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, 13 insertions, 1 deletions

diff --git a/include/net/dst.h b/include/net/dst.h
index 755ac6c..03a1c3d 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -112,10 +112,22 @@ dst_metric_raw(const struct dst_entry *dst, const int metric)
 static inline u32
 dst_metric(const struct dst_entry *dst, const int metric)
 {
-	WARN_ON_ONCE(metric == RTAX_HOPLIMIT);
+	WARN_ON_ONCE(metric == RTAX_HOPLIMIT ||
+		     metric == RTAX_ADVMSS);
 	return dst_metric_raw(dst, metric);
 }
 
+static inline u32
+dst_metric_advmss(const struct dst_entry *dst)
+{
+	u32 advmss = dst_metric_raw(dst, RTAX_ADVMSS);
+
+	if (!advmss)
+		advmss = dst->ops->default_advmss(dst);
+
+	return advmss;
+}
+
 static inline void dst_metric_set(struct dst_entry *dst, int metric, u32 val)
 {
 	dst->_metrics[metric-1] = val;