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, 17 insertions, 7 deletions

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 770f70427f0b..80997333db0c 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -139,6 +139,7 @@ static unsigned long expires_ljiffies;
  */
 
 static struct dst_entry *ipv4_dst_check(struct dst_entry *dst, u32 cookie);
+static unsigned int      ipv4_default_advmss(const struct dst_entry *dst);
 static void              ipv4_dst_destroy(struct dst_entry *dst);
 static struct dst_entry *ipv4_negative_advice(struct dst_entry *dst);
 static void              ipv4_link_failure(struct sk_buff *skb);
@@ -155,6 +156,7 @@ static struct dst_ops ipv4_dst_ops = {
         .protocol =             cpu_to_be16(ETH_P_IP),
         .gc =                   rt_garbage_collect,
         .check =                ipv4_dst_check,
+        .default_advmss =       ipv4_default_advmss,
         .destroy =              ipv4_dst_destroy,
         .ifdown =               ipv4_dst_ifdown,
         .negative_advice =      ipv4_negative_advice,
@@ -383,8 +385,7 @@ static int rt_cache_seq_show(struct seq_file *seq, void *v)
                         (__force u32)r->rt_gateway,
                         r->rt_flags, atomic_read(&r->dst.__refcnt),
                         r->dst.__use, 0, (__force u32)r->rt_src,
-                        (dst_metric(&r->dst, RTAX_ADVMSS) ?
-                             (int)dst_metric(&r->dst, RTAX_ADVMSS) + 40 : 0),
+                        dst_metric_advmss(&r->dst) + 40,
                         dst_metric(&r->dst, RTAX_WINDOW),
                         (int)((dst_metric(&r->dst, RTAX_RTT) >> 3) +
                               dst_metric(&r->dst, RTAX_RTTVAR)),
@@ -1798,6 +1799,19 @@ static void set_class_tag(struct rtable *rt, u32 tag)
 }
 #endif
 
+static unsigned int ipv4_default_advmss(const struct dst_entry *dst)
+{
+        unsigned int advmss = dst_metric_raw(dst, RTAX_ADVMSS);
+
+        if (advmss == 0) {
+                advmss = max_t(unsigned int, dst->dev->mtu - 40,
+                               ip_rt_min_advmss);
+                if (advmss > 65535 - 40)
+                        advmss = 65535 - 40;
+        }
+        return advmss;
+}
+
 static void rt_set_nexthop(struct rtable *rt, struct fib_result *res, u32 itag)
 {
         struct dst_entry *dst = &rt->dst;
@@ -1823,11 +1837,7 @@ static void rt_set_nexthop(struct rtable *rt, struct fib_result *res, u32 itag)
 
         if (dst_mtu(dst) > IP_MAX_MTU)
                 dst_metric_set(dst, RTAX_MTU, IP_MAX_MTU);
-        if (dst_metric(dst, RTAX_ADVMSS) == 0)
-                dst_metric_set(dst, RTAX_ADVMSS,
-                               max_t(unsigned int, dst->dev->mtu - 40,
-                                     ip_rt_min_advmss));
-        if (dst_metric(dst, RTAX_ADVMSS) > 65535 - 40)
+        if (dst_metric_raw(dst, RTAX_ADVMSS) > 65535 - 40)
                 dst_metric_set(dst, RTAX_ADVMSS, 65535 - 40);
 
 #ifdef CONFIG_NET_CLS_ROUTE