panic in tcp_do_segment()
Rick Macklem
rmacklem at uoguelph.ca
Sun Apr 7 21:28:56 UTC 2013
Juan Mojica wrote:
> Agree with Matt.
>
> Whenever there is an UNLOCK/LOCK like is present in soclose(), there
> is a window to allow something through. Unsetting SO_ACCEPTCONN was
> put in place because the LOCK/UNLOCK in soclose let a new socket to be
> added to the so_incomp list causing a different ASSERT to be hit - and
> memory to be leaked.
>
> As far as I can tell, because of the upcall performed in soabort(), we
> can't just hold the ACCEPT lock all the way through the close.
>
>
> The simplest thing to do in this case seemed to be to just drop the
> TCP segment - the connection is being closed anyway. Or like Matt
> said, having someone look at the lock logic and see if there is
> something there that can be exploited to prevent this would also help.
>
>
> -Juan
>
>
>
>
> On Fri, Apr 5, 2013 at 7:09 AM, Matt Miller < matt at matthewjmiller.net
> > wrote:
>
>
>
>
> Hey Rick,
>
> I believe Juan and I have root caused this crash recently. The
> t_state = 0x1, TCPS_LISTEN, in the link provided at the time of the
> assertion.
>
> In tcp_input(), if we're in TCPS_LISTEN, SO_ACCEPTCONN should be set
> on the socket and we should never enter tcp_do_segment() for this
> state. I think if you look in your corefile, you'll see the socket
> *doesn't* have this flag set in your case.
>
Thanks guys. I had missed the *tp near the end and mistakenly was
thinking it was the client socket. This sounds like a good explanation
to me.
Hopefully, one of the tcp stack guys will pick this up and commit
a patch, rick.
>
> 1043 /*
> 1044 * When the socket is accepting connections (the INPCB is
> in LISTEN
> 1045 * state) we look into the SYN cache if this is a new
> connection
> 1046 * attempt or the completion of a previous one. Because
> listen
> 1047 * sockets are never in TCPS_ESTABLISHED, the V_tcbinfo
> lock will be
> 1048 * held in this case.
> 1049 */
> 1050 if (so->so_options & SO_ACCEPTCONN) {
> 1051 struct in_conninfo inc;
> 1052
> 1053 KASSERT(tp->t_state == TCPS_LISTEN, ("%s: so
> accepting but "
> 1054 "tp not listening", __func__));
> ...
> 1356 syncache_add(&inc, &to, th, inp, &so, m, NULL,
> NULL);
> 1357 /*
> 1358 * Entry added to syncache and mbuf consumed.
> 1359 * Everything already unlocked by syncache_add().
> 1360 */
> 1361 INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
> 1362 return;
> 1363 }
> ...
> 1384 /*
> 1385 * Segment belongs to a connection in SYN_SENT,
> ESTABLISHED or later
> 1386 * state. tcp_do_segment() always consumes the mbuf
> chain, unlocks
> 1387 * the inpcb, and unlocks pcbinfo.
> 1388 */
> 1389 tcp_do_segment(m, th, so, tp, drop_hdrlen, tlen, iptos,
> ti_locked);
>
>
> I think this has to do with this patch in soclose() where
> SO_ACCEPTCONN is being turned off in soclose(). I suspect if you look
> at the other threads in your corefile, you'll see one at this point in
> soclose() operating on the same socket as the one in the
> tcp_do_segment() thread.
>
>
> http://svnweb.freebsd.org/base?view=revision&revision=243627
>
> 817 /*
> 818 * Prevent new additions to the accept queues due
> 819 * to ACCEPT_LOCK races while we are draining
> them.
> 820 */
> 821 so->so_options &= ~SO_ACCEPTCONN;
> 822 while ((sp = TAILQ_FIRST(&so->so_incomp)) !=
> NULL) {
> 823 TAILQ_REMOVE(&so->so_incomp, sp,
> so_list);
> 824 so->so_incqlen--;
> 825 sp->so_qstate &= ~SQ_INCOMP;
> 826 sp->so_head = NULL;
> 827 ACCEPT_UNLOCK();
> 828 soabort(sp);
> 829 ACCEPT_LOCK();
> 830 }
>
>
> Juan had evaluated this code path and it seemed safe to just drop the
> packet in this case:
>
>
> + /*
> + * In closing down the socket, the SO_ACCEPTCONN flag is removed
> to
> + * prevent new connections from being established. This means
> that
> + * any frames in that were in the midst of being processed could
> + * make it here. Need to just drop the frame.
> + */
> + if (TCPS_LISTEN == tp->t_state) {
> + TCPSTAT_INC(tcps_rcvwhileclosing);
> + goto drop;
> + }
> KASSERT(tp->t_state > TCPS_LISTEN, ("%s: TCPS_LISTEN",
> __func__));
>
>
> Or, if there's someone more familiar with the locking in these paths,
> they may be able to come up with a way to restructure the locks and
> logic to close this window.
>
>
>
> -Matt
>
>
>
>
>
>
>
>
>
> On Thu, Apr 4, 2013 at 6:33 PM, Rick Macklem < rmacklem at uoguelph.ca >
> wrote:
>
>
> Hi,
>
> When pho@ was doing some NFS testing, he got the
> following crash, which I can't figure out. (As far
> as I can see, INP_WLOCK() is always held when
> tp->t_state = TCPS_CLOSED and it is held from before
> the test for TCPS_CLOSED in tcp_input() up until
> the tcp_do_segment() call. As such, I don't see how
> tp->t_state can be TCPS_CLOSED, but that seems to
> be what causes the panic?)
>
> The "umount -f" will result in:
> soshutdown(so, SHUT_WR);
> soclose(so);
> being done by the krpc on the socket.
>
> Anyone have any ideas on this?
> pho@ wrote:
> > I continued running the NFS tests and got this "panic:
> > tcp_do_segment:
> > TCPS_LISTEN". It's the second time I get this panic with the same
> > test
> > scenario, so it seems to be reproducible. The scenario is "umount
> > -f"
> > of a mount point that is very active.
> >
> > http://people.freebsd.org/~pho/stress/log/kostik555.txt
>
> Thanks in advance for any help, rick
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>
>
>
> --
> Juan Mojica
> Email: jmojica at gmail.com
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