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, 1 insertions, 1 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 4fc3387aa994..f4011027543d 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1436,7 +1436,7 @@ struct sock *tcp_v4_syn_recv_sock(struct sock *sk, struct sk_buff *skb,
 
         tcp_mtup_init(newsk);
         tcp_sync_mss(newsk, dst_mtu(dst));
-        newtp->advmss = dst_metric(dst, RTAX_ADVMSS);
+        newtp->advmss = dst_metric_advmss(dst);
         if (tcp_sk(sk)->rx_opt.user_mss &&
             tcp_sk(sk)->rx_opt.user_mss < newtp->advmss)
                 newtp->advmss = tcp_sk(sk)->rx_opt.user_mss;