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Date:   Sat, 27 Aug 2016 11:55:59 +0200
From:   Miklos Szeredi <mszeredi@...hat.com>
To:     Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:     Nikolay Borisov <kernel@...p.com>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        netdev@...r.kernel.org
Subject: Re: kernel BUG at net/unix/garbage.c:149!"

On Wed, Aug 24, 2016 at 11:40 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On 24.08.2016 16:24, Nikolay Borisov wrote:
>> Hello,
>>
>> I hit the following BUG:
>>
>> [1851513.239831] ------------[ cut here ]------------
>> [1851513.240079] kernel BUG at net/unix/garbage.c:149!
>> [1851513.240313] invalid opcode: 0000 [#1] SMP
>> [1851513.248320] CPU: 37 PID: 11683 Comm: nginx Tainted: G           O    4.4.14-clouder3 #26
>> [1851513.248719] Hardware name: Supermicro X10DRi/X10DRi, BIOS 1.1 04/14/2015
>> [1851513.248966] task: ffff883b0f6f0000 ti: ffff880189cf0000 task.ti: ffff880189cf0000
>> [1851513.249361] RIP: 0010:[<ffffffff815f895d>]  [<ffffffff815f895d>] unix_notinflight+0x8d/0x90
>> [1851513.249846] RSP: 0018:ffff880189cf3cf8  EFLAGS: 00010246
>> [1851513.250082] RAX: ffff883b05491968 RBX: ffff883b05491680 RCX: ffff8807f9967330
>> [1851513.250476] RDX: 0000000000000001 RSI: ffff882e6d8bae00 RDI: ffffffff82073f10
>> [1851513.250886] RBP: ffff880189cf3d08 R08: ffff880cbc70e200 R09: 0000000180200001
>> [1851513.251280] R10: ffff883fff3b9dc0 R11: ffffea0032f1c380 R12: ffff883fbaf50000
>> [1851513.251674] R13: ffffffff815f6354 R14: ffff881a7c77b140 R15: ffff881a7c7792c0
>> [1851513.252083] FS:  00007f4f19573720(0000) GS:ffff883fff3a0000(0000) knlGS:0000000000000000
>> [1851513.252481] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [1851513.252717] CR2: 00000000013062d8 CR3: 0000001712f32000 CR4: 00000000001406e0
>> [1851513.253116] Stack:
>> [1851513.253345]  00000000ffffffff ffff880189cf3d40 ffff880189cf3d28 ffffffff815f4383
>> [1851513.254022]  ffff8839ee11a800 ffff8839ee11a800 ffff880189cf3d60 ffffffff815f53b8
>> [1851513.254685]  0000000000000000 ffff883406788de0 0000000000000000 0000000000000000
>> [1851513.255360] Call Trace:
>> [1851513.255594]  [<ffffffff815f4383>] unix_detach_fds.isra.19+0x43/0x50
>> [1851513.255851]  [<ffffffff815f53b8>] unix_destruct_scm+0x48/0x80
>> [1851513.256090]  [<ffffffff815384af>] skb_release_head_state+0x4f/0xb0
>> [1851513.256328]  [<ffffffff81538522>] skb_release_all+0x12/0x30
>> [1851513.256564]  [<ffffffff81538592>] kfree_skb+0x32/0xa0
>> [1851513.256810]  [<ffffffff815f6354>] unix_release_sock+0x1e4/0x2c0
>> [1851513.257046]  [<ffffffff815f6450>] unix_release+0x20/0x30
>> [1851513.257284]  [<ffffffff8152fbcf>] sock_release+0x1f/0x80
>> [1851513.257521]  [<ffffffff8152fc42>] sock_close+0x12/0x20
>> [1851513.257769]  [<ffffffff8119a8aa>] __fput+0xea/0x1f0
>> [1851513.258005]  [<ffffffff8119a9ee>] ____fput+0xe/0x10
>> [1851513.258244]  [<ffffffff8106fccf>] task_work_run+0x7f/0xb0
>> [1851513.258488]  [<ffffffff81002210>] exit_to_usermode_loop+0xc0/0xd0
>> [1851513.258728]  [<ffffffff81002a90>] syscall_return_slowpath+0x80/0xf0
>> [1851513.258983]  [<ffffffff816147b4>] int_ret_from_sys_call+0x25/0x9f
>> [1851513.259222] Code: 7e 5b 41 5c 5d c3 48 8b 8b e8 02 00 00 48 8b 93 f0 02 00 00 48 89 51 08 48 89 0a 48 89 83 e8 02 00 00 48 89 83 f0 02 00 00 eb b8 <0f> 0b 90 0f 1f 44 00 00 55 48 c7 c7 10 3f 07 82 48 89 e5 41 54
>> [1851513.268473] RIP  [<ffffffff815f895d>] unix_notinflight+0x8d/0x90
>> [1851513.268793]  RSP <ffff880189cf3cf8>
>>
>> That's essentially BUG_ON(list_empty(&u->link));
>>
>> I see that all the code involving the ->link member hasn't really been
>> touched since it was introduced in 2007. So this must be a latent bug.
>> This is the first time I've observed it. The state
>> of the struct unix_sock can be found here http://sprunge.us/WCMW . Evidently,
>> there are no inflight sockets.

Weird.  If it was the BUG_ON(!list_empty(&u->link)) I'd understand,
because the code in scan children looks fishy: what prevents "embryos"
from fledging to full socket status and going in-flight?

But this one, I cannot imagine any scenario.

Can we have access to the crashdump?

Thanks,
Miklos

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