Re: [syzbot] [fs?] WARNING in free_mnt_ns

[syzbot <syzbot+7d23dc5cd4fa132fb9f3@syzkaller.appspotmail.com>]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
WARNING in __ns_common_free

------------[ cut here ]------------
ida_free called for id=986 which is not allocated.
WARNING: CPU: 0 PID: 6550 at lib/idr.c:592 ida_free+0x1f9/0x2e0 lib/idr.c:592
Modules linked in:
CPU: 0 UID: 0 PID: 6550 Comm: syz.1.18 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
RIP: 0010:ida_free+0x1f9/0x2e0 lib/idr.c:592
Code: 77 f6 41 83 fe 3e 76 72 e8 d4 eb 77 f6 48 8b 7c 24 28 4c 89 ee e8 07 39 0d 00 90 48 c7 c7 e0 cf cf 8c 89 ee e8 a8 c5 36 f6 90 <0f> 0b 90 90 e8 ae eb 77 f6 48 b8 00 00 00 00 00 fc ff df 48 01 c3
RSP: 0018:ffffc90003f0f990 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 1ffff920007e1f33 RCX: ffffffff81796528
RDX: ffff8880272cdac0 RSI: ffffffff81796535 RDI: 0000000000000001
RBP: 00000000000003da R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: ffff88801e6db000
R13: 0000000000000293 R14: 00000000000003da R15: ffff88801e6db078
FS:  0000000000000000(0000) GS:ffff888124e79000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f2984627286 CR3: 000000007e5e2000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 __ns_common_free+0x7d/0xa0 kernel/nscommon.c:80
 free_mnt_ns fs/namespace.c:4096 [inline]
 namespace_unlock+0x7f9/0xa30 fs/namespace.c:1701
 put_mnt_ns fs/namespace.c:6135 [inline]
 put_mnt_ns+0xf5/0x120 fs/namespace.c:6126
 free_nsproxy+0x3a/0x400 kernel/nsproxy.c:188
 put_nsproxy include/linux/nsproxy.h:107 [inline]
 switch_task_namespaces+0xeb/0x100 kernel/nsproxy.c:241
 do_exit+0x86a/0x2bf0 kernel/exit.c:960
 do_group_exit+0xd3/0x2a0 kernel/exit.c:1102
 get_signal+0x2671/0x26d0 kernel/signal.c:3034
 arch_do_signal_or_restart+0x8f/0x790 arch/x86/kernel/signal.c:337
 exit_to_user_mode_loop+0x7a/0x100 kernel/entry/common.c:40
 exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
 syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
 do_syscall_64+0x419/0x4b0 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fc7c378eec9
Code: Unable to access opcode bytes at 0x7fc7c378ee9f.
RSP: 002b:00007fc7c461c038 EFLAGS: 00000246 ORIG_RAX: 0000000000000110
RAX: fffffffffffffff4 RBX: 00007fc7c39e5fa0 RCX: 00007fc7c378eec9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000020000
RBP: 00007fc7c3811f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fc7c39e6038 R14: 00007fc7c39e5fa0 R15: 00007ffe2b65def8
 </TASK>


Tested on:

commit:         50c19e20 Merge tag 'nolibc-20250928-for-6.18-1' of git..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11d95d04580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=b28601618dc289ee
dashboard link: https://syzkaller.appspot.com/bug?extid=7d23dc5cd4fa132fb9f3
compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
patch:          https://syzkaller.appspot.com/x/patch.diff?x=1292e942580000