netdev - Re: tcp hang when socket fills up ?

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Date:   Wed, 18 Apr 2018 12:27:48 +0200 (CEST)
From:   Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
To:     Dominique Martinet <asmadeus@...ewreck.org>
cc:     Florian Westphal <fw@...len.de>, Michal Kubecek <mkubecek@...e.cz>,
        netdev@...r.kernel.org,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: tcp hang when socket fills up ?

On Wed, 18 Apr 2018, Dominique Martinet wrote:

> Dominique Martinet wrote on Wed, Apr 18, 2018:
> > Jozsef Kadlecsik wrote on Wed, Apr 18, 2018:
> > > Yes, the state transition is wrong for simultaneous open, because the 
> > > tcp_conntracks table is not (cannot be) smart enough. Could you verify the 
> > > next untested patch?
> > 
> > Thanks for the patch; I'll give it a try (probably won't make it today
> > so will report tomorrow)
> 
> Actually had time; I can confirm (added printks) we did get in that if 
> that was pointed at, and we no longer get there now. The connection no 
> longer gets in invalid state, so that looks like it nailed it.
>
> I'm now confused what this has to do with tcp_timestamp though, since 
> setting that off also seemed to work around the issue, but if we get 
> something like that in I'll be happy anyway.

Thanks for the testing! One more line is required, however: we have to get 
the assured bit set for the connection, see the new patch below.

The tcp_conntracks state table could be fixed with introducing a new 
state, but that part is exposed to userspace (ctnetlink) and ugly 
compatibility code would be required for backward compatibility.
 
diff --git a/include/uapi/linux/netfilter/nf_conntrack_tcp.h b/include/uapi/linux/netfilter/nf_conntrack_tcp.h
index 74b9115..bcba72d 100644
--- a/include/uapi/linux/netfilter/nf_conntrack_tcp.h
+++ b/include/uapi/linux/netfilter/nf_conntrack_tcp.h
@@ -46,6 +46,9 @@ enum tcp_conntrack {
 /* Marks possibility for expected RFC5961 challenge ACK */
 #define IP_CT_EXP_CHALLENGE_ACK 		0x40
 
+/* Simultaneous open initialized */
+#define IP_CT_TCP_SIMULTANEOUS_OPEN		0x80
+
 struct nf_ct_tcp_flags {
 	__u8 flags;
 	__u8 mask;
diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index e97cdc1..2c1fc7e 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -981,6 +981,20 @@ static int tcp_packet(struct nf_conn *ct,
 			return NF_ACCEPT; /* Don't change state */
 		}
 		break;
+	case TCP_CONNTRACK_SYN_SENT2:
+		/* tcp_conntracks table is not smart enough to handle
+		 * simultaneous open.
+		 */
+		ct->proto.tcp.last_flags |= IP_CT_TCP_SIMULTANEOUS_OPEN;
+		break;
+	case TCP_CONNTRACK_SYN_RECV:
+		if (dir == IP_CT_DIR_REPLY && index == TCP_ACK_SET &&
+		    ct->proto.tcp.last_flags & IP_CT_TCP_SIMULTANEOUS_OPEN) {
+			/* We want to set the assured bit */
+			old_state = TCP_CONNTRACK_SYN_RECV;
+			new_state = TCP_CONNTRACK_ESTABLISHED;
+		}
+		break;
 	case TCP_CONNTRACK_CLOSE:
 		if (index == TCP_RST_SET
 		    && (ct->proto.tcp.seen[!dir].flags & IP_CT_TCP_FLAG_MAXACK_SET)

Best regards,
Jozsef
-
E-mail  : kadlec@...ckhole.kfki.hu, kadlecsik.jozsef@...ner.mta.hu
PGP key : http://www.kfki.hu/~kadlec/pgp_public_key.txt
Address : Wigner Research Centre for Physics, Hungarian Academy of Sciences
          H-1525 Budapest 114, POB. 49, Hungary