udp: add support for UDP_GRO cmsg

When UDP GRO is enabled, the UDP_GRO cmsg will carry the ingress
datagram size. User-space can use such info to compute the original
packets layout.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/udp.h

@@ -121,6 +121,17 @@ static inline bool udp_get_no_check6_rx(struct sock *sk)
 	return udp_sk(sk)->no_check6_rx;
 }
 
+static inline void udp_cmsg_recv(struct msghdr *msg, struct sock *sk,
+				 struct sk_buff *skb)
+{
+	int gso_size;
+
+	if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) {
+		gso_size = skb_shinfo(skb)->gso_size;
+		put_cmsg(msg, SOL_UDP, UDP_GRO, sizeof(gso_size), &gso_size);
+	}
+}
+
 #define udp_portaddr_for_each_entry(__sk, list) \
 	hlist_for_each_entry(__sk, list, __sk_common.skc_portaddr_node)
 

net/ipv4/udp.c

@@ -1711,6 +1711,10 @@ try_again:
 		memset(sin->sin_zero, 0, sizeof(sin->sin_zero));
 		*addr_len = sizeof(*sin);
 	}
+
+	if (udp_sk(sk)->gro_enabled)
+		udp_cmsg_recv(msg, sk, skb);
+
 	if (inet->cmsg_flags)
 		ip_cmsg_recv_offset(msg, sk, skb, sizeof(struct udphdr), off);
 

net/ipv6/udp.c

@@ -417,6 +417,9 @@ try_again:
 		*addr_len = sizeof(*sin6);
 	}
 
+	if (udp_sk(sk)->gro_enabled)
+		udp_cmsg_recv(msg, sk, skb);
+
 	if (np->rxopt.all)
 		ip6_datagram_recv_common_ctl(sk, msg, skb);