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

On 30.08.2016 11:18, Miklos Szeredi wrote:

On Tue, Aug 30, 2016 at 12:37 AM, Miklos Szeredi [off-list ref] wrote:

quoted

On Sat, Aug 27, 2016 at 11:55 AM, Miklos Szeredi [off-list ref] wrote:

quoted

crash> list -H gc_inflight_list unix_sock.link -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.

You can use spin_unlock_wait in unix_gc_barrier to make it a bit more
lightweight.

Anyway, all of the scans on the socket receive queues are actually
protected by the appropriate locks again, I didn't see a way were we
could result in such a crash because of concurrent modification of the
receive queue. Do you have any hints or looked into this more closely?

Thanks,
Hannes