==================================================================
BUG: KCSAN: data-race in __fat_write_inode / _copy_to_iter

write to 0xffff888129696c1c of 4 bytes by task 15442 on cpu 1:
 __fat_write_inode+0x235/0x540 fs/fat/inode.c:-1
 fat_write_inode+0xbb/0xd0 fs/fat/inode.c:928
 write_inode fs/fs-writeback.c:1607 [inline]
 __writeback_single_inode+0x327/0x660 fs/fs-writeback.c:1850
 writeback_single_inode+0x16d/0x400 fs/fs-writeback.c:1906
 sync_inode_metadata+0x6e/0xa0 fs/fs-writeback.c:3080
 writeback_inode fs/fat/inode.c:1886 [inline]
 fat_flush_inodes+0x77/0x150 fs/fat/inode.c:1906
 fat_truncate_blocks+0x536/0x5b0 fs/fat/file.c:399
 fat_write_failed fs/fat/inode.c:219 [inline]
 fat_write_begin+0xc0/0xe0 fs/fat/inode.c:234
 cont_expand_zero fs/buffer.c:2524 [inline]
 cont_write_begin+0x18d/0x920 fs/buffer.c:2586
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2487
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:227
 fat_setattr+0x2a4/0x850 fs/fat/file.c:551
 notify_change+0x842/0x8c0 fs/attr.c:556
 do_truncate fs/open.c:68 [inline]
 do_ftruncate+0x344/0x410 fs/open.c:194
 ksys_ftruncate fs/open.c:206 [inline]
 __do_sys_ftruncate fs/open.c:211 [inline]
 __se_sys_ftruncate fs/open.c:209 [inline]
 __x64_sys_ftruncate+0x65/0xc0 fs/open.c:209
 x64_sys_call+0x2beb/0x3020 arch/x86/include/generated/asm/syscalls_64.h:78
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888129696c00 of 512 bytes by task 14693 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:330 [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:204 [inline]
 filemap_read+0x3ed/0xa20 mm/filemap.c:2865
 blkdev_read_iter+0x21e/0x2d0 block/fops.c:830
 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+0x2886/0x3020 arch/x86/include/generated/asm/syscalls_64.h:1
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 14693 Comm: udevd Tainted: G        W           syzkaller #0 PREEMPT(lazy) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
==================================================================