[syzbot] [fs?] WARNING in vfs_open_tree

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: fe9e3edb6a21 Add linux-next specific files for 20260217
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=153fb652580000
kernel config: https://syzkaller.appspot.com/x/.config?x=3696efcd0f17d527
dashboard link: https://syzkaller.appspot.com/bug?extid=d09c0eed6e4176ba5c86
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=1267095a580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17036ffa580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/3a3b845d3361/disk-fe9e3edb.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/efa525980a65/vmlinux-fe9e3edb.xz
kernel image: https://storage.googleapis.com/syzbot-assets/5bf9ea774626/bzImage-fe9e3edb.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d09c0e...@syzkaller.appspotmail.com

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old_ns_root->mnt_id_unique != 1
WARNING: fs/namespace.c:3112 at create_new_namespace fs/namespace.c:3112 [inline], CPU#0: syz.0.17/6017
WARNING: fs/namespace.c:3112 at open_new_namespace fs/namespace.c:3172 [inline], CPU#0: syz.0.17/6017
WARNING: fs/namespace.c:3112 at vfs_open_tree+0xf35/0xfb0 fs/namespace.c:3221, CPU#0: syz.0.17/6017
Modules linked in:
CPU: 0 UID: 0 PID: 6017 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
RIP: 0010:create_new_namespace fs/namespace.c:3112 [inline]
RIP: 0010:open_new_namespace fs/namespace.c:3172 [inline]
RIP: 0010:vfs_open_tree+0xf35/0xfb0 fs/namespace.c:3221
Code: 39 78 ff 49 bf 00 00 00 00 00 fc ff df e9 1e fb ff ff e8 9e 39 78 ff 4d 63 ee 4c 8b 64 24 08 e9 64 ff ff ff e8 8c 39 78 ff 90 <0f> 0b 90 e9 d1 f5 ff ff e8 7e 39 78 ff 90 0f 0b 90 e9 18 f6 ff ff
RSP: 0018:ffffc90004237d80 EFLAGS: 00010293
RAX: ffffffff824db904 RBX: 00000000800002b1 RCX: ffff888031f41e40
RDX: 0000000000000000 RSI: 00000000800002b1 RDI: 0000000000000001
RBP: ffffc90004237eb0 R08: ffffffff8e7fa2eb R09: 1ffffffff1cff45d
R10: dffffc0000000000 R11: fffffbfff1cff45e R12: ffff88807cae1340
R13: 0000000000009902 R14: ffff888033097100 R15: dffffc0000000000
FS: 000055559209d500(0000) GS:ffff888124fff000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f7055417dac CR3: 000000002e056000 CR4: 00000000003526f0
Call Trace:
<TASK>
__do_sys_open_tree fs/namespace.c:3231 [inline]
__se_sys_open_tree fs/namespace.c:3229 [inline]
__x64_sys_open_tree+0x96/0x110 fs/namespace.c:3229
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: 0033:0x7f705519c629
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:00007fff9b19c798 EFLAGS: 00000246 ORIG_RAX: 00000000000001ac
RAX: ffffffffffffffda RBX: 00007f7055415fa0 RCX: 00007f705519c629
RDX: 0000000000009902 RSI: 0000200000000640 RDI: ffffffffffffff9c
RBP: 00007f7055232b39 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f7055415fac R14: 00007f7055415fa0 R15: 00007f7055415fa0
</TASK>


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