call_fault(const struct vm_fault *vmf)
--
mm/memory.c-3939- * When preparing to insert an anonymous page into a VMA from a
mm/memory.c:3940: * fault handler, call this function rather than anon_vma_prepare().
mm/memory.c-3941- * If this vma does not already have an associated anon_vma and we are
mm/memory.c-3942- * only protected by the per-VMA lock, the caller must retry with the
mm/memory.c:3943: * mmap_lock held. __anon_vma_prepare() will look at adjacent VMAs to
mm/memory.c-3944- * determine if this VMA can share its anon_vma, and that's not safe to
--
mm/memory.c=3950=vm_fault_t __vmf_anon_prepare(struct vm_fault *vmf)
--
mm/memory.c-3960- }
mm/memory.c:3961: if (__anon_vma_prepare(vma))
mm/memory.c-3962- ret = VM_FAULT_OOM;
--
mm/migrate_device.c=798=static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
--
mm/migrate_device.c-819-
mm/migrate_device.c:820: ret = anon_vma_prepare(vma);
mm/migrate_device.c-821- if (ret)
--
mm/migrate_device.c=976=static void migrate_vma_insert_page(struct migrate_vma *migrate,
--
mm/migrate_device.c-1028- goto abort;
mm/migrate_device.c:1029: if (unlikely(anon_vma_prepare(vma)))
mm/migrate_device.c-1030- goto abort;
--
mm/rmap.c=150=static void anon_vma_chain_assign(struct vm_area_struct *vma,
--
mm/rmap.c-159-/**
mm/rmap.c:160: * __anon_vma_prepare - attach an anon_vma to a memory region
mm/rmap.c-161- * @vma: the memory region in question
--
mm/rmap.c-167- * The common case will be that we already have one, which
mm/rmap.c:168: * is handled inline by anon_vma_prepare(). But if
mm/rmap.c-169- * not we either need to find an adjacent mapping that we
--
mm/rmap.c-184- */
mm/rmap.c:185:int __anon_vma_prepare(struct vm_area_struct *vma)
mm/rmap.c-186-{
--
mm/userfaultfd.c=109=struct vm_area_struct *find_vma_and_prepare_anon(struct mm_struct *mm,
--
mm/userfaultfd.c-118- else if (!(vma->vm_flags & VM_SHARED) &&
mm/userfaultfd.c:119: unlikely(anon_vma_prepare(vma)))
mm/userfaultfd.c-120- vma = ERR_PTR(-ENOMEM);
--
mm/vma.c=2106=static struct anon_vma *reusable_anon_vma(struct vm_area_struct *old,
--
mm/vma.c-2119-/*
mm/vma.c:2120: * find_mergeable_anon_vma is used by anon_vma_prepare, to check
mm/vma.c-2121- * neighbouring vmas for a suitable anon_vma, before it goes off
--
mm/vma.c=3222=int expand_upwards(struct vm_area_struct *vma, unsigned long address)
--
mm/vma.c-3262- /* We must make sure the anon_vma is allocated. */
mm/vma.c:3263: if (unlikely(anon_vma_prepare(vma))) {
mm/vma.c-3264- vma_iter_free(&vmi);
--
mm/vma.c=3307=int expand_downwards(struct vm_area_struct *vma, unsigned long address)
--
mm/vma.c-3340- /* We must make sure the anon_vma is allocated. */
mm/vma.c:3341: if (unlikely(anon_vma_prepare(vma))) {
mm/vma.c-3342- vma_iter_free(&vmi);
--
tools/testing/vma/include/custom.h=55=static inline int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src,
--
tools/testing/vma/include/custom.h-66-
tools/testing/vma/include/custom.h:67:static inline int __anon_vma_prepare(struct vm_area_struct *vma)
tools/testing/vma/include/custom.h-68-{
--
tools/testing/vma/include/custom.h-79-
tools/testing/vma/include/custom.h:80:static inline int anon_vma_prepare(struct vm_area_struct *vma)
tools/testing/vma/include/custom.h-81-{
--
tools/testing/vma/include/custom.h-84-
tools/testing/vma/include/custom.h:85: return __anon_vma_prepare(vma);
tools/testing/vma/include/custom.h-86-}
]
|
| 60/2 |
2026/09/07 00:53 |
llm |
expert |
0m
Model:
Tokens:
input: 60701
output: 33
thoughts: 13
Thoughts: Okay, let's dive into `__anon_vma_prepare`. I know this routine; it's a critical part of anonymous memory management in the kernel. I need to refresh my understanding of how it fits into the larger picture of allocating and managing anonymous virtual memory areas (VMAs). It's probably called during the process of a `mmap` or similar system call requesting anonymous memory, or perhaps during a fork or copy-on-write operation. I'm guessing it's responsible for setting up the necessary data structures: potentially associating the new VMA with an `anon_vma` structure, ensuring the page tables are appropriately configured (or prepared for future configuration) for the requested memory range, and generally ensuring the VMA is integrated into the anonymous memory tracking system. I'll be looking to see how it handles things like the `anon_vma` list management, page table creation, potential races with other processes accessing the same memory region, and any locking that's employed. Specifically, I'll be paying attention to the details of how it deals with the different VM_flags and how it interacts with the underlying page allocation mechanisms, such as `get_anon_vma` and the anonymous page cache itself. I want to see if it makes any assumptions about the calling context, such as whether it's called from interrupt context or process context, or if it deals with different memory architectures. It's time to put on my analysis hat. Let's dig in.
|
| 61/2 |
2026/09/07 00:53 |
tool |
read-file |
0m
Args: map[File:mm/rmap.c FirstLine:185 LineCount:50]
Results: map[Contents: 185: int __anon_vma_prepare(struct vm_area_struct *vma)
186: {
187: struct mm_struct *mm = vma->vm_mm;
188: struct anon_vma *anon_vma, *allocated;
189: struct anon_vma_chain *avc;
190:
191: mmap_assert_locked(mm);
192: might_sleep();
193:
194: avc = anon_vma_chain_alloc(GFP_KERNEL);
195: if (!avc)
196: goto out_enomem;
197:
198: anon_vma = find_mergeable_anon_vma(vma);
199: allocated = NULL;
200: if (!anon_vma) {
201: anon_vma = anon_vma_alloc();
202: if (unlikely(!anon_vma))
203: goto out_enomem_free_avc;
204: anon_vma->num_children++; /* self-parent link for new root */
205: allocated = anon_vma;
206: }
207:
208: anon_vma_lock_write(anon_vma);
209: /* page_table_lock to protect against threads */
210: spin_lock(&mm->page_table_lock);
211: if (likely(!vma->anon_vma)) {
212: vma->anon_vma = anon_vma;
213: anon_vma_chain_assign(vma, avc, anon_vma);
214: anon_rmap_tree_insert(avc, anon_vma);
215: anon_vma->num_active_vmas++;
216: allocated = NULL;
217: avc = NULL;
218: }
219: spin_unlock(&mm->page_table_lock);
220: anon_vma_unlock_write(anon_vma);
221:
222: if (unlikely(allocated))
223: put_anon_vma(allocated);
224: if (unlikely(avc))
225: anon_vma_chain_free(avc);
226:
227: return 0;
228:
229: out_enomem_free_avc:
230: anon_vma_chain_free(avc);
231: out_enomem:
232: return -ENOMEM;
233: }
234:
]
|
| 62/2 |
2026/09/07 00:53 |
llm |
expert |
0m
Model:
Tokens:
input: 61835
output: 35
thoughts: 43
Thoughts: Okay, let's see... I'm looking at `__anon_vma_prepare`, and it's calling `find_mergeable_anon_vma(vma)`. Right. That's a key interaction. I need to understand that call specifically to understand how the anonymous virtual memory areas are being managed.
Therefore, my next move is to dive straight into `find_mergeable_anon_vma`. I need to trace the logic there to see what it's doing, how it identifies mergeable areas, and what criteria it's applying. This is where the magic (or the potential bottleneck!) likely lies. Let's get to it.
|
| 63/2 |
2026/09/07 00:53 |
tool |
read-file |
0m
Args: map[File:mm/vma.c FirstLine:2120 LineCount:50]
Results: map[Contents:2120: * find_mergeable_anon_vma is used by anon_vma_prepare, to check
2121: * neighbouring vmas for a suitable anon_vma, before it goes off
2122: * to allocate a new anon_vma. It checks because a repetitive
2123: * sequence of mprotects and faults may otherwise lead to distinct
2124: * anon_vmas being allocated, preventing vma merge in subsequent
2125: * mprotect.
2126: */
2127: struct anon_vma *find_mergeable_anon_vma(struct vm_area_struct *vma)
2128: {
2129: struct anon_vma *anon_vma = NULL;
2130: struct vm_area_struct *prev, *next;
2131: VMA_ITERATOR(vmi, vma->vm_mm, vma->vm_end);
2132:
2133: /* Try next first. */
2134: next = vma_iter_load(&vmi);
2135: if (next) {
2136: anon_vma = reusable_anon_vma(next, vma, next);
2137: if (anon_vma)
2138: return anon_vma;
2139: }
2140:
2141: prev = vma_prev(&vmi);
2142: VM_BUG_ON_VMA(prev != vma, vma);
2143: prev = vma_prev(&vmi);
2144: /* Try prev next. */
2145: if (prev)
2146: anon_vma = reusable_anon_vma(prev, prev, vma);
2147:
2148: /*
2149: * We might reach here with anon_vma == NULL if we can't find
2150: * any reusable anon_vma.
2151: * There's no absolute need to look only at touching neighbours:
2152: * we could search further afield for "compatible" anon_vmas.
2153: * But it would probably just be a waste of time searching,
2154: * or lead to too many vmas hanging off the same anon_vma.
2155: * We're trying to allow mprotect remerging later on,
2156: * not trying to minimize memory used for anon_vmas.
2157: */
2158: return anon_vma;
2159: }
2160:
2161: static bool vm_ops_needs_writenotify(const struct vm_operations_struct *vm_ops)
2162: {
2163: return vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite);
2164: }
2165:
2166: static bool vma_is_shared_writable(struct vm_area_struct *vma)
2167: {
2168: return vma_test_all(vma, VMA_WRITE_BIT, VMA_SHARED_BIT);
2169: }
]
|
| 64/2 |
2026/09/07 00:53 |
llm |
expert |
0m
Model:
Tokens:
input: 62923
output: 36
thoughts: 958
Thoughts: **Analyzing the Impact of `mremap` with `MREMAP_DONTUNMAP` on Anonymous VMAs**
Okay, so I'm digging into this `reusable_anon_vma` function, and it's all about whether we can reuse an existing anonymous VMA. The crucial check here is if `vm_pgoff` matches. If it *doesn't* match, we're forced to allocate a brand new `anon_vma` using `__anon_vma_prepare`. That new VMA then goes through the `folio_add_new_anon_rmap` process, which ultimately calls `__folio_set_anon`. Inside `__folio_set_anon`, we have the important step of setting `folio->index`. This is where things get interesting. The index is calculated as `linear_anon_page_index(vma, address)`, and that translates to `linear_page_delta(vma, address) + vma_start_anon_pgoff(vma)`. Since we're dealing with anonymous VMAs, `vma_start_anon_pgoff` is simply `vma->vm_pgoff`. This boils down to `(address - vma->vm_start) >> PAGE_SHIFT + vma->vm_pgoff`.
In the specific scenario that triggered my investigation, a VMA was moved from `0x20a96000` to `0x20000000`. So the goal is to map the page at `0x20000000` to the original page at `0x20a96000`. The index calculation SHOULD be, and *IS*, correctly computed as `0x20a96`, because the VMA's `vm_pgoff` is also `0x20a96`. It aligns perfectly, and `folio->index` is, therefore, also perfect. It works!
So, the issue isn't the index calculation itself. Where's the problem? The problem is that `assert_sane_pgoff` makes this assumption: *if* `vma->anon_vma` is NULL, then `vma->vm_pgoff` *must* be `vma->vm_start >> PAGE_SHIFT`. But, the key is the behavior of `mremap` with the `MREMAP_DONTUNMAP` flag. This flag clears the `vma->anon_vma` field, but leaves the `vma->vm_pgoff` field untouched! Therefore, if `mremap` is called on a VMA, the assumptions are violated. `vma->vm_pgoff` no longer corresponds to `vma->vm_start >> PAGE_SHIFT`, and `anon_vma` is NULL. This is what breaks `assert_sane_pgoff`.
However, is `assert_sane_pgoff` the *only* place this assumption is made? I need to check for other assumptions in the kernel that rely on this. Specifically, I'm concerned with the code in `copy_vma`.
```c
/*
* If a vma has not yet been faulted, update its anonymous pgoff to
* match the new location to increase its chance of merging.
*/
if (!vma->anon_vma) {
anon_pgoff = addr >> PAGE_SHIFT;
if (vma_is_anonymous(vma)) {
pgoff = anon_pgoff;
can_self_merge = true;
}
}
```
If we `mremap` a VMA, and that VMA has `anon_vma == NULL` and `vm_pgoff = 0x20a96`, then `copy_vma` will *update* its `pgoff` to `addr >> PAGE_SHIFT`. But the old VMA had the pgoff of `0x20a96`. Is this an issue? For an anonymous VMA with `anon_vma == NULL`, there are NO pages faulted in. Why? Because if the pages were faulted, `anon_vma` would not be NULL. Right?
But wait! `mremap` with `MREMAP_DONTUNMAP` *clears* `anon_vma`, but does it actually unmap the pages? I need to review `dontunmap_complete` again to make certain.
|
| 65/2 |
2026/09/07 00:53 |
tool |
read-file |
0m
Args: map[File:mm/mremap.c FirstLine:1329 LineCount:50]
Results: map[Contents:1329: static void dontunmap_complete(struct vma_remap_struct *vrm,
1330: struct vm_area_struct *new_vma)
1331: {
1332: unsigned long start = vrm->addr;
1333: unsigned long end = vrm->addr + vrm->old_len;
1334: unsigned long old_start = vrm->vma->vm_start;
1335: unsigned long old_end = vrm->vma->vm_end;
1336:
1337: /* We always clear VMA_LOCKED[ONFAULT]_BIT on the old VMA. */
1338: vma_clear_flags_mask(vrm->vma, VMA_LOCKED_MASK);
1339:
1340: /*
1341: * anon_vma links of the old vma is no longer needed after its page
1342: * table has been moved.
1343: */
1344: if (new_vma != vrm->vma && start == old_start && end == old_end)
1345: unlink_anon_vmas(vrm->vma);
1346:
1347: /* Because we won't unmap we don't need to touch locked_vm. */
1348: }
1349:
1350: static unsigned long move_vma(struct vma_remap_struct *vrm)
1351: {
1352: struct mm_struct *mm = current->mm;
1353: struct vm_area_struct *new_vma;
1354: unsigned long hiwater_vm;
1355: int err;
1356:
1357: err = prep_move_vma(vrm);
1358: if (err)
1359: return err;
1360:
1361: /*
1362: * If accounted, determine the number of bytes the operation will
1363: * charge.
1364: */
1365: if (!vrm_calc_charge(vrm))
1366: return -ENOMEM;
1367:
1368: /* We don't want racing faults. */
1369: vma_start_write(vrm->vma);
1370:
1371: /* Perform copy step. */
1372: err = copy_vma_and_data(vrm, &new_vma);
1373: /*
1374: * If we established the copied-to VMA, we attempt to recover from the
1375: * error by setting the destination VMA to the source VMA and unmapping
1376: * it below.
1377: */
1378: if (err && !new_vma)
]
|
| 66/2 |
2026/09/07 00:53 |
llm |
expert |
0m
Model:
Tokens:
input: 65708
output: 19
thoughts: 410
Thoughts: **My Reasoning on VMA Page Table Management**
Okay, let me trace this through. So, `move_vma` calls `copy_vma_and_data`, which in turn, calls `move_page_tables`. Right. And `move_page_tables` is *moving* the page tables from the old VMA to the new one. That means the old VMA is essentially barren. It's got *nothing* left in terms of page tables. Absolutely empty.
If it's empty, then it can't possibly have any pages faulted in, by definition. Which means having `anon_vma == NULL` on the old VMA is perfectly acceptable. Makes total sense.
And if no pages have been faulted, the `vm_pgoff` field for anonymous memory seems...almost irrelevant. When a new page fault occurs, `do_anonymous_page` will spring into action and allocate a fresh zero page. It will call `anon_vma_prepare`, which allocates a new `anon_vma`. Then it will call `folio_add_new_anon_rmap` which will assign `folio->index` based on `vm_pgoff`.
But wait, hold on. For anonymous pages, what *does* `folio->index` even matter for?
Let's see. It's used for swapping, right? The page is placed in the swap cache at `folio->index`. But no, wait, the swap cache uses `swp_offset(entry)` as the index, not `folio->index`. Okay, so `folio->index` is *not* used to locate the page in the swap cache. Swap cache uses `swp_type` and `swp_offset`.
Aha! `folio->index` is used for the `rmap`! That's it. When the kernel needs to unmap a page ā for swap-out or migration, say ā it utilizes `rmap_walk_anon`. And `rmap_walk_anon` relies on `folio->index` to pinpoint the VMA that's mapping the page. Right, that makes total sense. Okay, I've got it now.
|
| 67/2 |
2026/09/07 00:54 |
tool |
grepper |
0m
Args: map[Expression:rmap_walk_anon]
Results: map[Output:mm/rmap.c=2966=void __put_anon_vma(struct anon_vma *anon_vma)
--
mm/rmap.c-2974-
mm/rmap.c:2975:static struct anon_vma *rmap_walk_anon_lock(const struct folio *folio,
mm/rmap.c-2976- struct rmap_walk_control *rwc)
--
mm/rmap.c-3007-/*
mm/rmap.c:3008: * rmap_walk_anon - do something to anonymous page using the object-based
mm/rmap.c-3009- * rmap method
--
mm/rmap.c-3016- */
mm/rmap.c:3017:static void rmap_walk_anon(struct folio *folio,
mm/rmap.c-3018- struct rmap_walk_control *rwc, bool locked)
--
mm/rmap.c-3034- } else {
mm/rmap.c:3035: anon_vma = rmap_walk_anon_lock(folio, rwc);
mm/rmap.c-3036- }
--
mm/rmap.c=3153=void rmap_walk(struct folio *folio, struct rmap_walk_control *rwc)
--
mm/rmap.c-3157- else if (folio_test_anon(folio))
mm/rmap.c:3158: rmap_walk_anon(folio, rwc, false);
mm/rmap.c-3159- else
--
mm/rmap.c=3164=void rmap_walk_locked(struct folio *folio, struct rmap_walk_control *rwc)
--
mm/rmap.c-3168- if (folio_test_anon(folio))
mm/rmap.c:3169: rmap_walk_anon(folio, rwc, true);
mm/rmap.c-3170- else
]
|
| 68/2 |
2026/09/07 00:54 |
llm |
expert |
0m
Model:
Tokens:
input: 67119
output: 34
thoughts: 11
Thoughts: Okay, let's dive into this. Here's my take on `rmap_walk_anon`:
**Analyzing `rmap_walk_anon`**
Alright, so I'm about to dissect `rmap_walk_anon`. It's time to get down to the nitty-gritty of how this anonymous memory region walker functions. I need to understand its role in memory management, particularly when dealing with pages that aren't backed by a file. I'll need to focus on how it iterates through the page table entries (PTEs) associated with these anonymous mappings. I'll be looking for how it handles different scenarios, perhaps including shared anonymous mappings (like those created with `MAP_SHARED`), and private mappings. I'm especially keen to see how it navigates the various data structures involved, such as the `mm_struct`, `vm_area_struct`, and the page tables themselves, to correctly identify and process the relevant memory regions. I'll be looking for any optimization techniques.
|
| 69/2 |
2026/09/07 00:54 |
tool |
read-file |
0m
Args: map[File:mm/rmap.c FirstLine:3017 LineCount:50]
Results: map[Contents:3017: static void rmap_walk_anon(struct folio *folio,
3018: struct rmap_walk_control *rwc, bool locked)
3019: {
3020: struct anon_vma *anon_vma;
3021: pgoff_t pgoff_start, pgoff_end;
3022: struct anon_vma_chain *avc;
3023:
3024: /*
3025: * The folio lock ensures that folio->mapping can't be changed under us
3026: * to an anon_vma with different root.
3027: */
3028: VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
3029:
3030: if (locked) {
3031: anon_vma = folio_anon_vma(folio);
3032: /* anon_vma disappear under us? */
3033: VM_BUG_ON_FOLIO(!anon_vma, folio);
3034: } else {
3035: anon_vma = rmap_walk_anon_lock(folio, rwc);
3036: }
3037: if (!anon_vma)
3038: return;
3039:
3040: pgoff_start = folio_pgoff(folio);
3041: pgoff_end = pgoff_start + folio_nr_pages(folio) - 1;
3042: anon_rmap_tree_foreach(avc, anon_vma, pgoff_start, pgoff_end) {
3043: struct vm_area_struct *vma = avc->vma;
3044: const unsigned long address = vma_anon_address(vma, pgoff_start,
3045: folio_nr_pages(folio));
3046:
3047: VM_WARN_ON_ONCE_VMA(address == -EFAULT, vma);
3048: cond_resched();
3049:
3050: if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
3051: continue;
3052:
3053: if (!rwc->rmap_one(folio, vma, address, rwc->arg))
3054: break;
3055: if (rwc->done && rwc->done(folio))
3056: break;
3057: }
3058:
3059: if (!locked)
3060: anon_vma_unlock_read(anon_vma);
3061: }
3062:
3063: /**
3064: * __rmap_walk_file() - Traverse the reverse mapping for a file-backed mapping
3065: * of a page mapped within a specified page cache object at a specified offset.
3066: *
]
|
| 70/2 |
2026/09/07 00:54 |
llm |
expert |
0m
Model:
Tokens:
input: 68195
output: 20
thoughts: 217
Thoughts: **Analyzing the Anon Rmap Walk**
Okay, let's break down this `rmap_walk_anon` function. It seems to be using `folio_pgoff(folio)` ā which is just the folio's index, `folio->index` ā as its starting point. It's then searching the `anon_rmap_tree` associated with an `anon_vma`. The goal is to identify VMAs that have overlapping page ranges defined by `pgoff_start` and `pgoff_end`.
The interesting part is how this `anon_rmap_tree` is indexed. It's keyed by `vma->vm_pgoff`. Now, the crucial connection: `anon_rmap_tree_insert` inserts VMAs into this tree using both the starting offset, `vma->vm_pgoff`, and the ending offset, calculated as `vma->vm_pgoff + vma_pages(vma) - 1`.
So, the eureka moment! If the `folio->index` ā which is our search starting point ā is, say, `0x20a96`, and we happen to find a VMA where `vma->vm_pgoff` is also `0x20a96`, it's going to find that VMA because we are searching a range.
Once it finds the right VMA, it calculates an address using `vma_anon_address(vma, pgoff_start, folio_nr_pages(folio))`. That makes sense; it's using the VMA's base address and the page offsets to determine the actual virtual address of the page within the VMA.
|
| 71/2 |
2026/09/07 00:54 |
tool |
grepper |
0m
Args: map[Expression:vma_anon_address]
Results: map[Output:mm/internal.h=1048=static inline unsigned long vma_filebacked_address(const struct vm_area_struct *vma,
--
mm/internal.h-1056-/**
mm/internal.h:1057: * vma_anon_address - Find the virtual address an anonymous page range is mapped
mm/internal.h-1058- * at.
--
mm/internal.h-1067- */
mm/internal.h:1068:static inline unsigned long vma_anon_address(const struct vm_area_struct *vma,
mm/internal.h-1069- pgoff_t pgoff_anon, unsigned long nr_pages)
--
mm/page_vma_mapped.c=355=unsigned long page_mapped_in_vma(const struct page *page,
--
mm/page_vma_mapped.c-367- if (folio_test_anon(folio))
mm/page_vma_mapped.c:368: pvmw.address = vma_anon_address(vma, pgoff, 1);
mm/page_vma_mapped.c-369- else
--
mm/rmap.c=856=unsigned long page_address_in_vma(const struct folio *folio,
--
mm/rmap.c-868- /* KSM folios don't reach here because of the !anon_vma check */
mm/rmap.c:869: return vma_anon_address(vma, page_pgoff(folio, page), 1);
mm/rmap.c-870- } else if (!vma->vm_file) {
--
mm/rmap.c=3017=static void rmap_walk_anon(struct folio *folio,
--
mm/rmap.c-3043- struct vm_area_struct *vma = avc->vma;
mm/rmap.c:3044: const unsigned long address = vma_anon_address(vma, pgoff_start,
mm/rmap.c-3045- folio_nr_pages(folio));
]
|
| 72/2 |
2026/09/07 00:54 |
llm |
expert |
0m
Model:
Tokens:
input: 69343
output: 34
thoughts: 12
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm an expert in this field, and speaking from the first person perspective:
**Diving into `vma_anon_address`**
Right, let's take a look at `vma_anon_address`. My immediate thought is, what context are we in? Are we debugging, exploring a new kernel feature, optimizing something, or perhaps responding to a crash? The "anon" part immediately flags an anonymous virtual memory area. So, likely something related to heap allocation, stack, shared memory without a file backing, or some other memory management interaction. Knowing the specific context will drastically change how I approach understanding the code surrounding `vma_anon_address`. Iām likely interested in its purpose, when it gets called, and how it fits into the larger picture of virtual memory management, possibly focusing on how it relates to page fault handling or memory allocation routines. Let's dig in and see what we can find.
|
| 73/2 |
2026/09/07 00:54 |
tool |
read-file |
0m
Args: map[File:mm/internal.h FirstLine:1068 LineCount:50]
Results: map[Contents:1068: static inline unsigned long vma_anon_address(const struct vm_area_struct *vma,
1069: pgoff_t pgoff_anon, unsigned long nr_pages)
1070: {
1071: VM_WARN_ON_ONCE(!vma_is_cow_mapping(vma));
1072:
1073: return __vma_address(vma, pgoff_anon, vma_start_anon_pgoff(vma), nr_pages);
1074: }
1075:
1076: /*
1077: * At what user virtual address will none of the range be found in vma?
1078: * Assumes that vma_address() already returned a good starting address.
1079: */
1080: static inline unsigned long vma_address_end(struct page_vma_mapped_walk *pvmw)
1081: {
1082: const pgoff_t pgoff_end = pvmw->pgoff + pvmw->nr_pages;
1083: const struct vm_area_struct *vma = pvmw->vma;
1084: pgoff_t pgoff_vma_start;
1085: unsigned long address;
1086:
1087: /* Common case, plus ->pgoff is invalid for KSM */
1088: if (pvmw->nr_pages == 1)
1089: return pvmw->address + PAGE_SIZE;
1090:
1091: if (pvmw->pgoff_is_anon)
1092: pgoff_vma_start = vma_start_anon_pgoff(vma);
1093: else
1094: pgoff_vma_start = vma_start_pgoff(vma);
1095:
1096: address = vma->vm_start +
1097: ((pgoff_end - pgoff_vma_start) << PAGE_SHIFT);
1098: /* Check for address beyond vma (or wrapped through 0?) */
1099: if (address < vma->vm_start || address > vma->vm_end)
1100: address = vma->vm_end;
1101: return address;
1102: }
1103:
1104: static inline struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf,
1105: struct file *fpin)
1106: {
1107: int flags = vmf->flags;
1108:
1109: if (fpin)
1110: return fpin;
1111:
1112: /*
1113: * FAULT_FLAG_RETRY_NOWAIT means we don't want to wait on page locks or
1114: * anything, so we only pin the file and drop the mmap_lock if only
1115: * FAULT_FLAG_ALLOW_RETRY is set, while this is the first attempt.
1116: */
1117: if (fault_flag_allow_retry_first(flags) &&
]
|
| 74/2 |
2026/09/07 00:54 |
llm |
expert |
0m
Model:
Tokens:
input: 70485
output: 29
thoughts: 797
Thoughts: **Analyzing the Implications of `vma_pgoff` in Anonymous VMAs**
Okay, let's break this down. I'm looking at how `__vma_address` handles anonymous VMAs, and I realize it uses `vma_start_anon_pgoff(vma)`. For anonymous VMAs, that's just `vma->vm_pgoff`. So, `__vma_address` calculates the address as `vma->vm_start + ((pgoff_anon - vma->vm_pgoff) << PAGE_SHIFT)`. Since `pgoff_anon` and `vma->vm_pgoff` are both `0x20a96`, this correctly returns `vma->vm_start`. Seems good so far. Everything *does* appear to work as intended, even when `vma->vm_pgoff` doesn't equal `vma->vm_start >> PAGE_SHIFT` for an anonymous VMA with `anon_vma == NULL`.
However, the real sticking point is `VM_WARN_ON_ONCE` in `assert_sane_pgoff`. Is there *any* other code that relies on this assumption that `vma->vm_pgoff == vma->vm_start >> PAGE_SHIFT`? Let's revisit `copy_vma`. If we `mremap` this VMA (with `anon_vma == NULL` and `vm_pgoff = 0x20a96`), `copy_vma` will *update* its `pgoff` to `addr >> PAGE_SHIFT`.
Wait a minute... If `copy_vma` updates the `pgoff` to `addr >> PAGE_SHIFT`, the new VMA will have `vm_pgoff = addr >> PAGE_SHIFT`. But the old VMA had `vm_pgoff = 0x20a96`. Does this matter? If the VMA hasn't had any pages faulted in (i.e., `anon_vma == NULL`), it has NO pages faulted in. So, the value of `vm_pgoff` is *irrelevant*! Any new page that's faulted in will just get a new `anon_vma`, and the `folio->index` will be set based on the NEW `vm_pgoff`. Since there are no existing pages, there are no existing folios with the old index. The update is perfectly fine.
What if the VMA *was* faulted in, but `MREMAP_DONTUNMAP` cleared its `anon_vma`? As we established, `MREMAP_DONTUNMAP` moves the page tables to the new VMA. So the old VMA has NO page tables and thus NO pages faulted in! It's effectively an unfaulted VMA! So, updating `vm_pgoff` to `addr >> PAGE_SHIFT` is completely safe here too. And it's fine for it to have `vm_pgoff != vm_start >> PAGE_SHIFT` during this process.
So the *only* issue is the overly strict `assert_sane_pgoff`, which assumes that if `anon_vma == NULL`, then `vm_pgoff` MUST equal `vm_start >> PAGE_SHIFT`. `MREMAP_DONTUNMAP` can create an unfaulted VMA with a different `vm_pgoff`.
This does sound like a known issue, actually. I think there might even be a patch for this. Let's check the git log for `assert_sane_pgoff`.
|
| 75/2 |
2026/09/07 00:54 |
tool |
git-log |
0m
Error:
Args: map[Count:10 SymbolName:assert_sane_pgoff]
Results:
|
| 76/2 |
2026/09/07 00:54 |
llm |
expert |
0m
Model:
Tokens:
input: 72134
output: 41
thoughts: 10
Thoughts: Okay, here's the summary, assuming I'm a subject-matter expert:
**Addressing a Parameter Gap**
Right, I've got the context, I understand the task at hand. Now, I see I'm missing a critical piece of information here: the `SourcePath`. This isn't just a detail, it's *essential*. Without knowing the `SourcePath`, I can't even begin to work on the problem. I need to make sure that this variable is defined, and known, before I can progress any further. It is the very foundation I require for the task.
|
| 77/2 |
2026/09/07 00:54 |
tool |
git-log |
0m
Args: map[Count:10 SourcePath:mm/vma.h SymbolName:assert_sane_pgoff]
Results: map[Output:613562c29fa6 mm/vma: introduce and use vma_set_pgoff()
]
|
| 78/2 |
2026/09/07 00:55 |
llm |
expert |
0m
Model:
Tokens:
input: 72355
output: 26
thoughts: 19
Thoughts: Okay, let's dive into this. I need to take a look at commit `613562c29fa6`. I'm mentally parsing the commit hash, which is, of course, critical for traceability. I'm going to quickly review the context, the diff, and try to understand the rationale behind it. I'll be looking for potential regressions, new features, or any significant changes in logic that may require a deeper analysis. Considering my expertise, I'm already anticipating where the potential hotspots might be, based on the project's structure and my past experience with similar code. Let's see what's actually in there. I'm also ready to trace the lineage and potentially identify related commits if necessary. This will be quick, thorough, and focused.
|
| 79/2 |
2026/09/07 00:55 |
tool |
git-show |
0m
Args:
Results: map[Output:commit 613562c29fa69840f9a6239f1c3e0c24270a795d
Author: Lorenzo Stoakes <ljs@kernel.org>
Date: Fri Jul 10 21:17:10 2026 +0100
mm/vma: introduce and use vma_set_pgoff()
In order to lay the foundation for work that permits us to track the
virtual page offset of MAP_PRIVATE file-backed mappings, we abstract the
assignment of vma->vm_pgoff to vma_set_pgoff().
We additionally add a lock check here using the newly introduced
vma_assert_can_modify(). This asserts the VMA write lock if the VMA is
attached.
We also assert that, if this is an anonymous VMA and unfaulted, that its
(virtual) page offset is equal to the page offset of the VMA's address.
We must be careful about MAP_PRIVATE-/dev/zero which violates fundamental
assumptions about anonymous memory, so we check for !vma->vm_file after
using vma_is_anonymous() which these mappings satisfy.
Additionally, we only perform the assert if CONFIG_MMU is defined, as nommu
does not set vma->vm_pgoff = addr >> PAGE_SHIFT. This isn't really relevant
to rmap as it has no anon rmap (nor needs it), but we must avoid it
asserting falsely.
All of this logic is kept in assert_sane_pgoff() to keep things clear.
In order to maintain correctness given this assert, we also update
__install_special_mapping() to invoke vma_set_range() after it's set
vma->vm_ops (which determine whether the VMA is anonymous or not).
We do not use vma_set_pgoff() in vm_area_init_from(), as at the point of
forking, we don't necessarily have correct locking state.
Updating vma_set_range() covers most cases, but in addition to this we also
update insert_vm_struct(), compat_set_vma_from_desc() and nommu callers.
We also update vma_add_pgoff() and vma_sub_pgoff() to use vma_set_pgoff().
While we're here, we drop a BUG_ON() and update insert_vm_struct()'s
comment to reflect the fact anonymous mappings can be added here.
Finally, we update the CONFIG_MMU, CONFIG_PER_VMA_LOCK defines in the VMA
userland tests so IS_ENABLED() will work correctly with them.
No functional change intended.
Link: https://lore.kernel.org/20260710-b4-pre-scalable-cow-v2-29-2a5aa403d977@kernel.org
Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Ackerley Tng <ackerleytng@google.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Kai Huang <kai.huang@intel.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: SJ Park <sj@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Liam R. Howlett (Oracle) <liam@infradead.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
diff --git a/mm/nommu.c b/mm/nommu.c
index dc64834e6834..82556301b356 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1062,7 +1062,7 @@ unsigned long do_mmap(struct file *file,
region->vm_pgoff = pgoff;
vm_flags_init(vma, vm_flags);
- vma->vm_pgoff = pgoff;
+ vma_set_pgoff(vma, pgoff);
if (file) {
region->vm_file = get_file(file);
diff --git a/mm/vma.c b/mm/vma.c
index 4f2c56c57040..c253c5498359 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -81,7 +81,7 @@ static void vma_set_range(struct vm_area_struct *vma, unsigned long start,
unsigned long end, pgoff_t pgoff)
{
__vma_set_range(vma, start, end);
- vma->vm_pgoff = pgoff;
+ vma_set_pgoff(vma, pgoff);
}
/* Was this VMA ever forked from a parent, i.e. maybe contains CoW mappings? */
@@ -3347,9 +3347,9 @@ int __vm_munmap(unsigned long start, size_t len, bool unlock)
return ret;
}
-/* Insert vm structure into process list sorted by address
- * and into the inode's i_mmap tree. If vm_file is non-NULL
- * then i_mmap_rwsem is taken here.
+/*
+ * Insert vm structure into process list sorted by address
+ * and into the inode's i_mmap tree if file-backed.
*/
int insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
{
@@ -3375,8 +3375,8 @@ int insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
* Similarly in do_mmap and in do_brk_flags.
*/
if (vma_is_anonymous(vma)) {
- BUG_ON(vma->anon_vma);
- vma->vm_pgoff = vma->vm_start >> PAGE_SHIFT;
+ WARN_ON_ONCE(vma->anon_vma);
+ vma_set_pgoff(vma, vma->vm_start >> PAGE_SHIFT);
}
if (vma_link(mm, vma)) {
@@ -3422,7 +3422,6 @@ struct vm_area_struct *__install_special_mapping(
if (unlikely(vma == NULL))
return ERR_PTR(-ENOMEM);
- vma_set_range(vma, addr, addr + len, 0);
vm_flags |= mm->def_flags | VM_DONTEXPAND;
if (pgtable_supports_soft_dirty())
vm_flags |= VM_SOFTDIRTY;
@@ -3431,6 +3430,7 @@ struct vm_area_struct *__install_special_mapping(
vma->vm_ops = ops;
vma->vm_private_data = priv;
+ vma_set_range(vma, addr, addr + len, 0);
ret = insert_vm_struct(mm, vma);
if (ret)
diff --git a/mm/vma.h b/mm/vma.h
index 40effaa3ebe4..58f48609ce22 100644
--- a/mm/vma.h
+++ b/mm/vma.h
@@ -247,16 +247,45 @@ static inline pgoff_t vmg_end_pgoff(const struct vma_merge_struct *vmg)
return vmg_start_pgoff(vmg) + vmg_pages(vmg);
}
+static inline void assert_sane_pgoff(struct vm_area_struct *vma, pgoff_t pgoff)
+{
+ /* nommu doesn't set a virtual pgoff for anon VMAs. */
+ if (!IS_ENABLED(CONFIG_MMU))
+ return;
+ /*
+ * File-backed VMAs have arbitrary page offset (either page offset into
+ * file or for pfnmap the PFN of the start of the range or drivers may
+ * set arbitrary page offset).
+ */
+ if (!vma_is_anonymous(vma))
+ return;
+ /* MAP_PRIVATE-/dev/zero is anon, non-NULL vm_file, but has file pgoff. */
+ if (vma->vm_file)
+ return;
+ /* If faulted in, could have been remapped. */
+ if (vma->anon_vma)
+ return;
+ /* OK this is really an anon VMA - expect virtual page offset. */
+ VM_WARN_ON_ONCE(pgoff != vma->vm_start >> PAGE_SHIFT);
+}
+
+static inline void vma_set_pgoff(struct vm_area_struct *vma, pgoff_t pgoff)
+{
+ vma_assert_can_modify(vma);
+ assert_sane_pgoff(vma, pgoff);
+ vma->vm_pgoff = pgoff;
+}
+
static inline void vma_add_pgoff(struct vm_area_struct *vma, pgoff_t delta)
{
vma_assert_can_modify(vma);
- vma->vm_pgoff += delta;
+ vma_set_pgoff(vma, vma_start_pgoff(vma) + delta);
}
static inline void vma_sub_pgoff(struct vm_area_struct *vma, pgoff_t delta)
{
vma_assert_can_modify(vma);
- vma->vm_pgoff -= delta;
+ vma_set_pgoff(vma, vma_start_pgoff(vma) - delta);
}
#define VMG_STATE(name, mm_, vmi_, start_, end_, vma_flags_, pgoff_) \
@@ -331,7 +360,7 @@ static inline void compat_set_vma_from_desc(struct vm_area_struct *vma,
*/
/* Mutable fields. Populated with initial state. */
- vma->vm_pgoff = desc->pgoff;
+ vma_set_pgoff(vma, desc->pgoff);
if (desc->vm_file != vma->vm_file)
vma_set_file(vma, desc->vm_file);
vma->flags = desc->vma_flags;
diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
index e12ab2c80f95..4f6c5666ac07 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -14,8 +14,8 @@
#include <stdlib.h>
-#define CONFIG_MMU
-#define CONFIG_PER_VMA_LOCK
+#define CONFIG_MMU 1
+#define CONFIG_PER_VMA_LOCK 1
#ifdef __CONCAT
#undef __CONCAT
]
|
| 80/2 |
2026/09/07 00:55 |
llm |
expert |
0m
Model:
Tokens:
input: 75600
output: 66
thoughts: 1191
Thoughts: **Analysis of the `assert_sane_pgoff` False Positive**
Okay, so this commit introduced `assert_sane_pgoff`, which aims to ensure the integrity of VMA page offsets. The code explicitly checks if an unfaulted anonymous VMA has a virtual page offset matching its actual address. My immediate thought is that this check misses a crucial scenario involving `MREMAP_DONTUNMAP`. It appears that `MREMAP_DONTUNMAP` can clear the `anon_vma` of a previously faulted VMA, effectively making it unfaulted, but with a page offset (`pgoff`) that doesn't align with its current virtual address. This leads to a false-positive warning.
Fortunately, this isn't a critical bug. It's a `VM_WARN_ON_ONCE`, meaning it's only active in `CONFIG_DEBUG_VM` builds. In production, it's disabled, so no kernel crash. Even if it *were* enabled, it's just a warning. There's no memory corruption, privilege escalation, or information leak. The kernel's `vm_pgoff` handling and the `anon_vma` logic are still correct; the warning is simply being too strict in this specific case. So, it's not exploitable. It's also not a denial-of-service in production, because it's not even compiled in.
Now, let's look for other consequences. If `vma->vm_pgoff` doesn't equal `vma->vm_start >> PAGE_SHIFT`, and `vma->anon_vma` is null, we can fault in a page and allocate a new `anon_vma` in `do_anonymous_page`. Then, `folio_add_new_anon_rmap` sets `folio->index` based on `vma->vm_pgoff`. If we fork, `anon_vma_fork` clones the `anon_vma`. If we unmap, `unmap_region` correctly unmaps the pages. If we swap out, `rmap_walk_anon` finds the VMA using `folio->index` and `vma->vm_pgoff`. Everything works as it should. The only minor side effect is that `vma_merge` might not merge this VMA with adjacent VMAs, because the page offsets may not be contiguous. But that's just a missed optimization, not a bug. Absolutely no security impact.
In summary, the bug is a false-positive warning in `assert_sane_pgoff`. The warning fires when an unfaulted anonymous VMA has a mismatched `vm_pgoff`. This can occur when `mremap` is used with `MREMAP_DONTUNMAP` on a previously faulted anonymous VMA that has been remapped. The steps are:
1. Create and fault in an anonymous VMA, getting an `anon_vma`.
2. Remap it to a new address using `MREMAP_MAYMOVE`, preserving its `vm_pgoff` and cloning its `anon_vma`.
3. Move it again using `MREMAP_DONTUNMAP`. This unlinks the `anon_vma` as part of its move, leaving `vma->anon_vma = NULL`.
4. The old VMA now has `anon_vma == NULL`, but `vm_pgoff` still has the original value, not matching `vm_start >> PAGE_SHIFT`.
5. Subsequent operations like splitting (e.g. `mbind`) call `vma_add_pgoff`, which triggers the warning.
Since it's a `VM_WARN_ON_ONCE`, it's only active in debug builds. In production, it's not compiled in, and in any case there's no memory corruption or logical errors. The mismatched page offset is properly handled, and the reverse mapping finds the VMA correctly. No security impact whatsoever.
Now, let's fill in the results:
* **Exploitable: false**
* **DenialOfService: false** (warning is compiled out in production, and it's just a warning even if enabled.)
* **Unprivileged: true** (can be triggered by an unprivileged user)
* **UserNamespace: false**
* **VMGuestTrigger: false**
* **VMHostTrigger: false**
* **NetworkTrigger: false**
* **RemoteTrigger: false**
* **PeripheralTrigger: false**
* **FilesystemTrigger: false**
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tool |
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0m
Args: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
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expert |
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Model:
Tokens:
input: 77926
output: 684
thoughts: 13
Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert user, rendered in the first person:
**Finalizing the Analysis**
Alright, the data's been crunched, the models have run, and the results are definitive. No more parameter tweaks, no more sensitivity analyses. This is it. Time to synthesize everything and craft the definitive response. It's that moment where the raw output becomes a coherent narrative, where I take the complex results and translate them into actionable insights. This isn't just about regurgitating numbers; it's about conveying the "so what?" ā what do these findings *mean* in the grand scheme of things? What are the implications? The limitations? Now, it's time to communicate.
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Results: map[Explanation:The bug report describes a `VM_WARN_ON_ONCE` triggering in `assert_sane_pgoff`
during a VMA split operation (in this case, initiated by `mbind`).
### Cause of the Warning
The `assert_sane_pgoff` function asserts that if an anonymous VMA is unfaulted
(i.e., `vma->anon_vma == NULL`), its page offset (`vma->vm_pgoff`) must be equal
to its virtual address page offset (`vma->vm_start >> PAGE_SHIFT`).
However, this assumption can be legitimately violated using `mremap` with the
`MREMAP_DONTUNMAP` flag:
1. An anonymous VMA is created and a page is faulted in, assigning it an
`anon_vma`.
2. The VMA is moved to a new address using `mremap(MREMAP_MAYMOVE)`. Its
`vm_pgoff` is preserved (so it no longer equals `vm_start >> PAGE_SHIFT`), and
its `anon_vma` is cloned.
3. The VMA is moved again using `mremap(MREMAP_DONTUNMAP)`. This operation moves
the page tables to a new VMA, leaving the old VMA intact but empty. As part of
this process, `dontunmap_complete()` calls `unlink_anon_vmas()`, which clears
`vma->anon_vma` of the old VMA.
4. The old VMA now has `anon_vma == NULL`, but its `vm_pgoff` retains the
preserved value, which does not equal `vm_start >> PAGE_SHIFT`.
5. A subsequent operation that splits or modifies the old VMA (like `mbind` in
the reproducer) calls `vma_add_pgoff()`, which triggers the `VM_WARN_ON_ONCE` in
`assert_sane_pgoff()`.
### Security Impact
This is a **benign false positive warning** with **no security impact**:
- **No Memory Corruption:** The mismatched `vm_pgoff` does not cause any memory
corruption or logical errors. If new pages are faulted into this empty VMA, they
will be assigned an index based on the existing `vm_pgoff`, and the reverse
mapping (`rmap`) will correctly find the VMA using this index. The kernel
handles this state perfectly fine.
- **Compiled Out in Production:** The warning is a `VM_WARN_ON_ONCE`, which is
only defined when `CONFIG_DEBUG_VM` is enabled. In production kernels, this
configuration is disabled, meaning the macro evaluates to a no-op
(`BUILD_BUG_ON_INVALID`) and generates no runtime code. It will not cause a
panic, oops, or even a log message in production environments.
- **No Denial of Service:** Since the warning is compiled out in production (and
is only a non-fatal warning even when compiled in), it cannot be used to crash
or hang the system.
Therefore, this bug is neither exploitable nor capable of causing a
denial-of-service in production builds.]
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