Re: [syzbot] [exfat?] WARNING in msdos_rmdir

[Dmitry Vyukov]

On Mon, 12 Jan 2026 at 17:15, OGAWA Hirofumi
<hirofumi@xxxxxxxxxxxxxxxxxx> wrote:
>
> syzbot <syzbot+66d24939ab3817bd81d0@xxxxxxxxxxxxxxxxxxxxxxxxx> writes:
>
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    9c7ef209cd0f Merge tag 'char-misc-6.19-rc5' of git://git.k..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=13cfec3a580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=7b058fb1d7dbe6b1
> > dashboard link: https://syzkaller.appspot.com/bug?extid=66d24939ab3817bd81d0
> > compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10d50052580000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16efd9fc580000
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/4fb19edb55d0/disk-9c7ef209.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/9b4c89c27788/vmlinux-9c7ef209.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/a728d410d528/bzImage-9c7ef209.xz
> > mounted in repro: https://storage.googleapis.com/syzbot-assets/83e4f46f8575/mount_0.gz
> >   fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=17550052580000)
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+66d24939ab3817bd81d0@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> This should be fixed by following patch.
>
> https://lore.kernel.org/all/20260101111148.1437-1-zhiyuzhang999@xxxxxxxxx/

Let's tell syzbot:

#syz fix: fat: avoid parent link count underflow in rmdir

> > ------------[ cut here ]------------
> > WARNING: fs/inode.c:417 at drop_nlink+0xc5/0x110 fs/inode.c:417, CPU#1: syz-executor/5973
> > Modules linked in:
> > CPU: 1 UID: 0 PID: 5973 Comm: syz-executor Not tainted syzkaller #0 PREEMPT_{RT,(full)}
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
> > RIP: 0010:drop_nlink+0xc5/0x110 fs/inode.c:417
> > Code: c0 08 00 00 be 08 00 00 00 e8 67 f9 eb ff f0 48 ff 83 c0 08 00 00 5b 41 5c 41 5e 41 5f 5d e9 42 90 9f 08 cc e8 1c bd 89 ff 90 <0f> 0b 90 eb 81 44 89 f1 80 e1 07 80 c1 03 38 c1 0f 8c 5b ff ff ff
> > RSP: 0018:ffffc90004877bf0 EFLAGS: 00010293
> > RAX: ffffffff8235efa4 RBX: ffff8880589f9d30 RCX: ffff8880274f3c80
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> > RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
> > R10: dffffc0000000000 R11: fffffbfff1db66cf R12: 1ffff1100b13f3af
> > R13: ffff8880589f9d30 R14: ffff8880589f9d78 R15: dffffc0000000000
> > FS:  000055559129d500(0000) GS:ffff888126dee000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00005555912d86c8 CR3: 0000000038956000 CR4: 00000000003526f0
> > Call Trace:
> >  <TASK>
> >  msdos_rmdir+0x3ca/0x4e0 fs/fat/namei_msdos.c:328
> >  vfs_rmdir+0x51b/0x670 fs/namei.c:5245
> >  do_rmdir+0x27f/0x4a0 fs/namei.c:5300
> >  __do_sys_unlinkat fs/namei.c:5477 [inline]
> >  __se_sys_unlinkat fs/namei.c:5471 [inline]
> >  __x64_sys_unlinkat+0xc2/0xf0 fs/namei.c:5471
> >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >  do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > RIP: 0033:0x7f0f8e67ed27
> > Code: 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 b8 07 01 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
> > RSP: 002b:00007fff98601628 EFLAGS: 00000207 ORIG_RAX: 0000000000000107
> > RAX: ffffffffffffffda RBX: 0000000000000065 RCX: 00007f0f8e67ed27
> > RDX: 0000000000000200 RSI: 00007fff986027d0 RDI: 00000000ffffff9c
> > RBP: 00007f0f8e703d7d R08: 00005555912c86ab R09: 0000000000000000
> > R10: 0000000000001000 R11: 0000000000000207 R12: 00007fff986027d0
> > R13: 00007f0f8e703d7d R14: 000000000001a887 R15: 00007fff98605a80
> >  </TASK>
> >
> >
> > ---
> > 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@xxxxxxxxxxxxxxxx.
> >
> > 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 syzbot to run the reproducer, reply with:
> > #syz test: git://repo/address.git branch-or-commit-hash
> > If you attach or paste a git patch, syzbot will apply it before testing.
> >
> > If you want to overwrite report's subsystems, reply with:
> > #syz set subsystems: new-subsystem
> > (See the list of subsystem names on the web dashboard)
> >
> > 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