[net PATCH] tcp: Set ECT0 bit in tos/tclass for synack when BPF needs ECN

[net PATCH] tcp: Set ECT0 bit in tos/tclass for synack when BPF needs ECN

[Alexander Duyck]

From: Alexander Duyck <alexanderduyck@fb.com>

When a BPF program is used to select between a type of TCP congestion
control algorithm that uses either ECN or not there is a case where the
synack for the frame was coming up without the ECT0 bit set. A bit of
research found that this was due to the final socket being configured to
dctcp while the listener socket was staying in cubic.

To reproduce it all that is needed is to monitor TCP traffic while running
the sample bpf program "samples/bpf/tcp_cong_kern.c". What is observed,
assuming tcp_dctcp module is loaded or compiled in and the traffic matches
the rules in the sample file, is that for all frames with the exception of
the synack the ECT0 bit is set.

To address that it is necessary to make one additional call to
tcp_bpf_ca_needs_ecn using the request socket and then use the output of
that to set the ECT0 bit for the tos/tclass of the packet.

Fixes: 91b5b21c7c16 ("bpf: Add support for changing congestion control")
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
---
 net/ipv4/tcp_ipv4.c |   12 ++++++++----
 net/ipv6/tcp_ipv6.c |    9 +++++++--
 2 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index c5f8b686aa82..d20d779b18f3 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -980,13 +980,17 @@ static int tcp_v4_send_synack(const struct sock *sk, struct dst_entry *dst,
 
 	skb = tcp_make_synack(sk, dst, req, foc, synack_type, syn_skb);
 
-	tos = sock_net(sk)->ipv4.sysctl_tcp_reflect_tos ?
-			tcp_rsk(req)->syn_tos & ~INET_ECN_MASK :
-			inet_sk(sk)->tos;
-
 	if (skb) {
 		__tcp_v4_send_check(skb, ireq->ir_loc_addr, ireq->ir_rmt_addr);
 
+		tos = sock_net(sk)->ipv4.sysctl_tcp_reflect_tos ?
+				tcp_rsk(req)->syn_tos & ~INET_ECN_MASK :
+				inet_sk(sk)->tos;
+
+		if (!INET_ECN_is_capable(tos) &&
+		    tcp_bpf_ca_needs_ecn((struct sock *)req))
+			tos |= INET_ECN_ECT_0;
+
 		rcu_read_lock();
 		err = ip_build_and_send_pkt(skb, sk, ireq->ir_loc_addr,
 					    ireq->ir_rmt_addr,
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 3d49e8d0afee..e2b360e9bec4 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -527,11 +527,16 @@ static int tcp_v6_send_synack(const struct sock *sk, struct dst_entry *dst,
 		if (np->repflow && ireq->pktopts)
 			fl6->flowlabel = ip6_flowlabel(ipv6_hdr(ireq->pktopts));
 
-		rcu_read_lock();
-		opt = ireq->ipv6_opt;
 		tclass = sock_net(sk)->ipv4.sysctl_tcp_reflect_tos ?
 				tcp_rsk(req)->syn_tos & ~INET_ECN_MASK :
 				np->tclass;
+
+		if (!INET_ECN_is_capable(tclass) &&
+		    tcp_bpf_ca_needs_ecn((struct sock *)req))
+			tclass |= INET_ECN_ECT_0;
+
+		rcu_read_lock();
+		opt = ireq->ipv6_opt;
 		if (!opt)
 			opt = rcu_dereference(np->opt);
 		err = ip6_xmit(sk, skb, fl6, sk->sk_mark, opt,

Re: [net PATCH] tcp: Set ECT0 bit in tos/tclass for synack when BPF needs ECN

[Jakub Kicinski]

On Fri, 20 Nov 2020 19:47:44 -0800 Alexander Duyck wrote:
> From: Alexander Duyck <alexanderduyck@fb.com>
> 
> When a BPF program is used to select between a type of TCP congestion
> control algorithm that uses either ECN or not there is a case where the
> synack for the frame was coming up without the ECT0 bit set. A bit of
> research found that this was due to the final socket being configured to
> dctcp while the listener socket was staying in cubic.
> 
> To reproduce it all that is needed is to monitor TCP traffic while running
> the sample bpf program "samples/bpf/tcp_cong_kern.c". What is observed,
> assuming tcp_dctcp module is loaded or compiled in and the traffic matches
> the rules in the sample file, is that for all frames with the exception of
> the synack the ECT0 bit is set.
> 
> To address that it is necessary to make one additional call to
> tcp_bpf_ca_needs_ecn using the request socket and then use the output of
> that to set the ECT0 bit for the tos/tclass of the packet.
> 
> Fixes: 91b5b21c7c16 ("bpf: Add support for changing congestion control")
> Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>

Applied, thank you!