tcp: gro: add checksuming helpers

skb with CHECKSUM_NONE cant currently be handled by GRO, and
we notice this deep in GRO stack in tcp[46]_gro_receive()

But there are cases where GRO can be a benefit, even with a lack
of checksums.

This preliminary work is needed to add GRO support
to tunnels.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -2803,6 +2803,8 @@ void tcp4_proc_exit(void)
 struct sk_buff **tcp4_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 {
 	const struct iphdr *iph = skb_gro_network_header(skb);
+	__wsum wsum;
+	__sum16 sum;
 
 	switch (skb->ip_summed) {
 	case CHECKSUM_COMPLETE:
@@ -2811,11 +2813,22 @@ struct sk_buff **tcp4_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 			skb->ip_summed = CHECKSUM_UNNECESSARY;
 			break;
 		}
-
-		/* fall through */
-	case CHECKSUM_NONE:
+flush:
 		NAPI_GRO_CB(skb)->flush = 1;
 		return NULL;
+
+	case CHECKSUM_NONE:
+		wsum = csum_tcpudp_nofold(iph->saddr, iph->daddr,
+					  skb_gro_len(skb), IPPROTO_TCP, 0);
+		sum = csum_fold(skb_checksum(skb,
+					     skb_gro_offset(skb),
+					     skb_gro_len(skb),
+					     wsum));
+		if (sum)
+			goto flush;
+
+		skb->ip_summed = CHECKSUM_UNNECESSARY;
+		break;
 	}
 
 	return tcp_gro_receive(head, skb);

net/ipv6/tcp_ipv6.c

@@ -763,6 +763,8 @@ static struct sk_buff **tcp6_gro_receive(struct sk_buff **head,
 					 struct sk_buff *skb)
 {
 	const struct ipv6hdr *iph = skb_gro_network_header(skb);
+	__wsum wsum;
+	__sum16 sum;
 
 	switch (skb->ip_summed) {
 	case CHECKSUM_COMPLETE:
@@ -771,11 +773,23 @@ static struct sk_buff **tcp6_gro_receive(struct sk_buff **head,
 			skb->ip_summed = CHECKSUM_UNNECESSARY;
 			break;
 		}
-
-		/* fall through */
-	case CHECKSUM_NONE:
+flush:
 		NAPI_GRO_CB(skb)->flush = 1;
 		return NULL;
+
+	case CHECKSUM_NONE:
+		wsum = ~csum_unfold(csum_ipv6_magic(&iph->saddr, &iph->daddr,
+						    skb_gro_len(skb),
+						    IPPROTO_TCP, 0));
+		sum = csum_fold(skb_checksum(skb,
+					     skb_gro_offset(skb),
+					     skb_gro_len(skb),
+					     wsum));
+		if (sum)
+			goto flush;
+
+		skb->ip_summed = CHECKSUM_UNNECESSARY;
+		break;
 	}
 
 	return tcp_gro_receive(head, skb);