Re: [syzbot] [exfat?] KCSAN: data-race in fat12_ent_put / fat_mirror_bhs

[Dmitry Vyukov]

On Fri, 6 Feb 2026 at 08:40, OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx> wrote:
>
> syzbot <syzbot+51b4c65bb770155d058f@xxxxxxxxxxxxxxxxxxxxxxxxx> writes:
>
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    de0674d9bc69 Merge tag 'for-6.19-rc8-tag' of git://git.ker..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=15f240aa580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=8e27f4588a0f2183
> > dashboard link: https://syzkaller.appspot.com/bug?extid=51b4c65bb770155d058f
> > compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/bfedab2f6279/disk-de0674d9.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/f012a4cb8d82/vmlinux-de0674d9.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/acb727c49110/bzImage-de0674d9.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+51b4c65bb770155d058f@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> This looks like same with the following thread.
>
>   https://lore.kernel.org/all/20250902081727.7146-1-anmuxixixi@xxxxxxxxx/
>
> And this temporary inconsistency will be fixed until unmount, so it
> should be no corruption by this race. Let me know if corruption remained
> after unmount.

Thanks for the link. I don't have means to test that, but the
explanation in the thread you referenced looks reasonable. So let's
just keep an AI to properly annotate the race according to LKMM rules.


> > ==================================================================
> > BUG: KCSAN: data-race in fat12_ent_put / fat_mirror_bhs
> >
> > read-write to 0xffff888129151032 of 1 bytes by task 4937 on cpu 1:
> >  fat12_ent_put+0xc4/0x170 fs/fat/fatent.c:165
> >  fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:417
> >  fat_chain_add+0x16c/0x490 fs/fat/misc.c:136
> >  fat_add_cluster fs/fat/inode.c:113 [inline]
> >  __fat_get_block fs/fat/inode.c:155 [inline]
> >  fat_get_block+0x46c/0x5e0 fs/fat/inode.c:190
> >  __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
> >  block_write_begin fs/buffer.c:2256 [inline]
> >  cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
> >  fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
> >  generic_perform_write+0x183/0x490 mm/filemap.c:4314
> >  __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
> >  generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
> >  new_sync_write fs/read_write.c:593 [inline]
> >  vfs_write+0x5a6/0x9f0 fs/read_write.c:686
> >  ksys_write+0xdc/0x1a0 fs/read_write.c:738
> >  __do_sys_write fs/read_write.c:749 [inline]
> >  __se_sys_write fs/read_write.c:746 [inline]
> >  __x64_sys_write+0x40/0x50 fs/read_write.c:746
> >  x64_sys_call+0x2847/0x3000 arch/x86/include/generated/asm/syscalls_64.h:2
> >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >  do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> > read to 0xffff888129151000 of 512 bytes by task 4940 on cpu 0:
> >  fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
> >  fat_alloc_clusters+0xb48/0xc50 fs/fat/fatent.c:543
> >  fat_add_cluster fs/fat/inode.c:108 [inline]
> >  __fat_get_block fs/fat/inode.c:155 [inline]
> >  fat_get_block+0x258/0x5e0 fs/fat/inode.c:190
> >  __block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
> >  block_write_begin fs/buffer.c:2256 [inline]
> >  cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
> >  fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
> >  generic_perform_write+0x183/0x490 mm/filemap.c:4314
> >  __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
> >  generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
> >  __kernel_write_iter+0x319/0x590 fs/read_write.c:619
> >  dump_emit_page fs/coredump.c:1298 [inline]
> >  dump_user_range+0xa7d/0xdb0 fs/coredump.c:1372
> >  elf_core_dump+0x21a2/0x2330 fs/binfmt_elf.c:2111
> >  coredump_write+0xacf/0xdf0 fs/coredump.c:1049
> >  do_coredump fs/coredump.c:1126 [inline]
> >  vfs_coredump+0x26bc/0x3120 fs/coredump.c:1200
> >  get_signal+0xd7b/0xf60 kernel/signal.c:3019
> >  arch_do_signal_or_restart+0x96/0x450 arch/x86/kernel/signal.c:337
> >  __exit_to_user_mode_loop kernel/entry/common.c:41 [inline]
> >  exit_to_user_mode_loop kernel/entry/common.c:75 [inline]
> >  __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
> >  irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:270 [inline]
> >  irqentry_exit_to_user_mode include/linux/irq-entry-common.h:339 [inline]
> >  irqentry_exit+0xf7/0x510 kernel/entry/common.c:196
> >  asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 0 UID: 0 PID: 4940 Comm: syz.3.398 Not tainted syzkaller #0 PREEMPT(voluntary)
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
> > ==================================================================
> > syz.3.398 (4940) used greatest stack depth: 8696 bytes left
> >
> >
> > ---
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> >
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