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Message-id: <175547723217.2234665.3959316236142184849@noble.neil.brown.name>
Date: Mon, 18 Aug 2025 10:33:52 +1000
From: "NeilBrown" <neil@...wn.name>
To: "Amir Goldstein" <amir73il@...il.com>
Cc: "syzbot" <syzbot+ec9fab8b7f0386b98a17@...kaller.appspotmail.com>,
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
On Mon, 18 Aug 2025, Amir Goldstein wrote:
> Neil,
>
> I will have a look tomorrow.
> If you have ideas I am open to hear them.
> The repro is mounting overlayfs all over each other in concurrent threads
> and one of the rmdir of "work" dir triggers this assertion
My guess is that by dropping and retaking the lock, we open the
possibility of a race so that by the time vfs_unlink() is called the
dentry has already been unlinked. In that case it would be unhashed.
So after retaking the lock we need to check d_unhashed() as well as
->d_parent.
So something like
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -1552,7 +1552,8 @@ void ovl_copyattr(struct inode *inode)
int ovl_parent_lock(struct dentry *parent, struct dentry *child)
{
inode_lock_nested(parent->d_inode, I_MUTEX_PARENT);
- if (!child || child->d_parent == parent)
+ if (!child ||
+ (!d_unhashed(child) && child->d_parent == parent))
return 0;
inode_unlock(parent->d_inode);
NeilBrown
>
> Thanks,
> Amir.
>
> On Sat, Aug 16, 2025 at 5:08 AM syzbot
> <syzbot+ec9fab8b7f0386b98a17@...kaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit: 0cc53520e68b Merge tag 'probes-fixes-v6.17-rc1' of git://g..
> > git tree: upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=14a003a2580000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=13f39c6a0380a209
> > dashboard link: https://syzkaller.appspot.com/bug?extid=ec9fab8b7f0386b98a17
> > compiler: Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1387bc34580000
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/1f4865acb167/disk-0cc53520.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/14540c5ef981/vmlinux-0cc53520.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/35534bfe1c7e/bzImage-0cc53520.xz
> >
> > Bisection is inconclusive: the first bad commit could be any of:
> >
> > 241062ae5d87 ovl: change ovl_workdir_cleanup() to take dir lock as needed.
> > a45ee87ded78 ovl: narrow locking in ovl_workdir_cleanup_recurse()
> > c69566b1d11d ovl: narrow locking on ovl_remove_and_whiteout()
> >
> > bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=130d1dbc580000
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+ec9fab8b7f0386b98a17@...kaller.appspotmail.com
> >
> > ------------[ cut here ]------------
> > WARNING: CPU: 1 PID: 9026 at fs/inode.c:417 drop_nlink+0xc5/0x110 fs/inode.c:417
> > Modules linked in:
> > CPU: 1 UID: 0 PID: 9026 Comm: syz.4.1430 Tainted: G W 6.17.0-rc1-syzkaller-00038-g0cc53520e68b #0 PREEMPT_{RT,(full)}
> > Tainted: [W]=WARN
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
> > RIP: 0010:drop_nlink+0xc5/0x110 fs/inode.c:417
> > Code: c8 08 00 00 be 08 00 00 00 e8 b7 90 ec ff f0 48 ff 83 c8 08 00 00 5b 41 5c 41 5e 41 5f 5d e9 82 9f c8 08 cc e8 dc 5a 8d 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:ffffc9000f5ef600 EFLAGS: 00010293
> > RAX: ffffffff82310064 RBX: ffff88803352c420 RCX: ffff88802cfcbb80
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> > RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
> > R10: dffffc0000000000 R11: fffff52001ebdeb5 R12: 1ffff110066a588d
> > R13: 00000000689e7afa R14: ffff88803352c468 R15: dffffc0000000000
> > FS: 00007fec6bd366c0(0000) GS:ffff8881269c5000(0000) knlGS:0000000000000000
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 0000555591d73608 CR3: 00000000274f4000 CR4: 00000000003526f0
> > Call Trace:
> > <TASK>
> > shmem_unlink+0x1f5/0x2d0 mm/shmem.c:4041
> > vfs_unlink+0x39a/0x660 fs/namei.c:4586
> > ovl_do_unlink fs/overlayfs/overlayfs.h:218 [inline]
> > ovl_cleanup_locked fs/overlayfs/dir.c:36 [inline]
> > ovl_cleanup+0x151/0x230 fs/overlayfs/dir.c:56
> > ovl_check_rename_whiteout fs/overlayfs/super.c:607 [inline]
> > ovl_make_workdir fs/overlayfs/super.c:704 [inline]
> > ovl_get_workdir+0xabd/0x17c0 fs/overlayfs/super.c:827
> > ovl_fill_super+0x1365/0x35b0 fs/overlayfs/super.c:1406
> > vfs_get_super fs/super.c:1325 [inline]
> > get_tree_nodev+0xbb/0x150 fs/super.c:1344
> > vfs_get_tree+0x8f/0x2b0 fs/super.c:1815
> > do_new_mount+0x2a2/0x9e0 fs/namespace.c:3805
> > do_mount fs/namespace.c:4133 [inline]
> > __do_sys_mount fs/namespace.c:4344 [inline]
> > __se_sys_mount+0x317/0x410 fs/namespace.c:4321
> > do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> > do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
> > entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > RIP: 0033:0x7fec6c6cebe9
> > 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:00007fec6bd36038 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
> > RAX: ffffffffffffffda RBX: 00007fec6c8f5fa0 RCX: 00007fec6c6cebe9
> > RDX: 0000200000000200 RSI: 0000200000000000 RDI: 0000000000000000
> > RBP: 00007fec6c751e19 R08: 0000200000000140 R09: 0000000000000000
> > R10: 00000000000000d4 R11: 0000000000000246 R12: 0000000000000000
> > R13: 00007fec6c8f6038 R14: 00007fec6c8f5fa0 R15: 00007ffc15eea8d8
> > </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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