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BUG: KCSAN: data-race in _copy_to_iter / fat_mirror_bhs

write to 0xffff88810ad12800 of 2048 bytes by task 20741 on cpu 1:
 fat_mirror_bhs+0x1df/0x330 fs/fat/fatent.c:399
 fat_ent_write+0xd0/0xe0 fs/fat/fatent.c:428
 fat_free fs/fat/file.c:367 [inline]
 fat_truncate_blocks+0x371/0x5b0 fs/fat/file.c:398
 fat_write_failed fs/fat/inode.c:219 [inline]
 fat_write_end+0xba/0x160 fs/fat/inode.c:247
 generic_perform_write+0x314/0x490 mm/filemap.c:4385
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4481
 generic_file_write_iter+0x77/0x1f0 mm/filemap.c:4507
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x57d/0x990 fs/read_write.c:687
 ksys_write+0xdc/0x1a0 fs/read_write.c:739
 __do_sys_write fs/read_write.c:750 [inline]
 __se_sys_write fs/read_write.c:747 [inline]
 __x64_sys_write+0x40/0x50 fs/read_write.c:747
 x64_sys_call+0x27e1/0x3020 arch/x86/include/generated/asm/syscalls_64.h:2
 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 0xffff88810ad12e00 of 512 bytes by task 3285 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:2871
 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: 3285 Comm: udevd Not tainted syzkaller #0 PREEMPT(lazy) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
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