Re: kernel BUG at net/unix/garbage.c:149!"

From: Miklos Szeredi
Date: Tue Aug 30 2016 - 05:40:07 EST

On Tue, Aug 30, 2016 at 11:31 AM, Nikolay Borisov <kernel@xxxxxxxx> wrote:
> On 08/30/2016 12:18 PM, Miklos Szeredi wrote:
>> On Tue, Aug 30, 2016 at 12:37 AM, Miklos Szeredi <mszeredi@xxxxxxxxxx> wrote:
>>> On Sat, Aug 27, 2016 at 11:55 AM, Miklos Szeredi <mszeredi@xxxxxxxxxx> wrote:
>>> crash> list -H gc_inflight_list -s unix_sock.inflight |
>>> grep counter | cut -d= -f2 | awk '{s+=$1} END {print s}'
>>> 130
>>> crash> p unix_tot_inflight
>>> unix_tot_inflight = $2 = 135
>>> We've lost track of a total of five inflight sockets, so it's not a
>>> one-off thing. Really weird... Now off to sleep, maybe I'll dream of
>>> the solution.
>> Okay, found one bug: gc assumes that in-flight sockets that don't have
>> an external ref can't gain one while unix_gc_lock is held. That is
>> true because unix_notinflight() will be called before detaching fds,
>> which takes unix_gc_lock. Only MSG_PEEK was somehow overlooked. That
>> one also clones the fds, also keeping them in the skb. But through
>> MSG_PEEK an external reference can definitely be gained without ever
>> touching unix_gc_lock.
>> Not sure whether the reported bug can be explained by this. Can you
>> confirm the MSG_PEEK was used in the setup?
>> Does someone want to write a stress test for SCM_RIGHTS + MSG_PEEK?
>> Anyway, attaching a fix that works by acquiring unix_gc_lock in case
>> of MSG_PEEK also. It is trivially correct, but I haven't tested it.
> I have no way of being 100% sure but looking through nginx's source code
> it seems they do utilize MSG_PEEK on several occasions. This issue has
> been apparently very hard to reproduce since I have 100s of servers
> running a lot of NGINX processes and this has been triggered only once.
> On a different note - if I inspect a live node without this patch should
> the discrepancy between the gc_inflight_list and the unix_tot_inflight
> be present VS with this patch applied?

May well be, since in the vmcore 4 in-flight sockets were "lost"
before triggering the bug. I guess the best way to check is with a
systemtap script that walks the list with the gc lock.