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, 0 deletions

diff --git a/include/net/dst_ops.h b/include/net/dst_ops.h
index 51665b3461b8..15fb7af08c42 100644
--- a/include/net/dst_ops.h
+++ b/include/net/dst_ops.h
@@ -16,6 +16,7 @@ struct dst_ops {
 
         int                     (*gc)(struct dst_ops *ops);
         struct dst_entry *      (*check)(struct dst_entry *, __u32 cookie);
+        unsigned int            (*default_advmss)(const struct dst_entry *);
         void                    (*destroy)(struct dst_entry *);
         void                    (*ifdown)(struct dst_entry *,
                                           struct net_device *dev, int how);