tcp: use dctcp if enabled on the route to the initiator

Currently, the following case doesn't use DCTCP, even if it should:
A responder has f.e. Cubic as system wide default, but for a specific
route to the initiating host, DCTCP is being set in RTAX_CC_ALGO. The
initiating host then uses DCTCP as congestion control, but since the
initiator sets ECT(0), tcp_ecn_create_request() doesn't set ecn_ok,
and we have to fall back to Reno after 3WHS completes.

We were thinking on how to solve this in a minimal, non-intrusive
way without bloating tcp_ecn_create_request() needlessly: lets cache
the CA ecn option flag in RTAX_FEATURES. In other words, when ECT(0)
is set on the SYN packet, set ecn_ok=1 iff route RTAX_FEATURES
contains the unexposed (internal-only) DST_FEATURE_ECN_CA. This allows
to only do a single metric feature lookup inside tcp_ecn_create_request().

Joint work with Florian Westphal.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 16 insertions, 6 deletions

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 115a08e70d43..992a9597daf8 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -879,6 +879,7 @@ static bool fib_valid_prefsrc(struct fib_config *cfg, __be32 fib_prefsrc)
 static int
 fib_convert_metrics(struct fib_info *fi, const struct fib_config *cfg)
 {
+	bool ecn_ca = false;
 	struct nlattr *nla;
 	int remaining;
 
@@ -898,7 +899,7 @@ fib_convert_metrics(struct fib_info *fi, const struct fib_config *cfg)
 			char tmp[TCP_CA_NAME_MAX];
 
 			nla_strlcpy(tmp, nla, sizeof(tmp));
-			val = tcp_ca_get_key_by_name(tmp);
+			val = tcp_ca_get_key_by_name(tmp, &ecn_ca);
 			if (val == TCP_CA_UNSPEC)
 				return -EINVAL;
 		} else {
@@ -913,6 +914,9 @@ fib_convert_metrics(struct fib_info *fi, const struct fib_config *cfg)
 		fi->fib_metrics[type - 1] = val;
 	}
 
+	if (ecn_ca)
+		fi->fib_metrics[RTAX_FEATURES - 1] |= DST_FEATURE_ECN_CA;
+
 	return 0;
 }
 
diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index a2ed23c595cf..93c4dc3ab23f 100644
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -114,16 +114,19 @@ void tcp_unregister_congestion_control(struct tcp_congestion_ops *ca)
 }
 EXPORT_SYMBOL_GPL(tcp_unregister_congestion_control);
 
-u32 tcp_ca_get_key_by_name(const char *name)
+u32 tcp_ca_get_key_by_name(const char *name, bool *ecn_ca)
 {
 	const struct tcp_congestion_ops *ca;
-	u32 key;
+	u32 key = TCP_CA_UNSPEC;
 
 	might_sleep();
 
 	rcu_read_lock();
 	ca = __tcp_ca_find_autoload(name);
-	key = ca ? ca->key : TCP_CA_UNSPEC;
+	if (ca) {
+		key = ca->key;
+		*ecn_ca = ca->flags & TCP_CONG_NEEDS_ECN;
+	}
 	rcu_read_unlock();
 
 	return key;
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index dc08e2352665..a8f515bb19c4 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6003,14 +6003,17 @@ static void tcp_ecn_create_request(struct request_sock *req,
 	const struct net *net = sock_net(listen_sk);
 	bool th_ecn = th->ece && th->cwr;
 	bool ect, ecn_ok;
+	u32 ecn_ok_dst;
 
 	if (!th_ecn)
 		return;
 
 	ect = !INET_ECN_is_not_ect(TCP_SKB_CB(skb)->ip_dsfield);
-	ecn_ok = net->ipv4.sysctl_tcp_ecn || dst_feature(dst, RTAX_FEATURE_ECN);
+	ecn_ok_dst = dst_feature(dst, DST_FEATURE_ECN_MASK);
+	ecn_ok = net->ipv4.sysctl_tcp_ecn || ecn_ok_dst;
 
-	if ((!ect && ecn_ok) || tcp_ca_needs_ecn(listen_sk))
+	if ((!ect && ecn_ok) || tcp_ca_needs_ecn(listen_sk) ||
+	    (ecn_ok_dst & DST_FEATURE_ECN_CA))
 		inet_rsk(req)->ecn_ok = 1;
 }