[RFC PATCH v2 03/10] udp: add support for UDP_GRO cmsg

[Paolo Abeni <pabeni@redhat.com>]

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>
---
CHECK: should we use a separate setsockopt to explicitly enable
gso_size cmsg reception? So that user space can enable UDP_GRO and
fetch cmsg without forcefully receiving GRO related info.
---
 include/linux/udp.h | 11 +++++++++++
 net/ipv4/udp.c      |  4 ++++
 net/ipv6/udp.c      |  3 +++
 3 files changed, 18 insertions(+)

diff --git a/include/linux/udp.h b/include/linux/udp.h
index f613b329852e..e23d5024f42f 100644
--- a/include/linux/udp.h
+++ b/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)
 
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 3c277378814f..2331ac9de954 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1714,6 +1714,10 @@ int udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int noblock,
 		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);
 
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 8bb50ba32a6f..05f723b98cab 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -421,6 +421,9 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 		*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);
 
-- 
2.17.2