Message-id: <176427972855.634289.8097806579329413784@noble.neil.brown.name>
Date: Fri, 28 Nov 2025 08:42:08 +1100
From: NeilBrown <neilb@...mail.net>
To: "syzbot" <syzbot+bfc9a0ccf0de47d04e8c@...kaller.appspotmail.com>
Cc: amir73il@...il.com, brauner@...nel.org, linux-kernel@...r.kernel.org,
 linux-unionfs@...r.kernel.org, miklos@...redi.hu,
 syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [overlayfs?] WARNING in shmem_unlink (2)

On Fri, 28 Nov 2025, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    30f09200cc4a Merge tag 'arm64-fixes' of git://git.kernel.o..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1047ee92580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=38a0c4cddc846161
> dashboard link: https://syzkaller.appspot.com/bug?extid=bfc9a0ccf0de47d04e8c
> 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=1626ae12580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1126ae12580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/a5630d1ab1eb/disk-30f09200.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/584408ed3fcf/vmlinux-30f09200.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/69749e493b1e/bzImage-30f09200.xz
> 
> The issue was bisected to:
> 
> commit d2c995581c7c5d0ff623b2700e76bf22499c66df
> Author: NeilBrown <neil@...wn.name>
> Date:   Wed Jul 16 00:44:14 2025 +0000
> 
>     ovl: Call ovl_create_temp() without lock held.
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=13db1e92580000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=103b1e92580000
> console output: https://syzkaller.appspot.com/x/log.txt?x=17db1e92580000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+bfc9a0ccf0de47d04e8c@...kaller.appspotmail.com
> Fixes: d2c995581c7c ("ovl: Call ovl_create_temp() without lock held.")

I think this was probably fixed by 
Commit 1f480a181137 ("Add start_renaming_two_dentries()")

That patch replaced the call to ovl_lock_rename_workdir()
with start_renaming_two_dentries()
The new function checks that the two dentries are still hashed.

ovl_lock_rename_workdir() should have been changed to check
that the dentries were still hashed before that patch that
the bisect found which changed the locking in ovl_cleanup_and_whiteout.

Can you please confirm the bug no longer exists after that patch?

Thanks,
NeilBrown


> 
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 6236 at fs/inode.c:417 drop_nlink+0xc5/0x110 fs/inode.c:417
> Modules linked in:
> CPU: 0 UID: 0 PID: 6236 Comm: syz.0.107 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 87 6b ec ff f0 48 ff 83 c0 08 00 00 5b 41 5c 41 5e 41 5f 5d e9 52 5c 90 08 cc e8 2c aa 8a 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:ffffc90003b0f288 EFLAGS: 00010293
> RAX: ffffffff82340314 RBX: ffff888023b292e0 RCX: ffff8880397ada00
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
> R10: dffffc0000000000 R11: fffff52000761e49 R12: 1ffff11004765265
> R13: 000000006926eb25 R14: ffff888023b29328 R15: dffffc0000000000
> FS:  00007f944571d6c0(0000) GS:ffff888126df6000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f944571cf98 CR3: 0000000028194000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  shmem_unlink+0x1f5/0x2d0 mm/shmem.c:3979
>  shmem_rename2+0x22d/0x360 mm/shmem.c:4065
>  vfs_rename+0xb34/0xe80 fs/namei.c:5216
>  ovl_do_rename+0x13c/0x210 fs/overlayfs/overlayfs.h:373
>  ovl_create_over_whiteout fs/overlayfs/dir.c:550 [inline]
>  ovl_create_or_link+0xaf7/0x1410 fs/overlayfs/dir.c:656
>  ovl_create_object+0x234/0x310 fs/overlayfs/dir.c:695
>  lookup_open fs/namei.c:3796 [inline]
>  open_last_lookups fs/namei.c:3895 [inline]
>  path_openat+0x1500/0x3840 fs/namei.c:4131
>  do_filp_open+0x1fa/0x410 fs/namei.c:4161
>  do_sys_openat2+0x121/0x1c0 fs/open.c:1437
>  do_sys_open fs/open.c:1452 [inline]
>  __do_sys_openat fs/open.c:1468 [inline]
>  __se_sys_openat fs/open.c:1463 [inline]
>  __x64_sys_openat+0x138/0x170 fs/open.c:1463
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f94460cf749
> Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 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:00007f944571d038 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
> RAX: ffffffffffffffda RBX: 00007f9446326090 RCX: 00007f94460cf749
> RDX: 0000000000105042 RSI: 0000200000000080 RDI: ffffffffffffff9c
> RBP: 00007f9446153f91 R08: 0000000000000000 R09: 0000000000000000
> R10: 00000000000001ff R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f9446326128 R14: 00007f9446326090 R15: 00007fff80085c88
>  </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@...glegroups.com.
> 
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> 
> 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
> 

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