gre: Add support for checksum unnecessary conversions

Call skb_checksum_try_convert and skb_gro_checksum_try_convert
after checksum is found present and validated in the GRE header
for normal and GRO paths respectively.

In GRO path, call skb_gro_checksum_try_convert
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/gre_demux.c

@@ -125,6 +125,10 @@ static int parse_gre_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 			*csum_err = true;
 			return -EINVAL;
 		}
+
+		skb_checksum_try_convert(skb, IPPROTO_GRE, 0,
+					 null_compute_pseudo);
+
 		options++;
 	}
 

net/ipv4/gre_offload.c

@@ -172,10 +172,14 @@ static struct sk_buff **gre_gro_receive(struct sk_buff **head,
 	}
 
 	/* Don't bother verifying checksum if we're going to flush anyway. */
-	if ((greh->flags & GRE_CSUM) && !NAPI_GRO_CB(skb)->flush &&
-	    skb_gro_checksum_simple_validate(skb))
+	if ((greh->flags & GRE_CSUM) && !NAPI_GRO_CB(skb)->flush) {
+		if (skb_gro_checksum_simple_validate(skb))
 			goto out_unlock;
 
+		skb_gro_checksum_try_convert(skb, IPPROTO_GRE, 0,
+					     null_compute_pseudo);
+	}
+
 	flush = 0;
 
 	for (p = *head; p; p = p->next) {