WARNING: proc registration bug in afs_manage_cell

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    c9c9e6a4 Merge tag 'trace-v5.9-rc5-2' of git://git.kernel...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13a34b1d900000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5f4c828c9e3cef97
dashboard link: https://syzkaller.appspot.com/bug?extid=b994ecf2b023f14832c1
compiler:       gcc (GCC) 10.1.0-syz 20200507
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=164ee99b900000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=165dd5c5900000

The issue was bisected to:

commit 0e7bbcc104baaade4f64205e9706b7d43c46db7d
Author: Julian Anastasov <ja@xxxxxx>
Date:   Wed Jul 27 06:56:50 2016 +0000

    neigh: allow admin to set NUD_STALE

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=16850bab900000
final oops:     https://syzkaller.appspot.com/x/report.txt?x=15850bab900000
console output: https://syzkaller.appspot.com/x/log.txt?x=11850bab900000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+b994ecf2b023f14832c1@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 0e7bbcc104ba ("neigh: allow admin to set NUD_STALE")

------------[ cut here ]------------
proc_dir_entry 'afs/^]$[+.](%{' already registered
WARNING: CPU: 1 PID: 2468 at fs/proc/generic.c:371 proc_register+0x34c/0x700 fs/proc/generic.c:371
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 2468 Comm: kworker/1:2 Not tainted 5.9.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: afs afs_manage_cell
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x198/0x1fd lib/dump_stack.c:118
 panic+0x382/0x7fb kernel/panic.c:231
 __warn.cold+0x20/0x4b kernel/panic.c:600
 report_bug+0x1bd/0x210 lib/bug.c:198
 handle_bug+0x38/0x90 arch/x86/kernel/traps.c:234
 exc_invalid_op+0x14/0x40 arch/x86/kernel/traps.c:254
 asm_exc_invalid_op+0x12/0x20 arch/x86/include/asm/idtentry.h:536
RIP: 0010:proc_register+0x34c/0x700 fs/proc/generic.c:371
Code: df 48 89 f9 48 c1 e9 03 80 3c 01 00 0f 85 5d 03 00 00 48 8b 44 24 28 48 c7 c7 60 61 9a 88 48 8b b0 d8 00 00 00 e8 46 1b 5d ff <0f> 0b 48 c7 c7 60 f6 1e 8a e8 76 78 3d 06 48 8b 4c 24 38 48 b8 00
RSP: 0018:ffffc90008387ac8 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff8880a06f0540 RSI: ffffffff815f5985 RDI: fffff52001070f4b
RBP: ffff8880a8237c50 R08: 0000000000000001 R09: ffff8880ae5318e7
R10: 0000000000000000 R11: 0000000000000000 R12: ffff888086ce9d98
R13: ffff888086ce9c40 R14: dffffc0000000000 R15: 000000000000000a
 proc_mkdir_data+0x140/0x1a0 fs/proc/generic.c:487
 proc_net_mkdir include/linux/proc_fs.h:201 [inline]
 afs_proc_cell_setup+0xb2/0x1f0 fs/afs/proc.c:620
 afs_activate_cell fs/afs/cell.c:615 [inline]
 afs_manage_cell+0x56a/0x11c0 fs/afs/cell.c:697
 process_one_work+0x94c/0x1670 kernel/workqueue.c:2269
 worker_thread+0x64c/0x1120 kernel/workqueue.c:2415
 kthread+0x3b5/0x4a0 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
Kernel Offset: disabled
Rebooting in 86400 seconds..


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