[syzbot] [net?] [nfs?] WARNING in svc_destroy

[syzbot <syzbot+8cbdf9aa6c281a4e6bbe@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    05441d697f7f Merge tag 'cocci-7.4-rc1' of git://git.kernel..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14721d2d580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=1cf189394a856f11
dashboard link: https://syzkaller.appspot.com/bug?extid=8cbdf9aa6c281a4e6bbe
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/e8f55ecc1d55/disk-05441d69.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/db66972c51fd/vmlinux-05441d69.xz
kernel image: https://storage.googleapis.com/syzbot-assets/6628bc03aa4a/bzImage-05441d69.xz

AI-generated root cause summary:
In fs/nfs/callback.c, nfs4_callback_up_net() first initializes an IPv4 TCP
listener transport via svc_xprt_create(..., PF_INET, ...), which links the
transport onto serv->sv_permsocks. When a subsequent call to
svc_xprt_create(..., PF_INET6, ...) fails with an error other than
-EAFNOSUPPORT, nfs4_callback_up_net() returns the error immediately without
tearing down the existing IPv4 transport. As a result, nfs_callback_up() unwinds
on error and calls xprt_svc_destroy_nullify_bc() -> svc_destroy(), which
triggers the WARN_ONCE(!list_empty(&serv->sv_permsocks)) warning because the
IPv4 transport is still attached to the service, and subsequently frees the
service while leaving the active listener with a dangling pointer.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=8cbdf9aa6c281a4e6bbe

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+8cbdf9aa6c281a4e6bbe@syzkaller.appspotmail.com

NFS: callback service start failed
------------[ cut here ]------------
SVC: permsocks remain for NFSv4 callback
WARNING: net/sunrpc/svc.c:532 at svc_destroy+0x299/0x330 net/sunrpc/svc.c:532, CPU#1: syz.2.321/6994
Modules linked in:
CPU: 1 UID: 0 PID: 6994 Comm: syz.2.321 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/15/2026
RIP: 0010:svc_destroy+0x29d/0x330 net/sunrpc/svc.c:532
Code: 48 b8 00 00 00 00 00 fc ff df 49 8b 5d 00 48 8d 7b 18 48 89 fa 48 c1 ea 03 80 3c 02 00 75 67 48 8d 3d 17 5e 78 07 48 8b 73 18 <67> 48 0f b9 3a e9 d0 fd ff ff 48 8b 7c 24 08 e8 bf 13 86 f7 e9 a5
RSP: 0018:ffffc900042af860 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffffffff9040ba80 RCX: ffffc900063d9000
RDX: 1ffffffff2081753 RSI: ffffffff8c37e800 RDI: ffffffff927aee30
RBP: 0000000000000000 R08: 00000000ffffffff R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffff888035744000
R13: ffff888055f04c00 R14: ffff88802de30000 R15: ffffffff9cd5b540
FS:  00007ff81118f6c0(0000) GS:ffff88812257c000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000110c29af61 CR3: 0000000056a04000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 nfs_callback_up+0x511/0xad0 fs/nfs/callback.c:236
 nfs4_init_callback fs/nfs/nfs4client.c:327 [inline]
 nfs4_init_client_minor_version fs/nfs/nfs4client.c:379 [inline]
 nfs4_init_client+0x236/0x420 fs/nfs/nfs4client.c:429
 nfs_get_client+0xf38/0x1230 fs/nfs/client.c:467
 nfs4_set_client+0x3f6/0x640 fs/nfs/nfs4client.c:763
 nfs4_init_server.isra.0+0x5be/0xf80 fs/nfs/nfs4client.c:1014
 nfs4_create_server+0x185/0x2b0 fs/nfs/nfs4client.c:1063
 nfs4_try_get_tree+0x93/0x140 fs/nfs/nfs4super.c:228
 nfs_get_tree+0xa61/0x1440 fs/nfs/fs_context.c:1621
 vfs_get_tree+0x92/0x320 fs/super.c:1881
 fc_mount fs/namespace.c:1282 [inline]
 do_new_mount_fc fs/namespace.c:3910 [inline]
 do_new_mount fs/namespace.c:3986 [inline]
 path_mount+0x7d0/0x2850 fs/namespace.c:4337
 do_mount fs/namespace.c:4350 [inline]
 __do_sys_mount fs/namespace.c:4565 [inline]
 __se_sys_mount fs/namespace.c:4542 [inline]
 __x64_sys_mount+0x293/0x310 fs/namespace.c:4542
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7ff81039fb19
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:00007ff81118f028 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007ff810625fa0 RCX: 00007ff81039fb19
RDX: 0000200000000480 RSI: 0000200000000440 RDI: 0000200000000400
RBP: 00007ff810437b70 R08: 00002000000004c0 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ff810626038 R14: 00007ff810625fa0 R15: 00007ffd2a563db8
 </TASK>
----------------
Code disassembly (best guess):
   0:	48 b8 00 00 00 00 00 	movabs $0xdffffc0000000000,%rax
   7:	fc ff df
   a:	49 8b 5d 00          	mov    0x0(%r13),%rbx
   e:	48 8d 7b 18          	lea    0x18(%rbx),%rdi
  12:	48 89 fa             	mov    %rdi,%rdx
  15:	48 c1 ea 03          	shr    $0x3,%rdx
  19:	80 3c 02 00          	cmpb   $0x0,(%rdx,%rax,1)
  1d:	75 67                	jne    0x86
  1f:	48 8d 3d 17 5e 78 07 	lea    0x7785e17(%rip),%rdi        # 0x7785e3d
  26:	48 8b 73 18          	mov    0x18(%rbx),%rsi
* 2a:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2f:	e9 d0 fd ff ff       	jmp    0xfffffe04
  34:	48 8b 7c 24 08       	mov    0x8(%rsp),%rdi
  39:	e8 bf 13 86 f7       	call   0xf78613fd
  3e:	e9                   	.byte 0xe9
  3f:	a5                   	movsl  %ds:(%rsi),%es:(%rdi)


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