udp6: set rx_dst_cookie on rx_dst updates

Currently, in the udp6 code, the dst cookie is not initialized/updated
concurrently with the RX dst used by early demux.

As a result, the dst_check() in the early_demux path always fails,
the rx dst cache is always invalidated, and we can't really
leverage significant gain from the demux lookup.

Fix it adding udp6 specific variant of sk_rx_dst_set() and use it
to set the dst cookie when the dst entry is really changed.

The issue is there since the introduction of early demux for ipv6.

Fixes: 5425077d73 ("net: ipv6: Add early demux handler for UDP unicast")
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/udp.h

@@ -260,7 +260,7 @@ static inline struct sk_buff *skb_recv_udp(struct sock *sk, unsigned int flags,
 }
 
 void udp_v4_early_demux(struct sk_buff *skb);
-void udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst);
+bool udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst);
 int udp_get_port(struct sock *sk, unsigned short snum,
 		 int (*saddr_cmp)(const struct sock *,
 				  const struct sock *));

net/ipv4/udp.c

@@ -1929,14 +1929,16 @@ drop:
 /* For TCP sockets, sk_rx_dst is protected by socket lock
  * For UDP, we use xchg() to guard against concurrent changes.
  */
-void udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst)
+bool udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst)
 {
 	struct dst_entry *old;
 
 	if (dst_hold_safe(dst)) {
 		old = xchg(&sk->sk_rx_dst, dst);
 		dst_release(old);
+		return old != dst;
 	}
+	return false;
 }
 EXPORT_SYMBOL(udp_sk_rx_dst_set);
 

net/ipv6/udp.c

@@ -768,6 +768,15 @@ start_lookup:
 	return 0;
 }
 
+static void udp6_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst)
+{
+	if (udp_sk_rx_dst_set(sk, dst)) {
+		const struct rt6_info *rt = (const struct rt6_info *)dst;
+
+		inet6_sk(sk)->rx_dst_cookie = rt6_get_cookie(rt);
+	}
+}
+
 int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 		   int proto)
 {
@@ -817,7 +826,7 @@ int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 		int ret;
 
 		if (unlikely(sk->sk_rx_dst != dst))
-			udp_sk_rx_dst_set(sk, dst);
+			udp6_sk_rx_dst_set(sk, dst);
 
 		ret = udpv6_queue_rcv_skb(sk, skb);
 		sock_put(sk);