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 97041f24cd27..2d390669d406 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;