syzbot ci has tested the following series

[v1] nstree: listns()
https://lore.kernel.org/all/20251021-work-namespace-nstree-listns-v1-0-ad44261a8a5b@kernel.org (local)
* [PATCH RFC DRAFT 01/50] libfs: allow to specify s_d_flags
* [PATCH RFC DRAFT 02/50] nsfs: use inode_just_drop()
* [PATCH RFC DRAFT 03/50] nsfs: raise DCACHE_DONTCACHE explicitly
* [PATCH RFC DRAFT 04/50] pidfs: raise DCACHE_DONTCACHE explicitly
* [PATCH RFC DRAFT 05/50] nsfs: raise SB_I_NODEV and SB_I_NOEXEC
* [PATCH RFC DRAFT 06/50] nstree: simplify return
* [PATCH RFC DRAFT 07/50] ns: initialize ns_list_node for initial namespaces
* [PATCH RFC DRAFT 08/50] ns: add __ns_ref_read()
* [PATCH RFC DRAFT 09/50] ns: add active reference count
* [PATCH RFC DRAFT 10/50] ns: use anonymous struct to group list member
* [PATCH RFC DRAFT 11/50] nstree: introduce a unified tree
* [PATCH RFC DRAFT 12/50] nstree: allow lookup solely based on inode
* [PATCH RFC DRAFT 13/50] nstree: assign fixed ids to the initial namespaces
* [PATCH RFC DRAFT 14/50] ns: maintain list of owned namespaces
* [PATCH RFC DRAFT 15/50] nstree: add listns()
* [PATCH RFC DRAFT 16/50] arch: hookup listns() system call
* [PATCH RFC DRAFT 17/50] nsfs: update tools header
* [PATCH RFC DRAFT 18/50] selftests/filesystems: remove CLONE_NEWPIDNS from setup_userns() helper
* [PATCH RFC DRAFT 19/50] selftests/namespaces: first active reference count tests
* [PATCH RFC DRAFT 20/50] selftests/namespaces: second active reference count tests
* [PATCH RFC DRAFT 21/50] selftests/namespaces: third active reference count tests
* [PATCH RFC DRAFT 22/50] selftests/namespaces: fourth active reference count tests
* [PATCH RFC DRAFT 23/50] selftests/namespaces: fifth active reference count tests
* [PATCH RFC DRAFT 24/50] selftests/namespaces: sixth active reference count tests
* [PATCH RFC DRAFT 25/50] selftests/namespaces: seventh active reference count tests
* [PATCH RFC DRAFT 26/50] selftests/namespaces: eigth active reference count tests
* [PATCH RFC DRAFT 27/50] selftests/namespaces: ninth active reference count tests
* [PATCH RFC DRAFT 28/50] selftests/namespaces: tenth active reference count tests
* [PATCH RFC DRAFT 29/50] selftests/namespaces: eleventh active reference count tests
* [PATCH RFC DRAFT 30/50] selftests/namespaces: twelth active reference count tests
* [PATCH RFC DRAFT 31/50] selftests/namespaces: thirteenth active reference count tests
* [PATCH RFC DRAFT 32/50] selftests/namespaces: fourteenth active reference count tests
* [PATCH RFC DRAFT 33/50] selftests/namespaces: fifteenth active reference count tests
* [PATCH RFC DRAFT 34/50] selftests/namespaces: add listns() wrapper
* [PATCH RFC DRAFT 35/50] selftests/namespaces: first listns() test
* [PATCH RFC DRAFT 36/50] selftests/namespaces: second listns() test
* [PATCH RFC DRAFT 37/50] selftests/namespaces: third listns() test
* [PATCH RFC DRAFT 38/50] selftests/namespaces: fourth listns() test
* [PATCH RFC DRAFT 39/50] selftests/namespaces: fifth listns() test
* [PATCH RFC DRAFT 40/50] selftests/namespaces: sixth listns() test
* [PATCH RFC DRAFT 41/50] selftests/namespaces: seventh listns() test
* [PATCH RFC DRAFT 42/50] selftests/namespaces: ninth listns() test
* [PATCH RFC DRAFT 43/50] selftests/namespaces: ninth listns() test
* [PATCH RFC DRAFT 44/50] selftests/namespaces: first listns() permission test
* [PATCH RFC DRAFT 45/50] selftests/namespaces: second listns() permission test
* [PATCH RFC DRAFT 46/50] selftests/namespaces: third listns() permission test
* [PATCH RFC DRAFT 47/50] selftests/namespaces: fourth listns() permission test
* [PATCH RFC DRAFT 48/50] selftests/namespaces: fifth listns() permission test
* [PATCH RFC DRAFT 49/50] selftests/namespaces: sixth listns() permission test
* [PATCH RFC DRAFT 50/50] selftests/namespaces: seventh listns() permission test

and found the following issue:
WARNING in __ns_tree_add_raw

Full report is available here:
https://ci.syzbot.org/series/03ca38c3-876c-4231-aa06-ddb0bc8a30ad

***

WARNING in __ns_tree_add_raw

tree:      bpf
URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/bpf/bpf.git
base:      5fb750e8a9ae123b2034771b864b8a21dbef65cd
arch:      amd64
compiler:  Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
config:    https://ci.syzbot.org/builds/156cf21b-68f9-423c-807a-3dd094e6aed8/config

------------[ cut here ]------------
WARNING: CPU: 1 PID: 5816 at kernel/nstree.c:189 __ns_tree_add_raw+0xa92/0xb30
Modules linked in:
CPU: 1 UID: 0 PID: 5816 Comm: syz-executor Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:__ns_tree_add_raw+0xa92/0xb30
Code: 32 00 90 0f 0b 90 42 80 3c 23 00 0f 85 1e fc ff ff e9 21 fc ff ff e8 ed 78 32 00 90 0f 0b 90 e9 66 fc ff ff e8 df 78 32 00 90 <0f> 0b 90 e9 53 ff ff ff 44 89 f9 80 e1 07 80 c1 03 38 c1 0f 8c ef
RSP: 0018:ffffc90003f27c30 EFLAGS: 00010293
RAX: ffffffff818e0051 RBX: 1ffffffff16db871 RCX: ffff88810ffe0000
RDX: 0000000000000000 RSI: ffff8881bbf5e9a8 RDI: ffff88816d0e6e00
RBP: ffff88816d0e6e00 R08: ffff88816d0e6e3f R09: 0000000000000000
R10: ffff88816d0e6e30 R11: ffffffff81b988c0 R12: dffffc0000000000
R13: ffff88816d0e6e40 R14: ffffffff8b6dc388 R15: ffff8881bbf5e9a8
FS:  000055558630f500(0000) GS:ffff8882a9d04000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f5c3f03529c CR3: 000000010b5bc000 CR4: 00000000000006f0
Call Trace:
 <TASK>
 copy_cgroup_ns+0x373/0x5f0
 create_new_namespaces+0x358/0x720
 unshare_nsproxy_namespaces+0x11c/0x170
 ksys_unshare+0x4c8/0x8c0
 __x64_sys_unshare+0x38/0x50
 do_syscall_64+0xfa/0xfa0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f5c3ef907c7
Code: 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 b8 10 01 00 00 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:00007ffc62c39c88 EFLAGS: 00000246 ORIG_RAX: 0000000000000110
RAX: ffffffffffffffda RBX: 00007ffc62c39c90 RCX: 00007f5c3ef907c7
RDX: 0000000000000000 RSI: 00007f5c3f03529c RDI: 0000000002000000
RBP: 00007ffc62c39d20 R08: 0000000000000000 R09: 00007f5c3fd1d6c0
R10: 0000000000044000 R11: 0000000000000246 R12: 00007ffc62c39d20
R13: 00007ffc62c39d28 R14: 0000000000000009 R15: 0000000000000000
 </TASK>


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
  Tested-by: syzbot@syzkaller.appspotmail.com

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