net: Abstract default ADVMSS behind an accessor.

Make all RTAX_ADVMSS metric accesses go through a new helper function,
dst_metric_advmss().

Leave the actual default metric as "zero" in the real metric slot,
and compute the actual default value dynamically via a new dst_ops
AF specific callback.

For stacked IPSEC routes, we use the advmss of the path which
preserves existing behavior.

Unlike ipv4/ipv6, DecNET ties the advmss to the mtu and thus updates
advmss on pmtu updates.  This inconsistency in advmss handling
results in more raw metric accesses than I wish we ended up with.

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 5 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 97041f24cd2..2d390669d40 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -119,9 +119,13 @@ static __u16 tcp_advertise_mss(struct sock *sk)
 	struct dst_entry *dst = __sk_dst_get(sk);
 	int mss = tp->advmss;
 
-	if (dst && dst_metric(dst, RTAX_ADVMSS) < mss) {
-		mss = dst_metric(dst, RTAX_ADVMSS);
-		tp->advmss = mss;
+	if (dst) {
+		unsigned int metric = dst_metric_advmss(dst);
+
+		if (metric < mss) {
+			mss = metric;
+			tp->advmss = mss;
+		}
 	}
 
 	return (__u16)mss;
@@ -2422,7 +2426,7 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
 
 	skb_dst_set(skb, dst_clone(dst));
 
-	mss = dst_metric(dst, RTAX_ADVMSS);
+	mss = dst_metric_advmss(dst);
 	if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss)
 		mss = tp->rx_opt.user_mss;
 
@@ -2556,7 +2560,7 @@ static void tcp_connect_init(struct sock *sk)
 
 	if (!tp->window_clamp)
 		tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
-	tp->advmss = dst_metric(dst, RTAX_ADVMSS);
+	tp->advmss = dst_metric_advmss(dst);
 	if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->advmss)
 		tp->advmss = tp->rx_opt.user_mss;