gro: Fix COMPLETE checksum handling

On a brand new GRO skb, we cannot call ip_hdr since the header
may lie in the non-linear area.  This patch adds the helper
skb_gro_network_header to handle this.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1142,6 +1142,13 @@ static inline void *skb_gro_mac_header(struct sk_buff *skb)
 	       skb_shinfo(skb)->frags[0].page_offset;
 }
 
+static inline void *skb_gro_network_header(struct sk_buff *skb)
+{
+	return skb_headlen(skb) ? skb_network_header(skb) :
+	       page_address(skb_shinfo(skb)->frags[0].page) +
+	       skb_shinfo(skb)->frags[0].page_offset + skb_network_offset(skb);
+}
+
 static inline int dev_hard_header(struct sk_buff *skb, struct net_device *dev,
 				  unsigned short type,
 				  const void *daddr, const void *saddr,

net/ipv4/tcp_ipv4.c

@@ -2343,7 +2343,7 @@ void tcp4_proc_exit(void)
 
 struct sk_buff **tcp4_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 {
-	struct iphdr *iph = ip_hdr(skb);
+	struct iphdr *iph = skb_gro_network_header(skb);
 
 	switch (skb->ip_summed) {
 	case CHECKSUM_COMPLETE:

net/ipv6/tcp_ipv6.c

@@ -943,7 +943,7 @@ static int tcp_v6_gso_send_check(struct sk_buff *skb)
 
 struct sk_buff **tcp6_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 {
-	struct ipv6hdr *iph = ipv6_hdr(skb);
+	struct ipv6hdr *iph = skb_gro_network_header(skb);
 
 	switch (skb->ip_summed) {
 	case CHECKSUM_COMPLETE: