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

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

syzbot has found a reproducer for the following issue on:

HEAD commit:    d164c7d754f0 Merge tag 'devicetree-fixes-for-7.4-1' of git..
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=13ac11a1580000
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
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1494d369580000

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-executor154/6024
Modules linked in:
CPU: 1 UID: 0 PID: 6024 Comm: syz-executor154 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/06/2026
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 e7 6d 77 07 48 8b 73 18 <67> 48 0f b9 3a e9 d0 fd ff ff 48 8b 7c 24 08 e8 7f 4d 85 f7 e9 a5
RSP: 0018:ffffc9000393f860 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffffffff9040bf80 RCX: 0000000000000000
RDX: 1ffffffff20817f3 RSI: ffffffff8c37ebc0 RDI: ffffffff927b0660
RBP: 0000000000000000 R08: 00000000ffffffff R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffff888020bcc000
R13: ffff8880274a0000 R14: ffff88802c3c4000 R15: ffffffff9cd624c0
FS:  000055556ff69400(0000) GS:ffff888122575000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055fdb0009fd0 CR3: 00000000377ff000 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: 0033:0x7fd324de7f1a
Code: 48 c7 c2 d8 ff ff ff f7 d8 64 89 02 b8 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 d8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffca5d506e8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fd324de7f1a
RDX: 00007fd324e4603f RSI: 00007fd324e45ffe RDI: 00007fd324e46044
RBP: 0000000000000004 R08: 00007ffca5d50760 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fd324e45ffe
R13: 0000000000001789 R14: 0000000000000801 R15: 00007ffca5d50760
 </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 e7 6d 77 07 	lea    0x7776de7(%rip),%rdi        # 0x7776e0d
  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 7f 4d 85 f7       	call   0xf7854dbd
  3e:	e9                   	.byte 0xe9
  3f:	a5                   	movsl  %ds:(%rsi),%es:(%rdi)


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