[UDP]: Return true length if user specifies MSG_TRUNC.

net/ipv4/udp.c

@@ -828,7 +828,10 @@ int udp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
   	}
 	if (inet->cmsg_flags)
 		ip_cmsg_recv(msg, skb);
+
 	err = copied;
+	if (flags & MSG_TRUNC)
+		err = skb->len - sizeof(struct udphdr);
   
 out_free:
   	skb_free_datagram(sk, skb);

net/ipv6/udp.c

@@ -436,7 +436,10 @@ static int udpv6_recvmsg(struct kiocb *iocb, struct sock *sk,
 				sin6->sin6_scope_id = IP6CB(skb)->iif;
 		}
   	}
+
 	err = copied;
+	if (flags & MSG_TRUNC)
+		err = skb->len - sizeof(struct udphdr);
 
 out_free:
 	skb_free_datagram(sk, skb);