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BUG: KCSAN: data-race in _copy_to_iter / fat12_ent_put

read-write to 0xffff888125eb9f69 of 1 bytes by task 8989 on cpu 1:
 fat12_ent_put+0xc4/0x180 fs/fat/fatent.c:165
 __fat_alloc_clusters+0x55d/0xd80 fs/fat/fatent.c:542
 fat_alloc_chain+0x25/0x30 fs/fat/fatent.c:610
 fat_add_clusters fs/fat/inode.c:109 [inline]
 __fat_get_block fs/fat/inode.c:156 [inline]
 fat_get_block+0x252/0x5e0 fs/fat/inode.c:191
 __block_write_begin_int+0x421/0x1060 fs/buffer.c:2031
 block_write_begin fs/buffer.c:2145 [inline]
 cont_write_begin+0x5bf/0x920 fs/buffer.c:2507
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:231
 cont_expand_zero fs/buffer.c:2435 [inline]
 cont_write_begin+0x18d/0x920 fs/buffer.c:2497
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:231
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2398
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:226
 fat_fallocate+0x168/0x1a0 fs/fat/file.c:302
 vfs_fallocate+0x3ac/0x400 fs/open.c:338
 ksys_fallocate fs/open.c:362 [inline]
 __do_sys_fallocate fs/open.c:367 [inline]
 __se_sys_fallocate fs/open.c:365 [inline]
 __x64_sys_fallocate+0x7a/0xd0 fs/open.c:365
 x64_sys_call+0x1e1d/0x2550 arch/x86/include/generated/asm/syscalls_64.h:286
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888125eb9e00 of 512 bytes by task 6908 on cpu 0:
 instrument_copy_to_user include/linux/instrumented.h:130 [inline]
 copy_to_user_iter lib/iov_iter.c:24 [inline]
 iterate_ubuf include/linux/iov_iter.h:30 [inline]
 iterate_and_advance2 include/linux/iov_iter.h:302 [inline]
 iterate_and_advance include/linux/iov_iter.h:332 [inline]
 _copy_to_iter+0x130/0xea0 lib/iov_iter.c:197
 copy_page_to_iter+0x174/0x210 lib/iov_iter.c:374
 copy_folio_to_iter include/linux/uio.h:212 [inline]
 filemap_read+0x3f8/0xad0 mm/filemap.c:2893
 blkdev_read_iter+0x29c/0x390 block/fops.c:896
 new_sync_read fs/read_write.c:493 [inline]
 vfs_read+0x699/0x7c0 fs/read_write.c:574
 ksys_read+0xdc/0x1a0 fs/read_write.c:716
 __do_sys_read fs/read_write.c:725 [inline]
 __se_sys_read fs/read_write.c:723 [inline]
 __x64_sys_read+0x40/0x50 fs/read_write.c:723
 x64_sys_call+0x2135/0x2550 arch/x86/include/generated/asm/syscalls_64.h:1
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 6908 Comm: udevd Tainted: G        W           syzkaller #0 PREEMPT(lazy) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/07/2026
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