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, 7 insertions, 0 deletions

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 6e50ccd8c532..36936c8ae961 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -2361,6 +2361,11 @@ static int xfrm_bundle_ok(struct xfrm_policy *pol, struct xfrm_dst *first,
         return 1;
 }
 
+static unsigned int xfrm_default_advmss(const struct dst_entry *dst)
+{
+        return dst_metric_advmss(dst->path);
+}
+
 int xfrm_policy_register_afinfo(struct xfrm_policy_afinfo *afinfo)
 {
         struct net *net;
@@ -2378,6 +2383,8 @@ int xfrm_policy_register_afinfo(struct xfrm_policy_afinfo *afinfo)
                         dst_ops->kmem_cachep = xfrm_dst_cache;
                 if (likely(dst_ops->check == NULL))
                         dst_ops->check = xfrm_dst_check;
+                if (likely(dst_ops->default_advmss == NULL))
+                        dst_ops->default_advmss = xfrm_default_advmss;
                 if (likely(dst_ops->negative_advice == NULL))
                         dst_ops->negative_advice = xfrm_negative_advice;
                 if (likely(dst_ops->link_failure == NULL))