ipv4: Add helper inet_csk_update_pmtu().

This abstracts away the call to dst_ops->update_pmtu() so that we can
transparently handle the fact that, in the future, the dst itself can
be invalidated by the PMTU update (when we have non-host routes cached
in sockets).

So we try to rebuild the socket cached route after the method
invocation if necessary.

This isn't used by SCTP because it needs to cache dsts per-transport,
and thus will need it's own local version of this helper.

Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 48 insertions, 9 deletions

diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 76825be3b643..200d21809379 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -803,3 +803,49 @@ int inet_csk_compat_setsockopt(struct sock *sk, int level, int optname,
 }
 EXPORT_SYMBOL_GPL(inet_csk_compat_setsockopt);
 #endif
+
+static struct dst_entry *inet_csk_rebuild_route(struct sock *sk, struct flowi *fl)
+{
+        struct inet_sock *inet = inet_sk(sk);
+        struct ip_options_rcu *inet_opt;
+        __be32 daddr = inet->inet_daddr;
+        struct flowi4 *fl4;
+        struct rtable *rt;
+
+        rcu_read_lock();
+        inet_opt = rcu_dereference(inet->inet_opt);
+        if (inet_opt && inet_opt->opt.srr)
+                daddr = inet_opt->opt.faddr;
+        fl4 = &fl->u.ip4;
+        rt = ip_route_output_ports(sock_net(sk), fl4, sk, daddr,
+                                   inet->inet_saddr, inet->inet_dport,
+                                   inet->inet_sport, sk->sk_protocol,
+                                   RT_CONN_FLAGS(sk), sk->sk_bound_dev_if);
+        if (IS_ERR(rt))
+                rt = NULL;
+        if (rt)
+                sk_setup_caps(sk, &rt->dst);
+        rcu_read_unlock();
+
+        return &rt->dst;
+}
+
+struct dst_entry *inet_csk_update_pmtu(struct sock *sk, u32 mtu)
+{
+        struct dst_entry *dst = __sk_dst_check(sk, 0);
+        struct inet_sock *inet = inet_sk(sk);
+
+        if (!dst) {
+                dst = inet_csk_rebuild_route(sk, &inet->cork.fl);
+                if (!dst)
+                        goto out;
+        }
+        dst->ops->update_pmtu(dst, mtu);
+
+        dst = __sk_dst_check(sk, 0);
+        if (!dst)
+                dst = inet_csk_rebuild_route(sk, &inet->cork.fl);
+out:
+        return dst;
+}
+EXPORT_SYMBOL_GPL(inet_csk_update_pmtu);
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 7a0062cb4ed0..b8e7e0595407 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -289,17 +289,10 @@ static void do_pmtu_discovery(struct sock *sk, const struct iphdr *iph, u32 mtu)
         if (sk->sk_state == TCP_LISTEN)
                 return;
 
-        /* We don't check in the destentry if pmtu discovery is forbidden
-         * on this route. We just assume that no packet_to_big packets
-         * are send back when pmtu discovery is not active.
-         * There is a small race when the user changes this flag in the
-         * route, but I think that's acceptable.
-         */
-        if ((dst = __sk_dst_check(sk, 0)) == NULL)
+        dst = inet_csk_update_pmtu(sk, mtu);
+        if (!dst)
                 return;
 
-        dst->ops->update_pmtu(dst, mtu);
-
         /* Something is about to be wrong... Remember soft error
          * for the case, if this connection will not able to recover.
          */