[syzbot] [gfs2?] WARNING in filename_mkdirat

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 0f2acd3148e0 Merge tag 'm68knommu-for-v7.0' of git://git.k..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15331c02580000
kernel config: https://syzkaller.appspot.com/x/.config?x=ac00553de86d6bf0
dashboard link: https://syzkaller.appspot.com/bug?extid=0ea5108a1f5fb4fcc2d8
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=146b295a580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-0f2acd31.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/b7d134e71e9c/vmlinux-0f2acd31.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b18643058ceb/bzImage-0f2acd31.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/bbfed09077d3/mount_1.gz
fsck result: OK (log: https://syzkaller.appspot.com/x/fsck.log?x=106b295a580000)

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+0ea510...@syzkaller.appspotmail.com

------------[ cut here ]------------
DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) && !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE)): count = 0x0, magic = 0xffff88804a18c9b8, owner = 0x0, curr 0xffff888000ec2480, list empty
WARNING: kernel/locking/rwsem.c:1381 at __up_write kernel/locking/rwsem.c:1380 [inline], CPU#0: syz.0.53/5774
WARNING: kernel/locking/rwsem.c:1381 at up_write+0x2d6/0x410 kernel/locking/rwsem.c:1643, CPU#0: syz.0.53/5774
Modules linked in:
CPU: 0 UID: 0 PID: 5774 Comm: syz.0.53 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:__up_write kernel/locking/rwsem.c:1380 [inline]
RIP: 0010:up_write+0x388/0x410 kernel/locking/rwsem.c:1643
Code: cc 8b 49 c7 c2 c0 eb cc 8b 4c 0f 44 d0 48 8b 7c 24 08 48 c7 c6 20 ee cc 8b 48 8b 14 24 4c 89 f1 4d 89 e0 4c 8b 4c 24 10 41 52 <67> 48 0f b9 3a 48 83 c4 08 e8 ea 60 0a 03 e9 67 fd ff ff 48 c7 c1
RSP: 0000:ffffc90006407d80 EFLAGS: 00010246
RAX: ffffffff8bcceba0 RBX: ffff88804a18c9b8 RCX: ffff88804a18c9b8
RDX: 0000000000000000 RSI: ffffffff8bccee20 RDI: ffffffff9014bf50
RBP: ffff88804a18ca10 R08: 0000000000000000 R09: ffff888000ec2480
R10: ffffffff8bcceba0 R11: ffffed1009431939 R12: 0000000000000000
R13: dffffc0000000000 R14: ffff88804a18c9b8 R15: 1ffff11009431938
FS: 00007f9e11bfe6c0(0000) GS:ffff88808ca62000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000c000d54e20 CR3: 0000000041f2c000 CR4: 0000000000352ef0
Call Trace:
<TASK>
inode_unlock include/linux/fs.h:1038 [inline]
end_dirop fs/namei.c:2947 [inline]
end_creating include/linux/namei.h:126 [inline]
end_creating_path fs/namei.c:4962 [inline]
filename_mkdirat+0x305/0x510 fs/namei.c:5271
__do_sys_mkdirat fs/namei.c:5287 [inline]
__se_sys_mkdirat+0x35/0x150 fs/namei.c:5284
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f9e10d9bf79
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f9e11bfe028 EFLAGS: 00000246 ORIG_RAX: 0000000000000102
RAX: ffffffffffffffda RBX: 00007f9e11016090 RCX: 00007f9e10d9bf79
RDX: 00000000000001c0 RSI: 0000200000000140 RDI: ffffffffffffff9c
RBP: 00007f9e10e327e0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f9e11016128 R14: 00007f9e11016090 R15: 00007ffffd54f8b8
</TASK>
----------------
Code disassembly (best guess), 2 bytes skipped:
0: 49 c7 c2 c0 eb cc 8b mov $0xffffffff8bccebc0,%r10
7: 4c 0f 44 d0 cmove %rax,%r10
b: 48 8b 7c 24 08 mov 0x8(%rsp),%rdi
10: 48 c7 c6 20 ee cc 8b mov $0xffffffff8bccee20,%rsi
17: 48 8b 14 24 mov (%rsp),%rdx
1b: 4c 89 f1 mov %r14,%rcx
1e: 4d 89 e0 mov %r12,%r8
21: 4c 8b 4c 24 10 mov 0x10(%rsp),%r9
26: 41 52 push %r10
* 28: 67 48 0f b9 3a ud1 (%edx),%rdi <-- trapping instruction
2d: 48 83 c4 08 add $0x8,%rsp
31: e8 ea 60 0a 03 call 0x30a6120
36: e9 67 fd ff ff jmp 0xfffffda2
3b: 48 rex.W
3c: c7 .byte 0xc7
3d: c1 .byte 0xc1


---
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[Christian Brauner]

On Mon, Feb 16, 2026 at 04:30:27PM -0800, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 0f2acd3148e0 Merge tag 'm68knommu-for-v7.0' of git://git.k..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=15331c02580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=ac00553de86d6bf0
> dashboard link: https://syzkaller.appspot.com/bug?extid=0ea5108a1f5fb4fcc2d8
> compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=146b295a580000
>
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-0f2acd31.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/b7d134e71e9c/vmlinux-0f2acd31.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/b18643058ceb/bzImage-0f2acd31.xz
> mounted in repro: https://storage.googleapis.com/syzbot-assets/bbfed09077d3/mount_1.gz
> fsck result: OK (log: https://syzkaller.appspot.com/x/fsck.log?x=106b295a580000)
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+0ea510...@syzkaller.appspotmail.com

Neil, is this something you have time to look into?

[NeilBrown]

On Tue, 17 Feb 2026, Christian Brauner wrote:
> On Mon, Feb 16, 2026 at 04:30:27PM -0800, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit: 0f2acd3148e0 Merge tag 'm68knommu-for-v7.0' of git://git.k..
> > git tree: upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=15331c02580000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=ac00553de86d6bf0
> > dashboard link: https://syzkaller.appspot.com/bug?extid=0ea5108a1f5fb4fcc2d8
> > compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=146b295a580000
> >
> > Downloadable assets:
> > disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-0f2acd31.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/b7d134e71e9c/vmlinux-0f2acd31.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/b18643058ceb/bzImage-0f2acd31.xz
> > mounted in repro: https://storage.googleapis.com/syzbot-assets/bbfed09077d3/mount_1.gz
> > fsck result: OK (log: https://syzkaller.appspot.com/x/fsck.log?x=106b295a580000)
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+0ea510...@syzkaller.appspotmail.com
>
> Neil, is this something you have time to look into?

The reproducer appears to mount a gfs2 filesystem and mkdir 3
directories:
./file1
./file1/file4
./file1/file4/file7

and somewhere in there it crashes because vfs_mkdir() returns a
non-error dentry for which ->d_parent->d_inode is not locked and
end_creating_path() tries to up_write().

Presumably either ->d_parent has changed or the inode was unlocked?

gfs2_mkdir() never returns a dentry, so it must be returning NULL.

It's weird - but that is no surprise.

I'll try building a kernel myself and see if the reproducer still fires.
if so some printk tracing my reveal something.

NeilBrown