udp: Fix the SNMP counter of UDP_MIB_INDATAGRAMS

If UDP echo is sent to xinetd/echo-dgram, the UDP reply will be received
at the sender. But the SNMP counter of UDP_MIB_INDATAGRAMS will be not
increased, UDP6_MIB_INDATAGRAMS will be increased instead.

  Endpoint A                      Endpoint B
  UDP Echo request ----------->
  (IPv4, Dst port=7)
                   <----------    UDP Echo Reply
                                  (IPv4, Src port=7)

This bug is come from this patch cb75994e.

It do counter UDP[6]_MIB_INDATAGRAMS until udp[v6]_recvmsg. Because
xinetd used IPv6 socket to receive UDP messages, thus, when received
UDP packet, the UDP6_MIB_INDATAGRAMS will be increased in function
udpv6_recvmsg() even if the packet is a IPv4 UDP packet.

This patch fixed the problem.
Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/udp.c

@@ -138,6 +138,7 @@ int udpv6_recvmsg(struct kiocb *iocb, struct sock *sk,
 	int peeked;
 	int err;
 	int is_udplite = IS_UDPLITE(sk);
+	int is_udp4;
 
 	if (addr_len)
 		*addr_len=sizeof(struct sockaddr_in6);
@@ -158,6 +159,8 @@ int udpv6_recvmsg(struct kiocb *iocb, struct sock *sk,
 	else if (copied < ulen)
 		msg->msg_flags |= MSG_TRUNC;
 
+	is_udp4 = (skb->protocol == htons(ETH_P_IP));
+
 	/*
 	 * If checksum is needed at all, try to do it while copying the
 	 * data.  If the data is truncated, or if we only want a partial
@@ -180,9 +183,14 @@ int udpv6_recvmsg(struct kiocb *iocb, struct sock *sk,
 	if (err)
 		goto out_free;
 
-	if (!peeked)
+	if (!peeked) {
+		if (is_udp4)
+			UDP_INC_STATS_USER(sock_net(sk),
+					UDP_MIB_INDATAGRAMS, is_udplite);
+		else
 			UDP6_INC_STATS_USER(sock_net(sk),
 					UDP_MIB_INDATAGRAMS, is_udplite);
+	}
 
 	sock_recv_timestamp(msg, sk, skb);
 
@@ -196,7 +204,7 @@ int udpv6_recvmsg(struct kiocb *iocb, struct sock *sk,
 		sin6->sin6_flowinfo = 0;
 		sin6->sin6_scope_id = 0;
 
-		if (skb->protocol == htons(ETH_P_IP))
+		if (is_udp4)
 			ipv6_addr_set(&sin6->sin6_addr, 0, 0,
 				      htonl(0xffff), ip_hdr(skb)->saddr);
 		else {
@@ -207,7 +215,7 @@ int udpv6_recvmsg(struct kiocb *iocb, struct sock *sk,
 		}
 
 	}
-	if (skb->protocol == htons(ETH_P_IP)) {
+	if (is_udp4) {
 		if (inet->cmsg_flags)
 			ip_cmsg_recv(msg, skb);
 	} else {