net: Move mtu handling down to the protocol depended handlers

We move all mtu handling from dst_mtu() down to the protocol
layer. So each protocol can implement the mtu handling in
a different manner.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dst.h

@@ -205,12 +205,7 @@ dst_feature(const struct dst_entry *dst, u32 feature)
 
 static inline u32 dst_mtu(const struct dst_entry *dst)
 {
-	u32 mtu = dst_metric_raw(dst, RTAX_MTU);
-
-	if (!mtu)
-		mtu = dst->ops->mtu(dst);
-
-	return mtu;
+	return dst->ops->mtu(dst);
 }
 
 /* RTT metrics are stored in milliseconds for user ABI, but used as jiffies */

net/decnet/dn_route.c

@@ -827,7 +827,9 @@ static unsigned int dn_dst_default_advmss(const struct dst_entry *dst)
 
 static unsigned int dn_dst_mtu(const struct dst_entry *dst)
 {
-	return dst->dev->mtu;
+	unsigned int mtu = dst_metric_raw(dst, RTAX_MTU);
+
+	return mtu ? : dst->dev->mtu;
 }
 
 static struct neighbour *dn_dst_neigh_lookup(const struct dst_entry *dst, const void *daddr)

net/ipv4/route.c

@@ -1816,7 +1816,12 @@ static unsigned int ipv4_default_advmss(const struct dst_entry *dst)
 
 static unsigned int ipv4_mtu(const struct dst_entry *dst)
 {
-	unsigned int mtu = dst->dev->mtu;
+	unsigned int mtu = dst_metric_raw(dst, RTAX_MTU);
+
+	if (mtu)
+		return mtu;
+
+	mtu = dst->dev->mtu;
 
 	if (unlikely(dst_metric_locked(dst, RTAX_MTU))) {
 		const struct rtable *rt = (const struct rtable *) dst;
@@ -2757,7 +2762,9 @@ static struct dst_entry *ipv4_blackhole_dst_check(struct dst_entry *dst, u32 coo
 
 static unsigned int ipv4_blackhole_mtu(const struct dst_entry *dst)
 {
-	return dst->dev->mtu;
+	unsigned int mtu = dst_metric_raw(dst, RTAX_MTU);
+
+	return mtu ? : dst->dev->mtu;
 }
 
 static void ipv4_rt_blackhole_update_pmtu(struct dst_entry *dst, u32 mtu)

net/ipv6/route.c

@@ -157,7 +157,9 @@ static struct dst_ops ip6_dst_ops_template = {
 
 static unsigned int ip6_blackhole_mtu(const struct dst_entry *dst)
 {
-	return dst->dev->mtu;
+	unsigned int mtu = dst_metric_raw(dst, RTAX_MTU);
+
+	return mtu ? : dst->dev->mtu;
 }
 
 static void ip6_rt_blackhole_update_pmtu(struct dst_entry *dst, u32 mtu)
@@ -1043,8 +1045,13 @@ static unsigned int ip6_default_advmss(const struct dst_entry *dst)
 
 static unsigned int ip6_mtu(const struct dst_entry *dst)
 {
-	unsigned int mtu = IPV6_MIN_MTU;
 	struct inet6_dev *idev;
+	unsigned int mtu = dst_metric_raw(dst, RTAX_MTU);
+
+	if (mtu)
+		return mtu;
+
+	mtu = IPV6_MIN_MTU;
 
 	rcu_read_lock();
 	idev = __in6_dev_get(dst->dev);

net/xfrm/xfrm_policy.c

@@ -2384,7 +2384,9 @@ static unsigned int xfrm_default_advmss(const struct dst_entry *dst)
 
 static unsigned int xfrm_mtu(const struct dst_entry *dst)
 {
-	return dst_mtu(dst->path);
+	unsigned int mtu = dst_metric_raw(dst, RTAX_MTU);
+
+	return mtu ? : dst_mtu(dst->path);
 }
 
 static struct neighbour *xfrm_neigh_lookup(const struct dst_entry *dst, const void *daddr)