udp4: fix IP_CMSG_CHECKSUM for connected sockets

commit 2abb7cd ("udp: Add support for doing checksum
unnecessary conversion") left out the early demux path for
connected sockets. As a result IP_CMSG_CHECKSUM gives wrong
values for such socket when GRO is not enabled/available.

This change addresses the issue by moving the csum conversion to a
common helper and using such helper in both the default and the
early demux rx path.

Fixes: 2abb7cd ("udp: Add support for doing checksum unnecessary conversion")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -2124,6 +2124,28 @@ static inline int udp4_csum_init(struct sk_buff *skb, struct udphdr *uh,
 							 inet_compute_pseudo);
 }
 
+/* wrapper for udp_queue_rcv_skb tacking care of csum conversion and
+ * return code conversion for ip layer consumption
+ */
+static int udp_unicast_rcv_skb(struct sock *sk, struct sk_buff *skb,
+			       struct udphdr *uh)
+{
+	int ret;
+
+	if (inet_get_convert_csum(sk) && uh->check && !IS_UDPLITE(sk))
+		skb_checksum_try_convert(skb, IPPROTO_UDP, uh->check,
+					 inet_compute_pseudo);
+
+	ret = udp_queue_rcv_skb(sk, skb);
+
+	/* a return value > 0 means to resubmit the input, but
+	 * it wants the return to be -protocol, or 0
+	 */
+	if (ret > 0)
+		return -ret;
+	return 0;
+}
+
 /*
  *	All we need to do is get the socket, and then do a checksum.
  */
@@ -2170,37 +2192,18 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 		if (unlikely(sk->sk_rx_dst != dst))
 			udp_sk_rx_dst_set(sk, dst);
 
-		ret = udp_queue_rcv_skb(sk, skb);
+		ret = udp_unicast_rcv_skb(sk, skb, uh);
 		sock_put(sk);
-		/* a return value > 0 means to resubmit the input, but
-		 * it wants the return to be -protocol, or 0
-		 */
-		if (ret > 0)
-			return -ret;
-		return 0;
+		return ret;
 	}
 
 	if (rt->rt_flags & (RTCF_BROADCAST|RTCF_MULTICAST))
 		return __udp4_lib_mcast_deliver(net, skb, uh,
 						saddr, daddr, udptable, proto);
 
 	sk = __udp4_lib_lookup_skb(skb, uh->source, uh->dest, udptable);
-	if (sk) {
-		int ret;
-
-		if (inet_get_convert_csum(sk) && uh->check && !IS_UDPLITE(sk))
-			skb_checksum_try_convert(skb, IPPROTO_UDP, uh->check,
-						 inet_compute_pseudo);
-
-		ret = udp_queue_rcv_skb(sk, skb);
-
-		/* a return value > 0 means to resubmit the input, but
-		 * it wants the return to be -protocol, or 0
-		 */
-		if (ret > 0)
-			return -ret;
-		return 0;
-	}
+	if (sk)
+		return udp_unicast_rcv_skb(sk, skb, uh);
 
 	if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb))
 		goto drop;