Message-ID: <87h5rue2vd.fsf@mail.parknet.co.jp>
Date: Fri, 06 Feb 2026 16:40:38 +0900
From: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To: syzbot <syzbot+51b4c65bb770155d058f@...kaller.appspotmail.com>
Cc: linkinjeon@...nel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, sj1557.seo@...sung.com,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [exfat?] KCSAN: data-race in fat12_ent_put /
 fat_mirror_bhs

syzbot <syzbot+51b4c65bb770155d058f@...kaller.appspotmail.com> 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@...kaller.appspotmail.com

This looks like same with the following thread.

  https://lore.kernel.org/all/20250902081727.7146-1-anmuxixixi@gmail.com/

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.

> ==================================================================
> 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
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@...glegroups.com.
>
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
> If the report is already addressed, let syzbot know by replying with:
> #syz fix: exact-commit-title
>
> If you want to overwrite report's subsystems, reply with:
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>
> If the report is a duplicate of another one, reply with:
> #syz dup: exact-subject-of-another-report
>
> If you want to undo deduplication, reply with:
> #syz undup

-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>

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