Re: [PATCH v2 18/33] mnt: support ns lookup

[Kuniyuki Iwashima <kuniyu@google.com>]

Hi,

From: Christian Brauner <brauner@kernel.org>
Date: Fri, 12 Sep 2025 13:52:41 +0200
[...]
> @@ -4275,7 +4197,7 @@ struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
>  	if (pwdmnt)
>  		mntput(pwdmnt);
>  
> -	mnt_ns_tree_add(new_ns);
> +	ns_tree_add_raw(new_ns);

When copy_tree() fails, new_ns's rb tree could be empty,

	guard(namespace_excl)();
...
	new = copy_tree(old, old->mnt.mnt_root, copy_flags);
	if (IS_ERR(new)) {
		emptied_ns = new_ns;
		return ERR_CAST(new);
	}

which seems to trigger this warning in __ns_tree_remove().

	VFS_WARN_ON_ONCE(RB_EMPTY_NODE(&ns->ns_tree_node));


stack trace captured by syzbot:

WARNING: kernel/nstree.c:115 at __ns_tree_remove+0x28d/0x330 kernel/nstree.c:115, CPU#0: syz.5.2042/14092
Modules linked in:
CPU: 0 UID: 0 PID: 14092 Comm: syz.5.2042 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
RIP: 0010:__ns_tree_remove+0x28d/0x330 kernel/nstree.c:115
Code: 0f 85 a4 00 00 00 48 8b 04 24 ff 00 48 8b 7c 24 18 48 83 c4 30 5b 41 5c 41 5d 41 5e 41 5f 5d e9 e9 b4 f8 09 e8 94 f1 31 00 90 <0f> 0b 90 e9 cc fd ff ff e8 86 f1 31 00 90 0f 0b 90 e9 ee fd ff ff
RSP: 0018:ffffc9001040faa8 EFLAGS: 00010287
RAX: ffffffff818de18c RBX: ffff8880565d4620 RCX: 0000000000080000
RDX: ffffc9001ca51000 RSI: 00000000000003f8 RDI: 00000000000003f9
RBP: dffffc0000000000 R08: ffffffff8e1eff53 R09: 1ffffffff1c3dfea
R10: dffffc0000000000 R11: fffffbfff1c3dfeb R12: ffff8880565d4620
R13: 1ffff1100acba8c3 R14: ffff8880565d4600 R15: ffffffff8e1ea7c0
FS:  00007fba829d56c0(0000) GS:ffff8881259e0000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000110c3b8de5 CR3: 000000004bb54000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 mnt_ns_tree_remove fs/namespace.c:165 [inline]
 free_mnt_ns+0xd1/0x110 fs/namespace.c:4074
 namespace_unlock+0x529/0x760 fs/namespace.c:1710
 class_namespace_excl_destructor fs/namespace.c:96 [inline]
 copy_mnt_ns+0x6e0/0x870 fs/namespace.c:4180
 create_new_namespaces+0xd1/0x720 kernel/nsproxy.c:78
 unshare_nsproxy_namespaces+0x11c/0x170 kernel/nsproxy.c:218
 ksys_unshare+0x4c8/0x8c0 kernel/fork.c:3144
 __do_sys_unshare kernel/fork.c:3215 [inline]
 __se_sys_unshare kernel/fork.c:3213 [inline]
 __x64_sys_unshare+0x38/0x50 kernel/fork.c:3213
 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: 0033:0x7fba8478eba9
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:00007fba829d5038 EFLAGS: 00000246 ORIG_RAX: 0000000000000110
RAX: ffffffffffffffda RBX: 00007fba849d6180 RCX: 00007fba8478eba9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000028060400
RBP: 00007fba84811e19 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fba849d6218 R14: 00007fba849d6180 R15: 00007ffe311606c8
 </TASK>