gre: Use GSO flags to determine csum need instead of GRE flags

This patch updates the gre checksum path to follow something much closer to
the UDP checksum path.  By doing this we can avoid needing to do as much
header inspection and can just make use of the fields we were already
reading in the sk_buff structure.
Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/gre_offload.c

@@ -18,14 +18,14 @@
 static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 				       netdev_features_t features)
 {
+	int tnl_hlen = skb_inner_mac_header(skb) - skb_transport_header(skb);
 	struct sk_buff *segs = ERR_PTR(-EINVAL);
-	int ghl;
 	struct gre_base_hdr *greh;
 	u16 mac_offset = skb->mac_header;
-	int mac_len = skb->mac_len;
 	__be16 protocol = skb->protocol;
-	int tnl_hlen;
-	bool csum;
+	u16 mac_len = skb->mac_len;
+	int gre_offset, outer_hlen;
+	bool need_csum;
 
 	if (unlikely(skb_shinfo(skb)->gso_type &
 				~(SKB_GSO_TCPV4 |
@@ -42,64 +42,60 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 	if (!skb->encapsulation)
 		goto out;
 
-	if (unlikely(!pskb_may_pull(skb, sizeof(*greh))))
+	if (unlikely(tnl_hlen < sizeof(struct gre_base_hdr)))
 		goto out;
 
-	greh = (struct gre_base_hdr *)skb_transport_header(skb);
-
-	ghl = skb_inner_mac_header(skb) - skb_transport_header(skb);
-	if (unlikely(ghl < sizeof(*greh)))
+	if (unlikely(!pskb_may_pull(skb, tnl_hlen)))
 		goto out;
 
-	csum = !!(greh->flags & GRE_CSUM);
-	if (csum)
-		skb->encap_hdr_csum = 1;
+	greh = (struct gre_base_hdr *)skb_transport_header(skb);
 
 	/* setup inner skb. */
 	skb->protocol = greh->protocol;
 	skb->encapsulation = 0;
-
-	if (unlikely(!pskb_may_pull(skb, ghl)))
-		goto out;
-
-	__skb_pull(skb, ghl);
+	__skb_pull(skb, tnl_hlen);
 	skb_reset_mac_header(skb);
 	skb_set_network_header(skb, skb_inner_network_offset(skb));
 	skb->mac_len = skb_inner_network_offset(skb);
 
+	need_csum = !!(skb_shinfo(skb)->gso_type & SKB_GSO_GRE_CSUM);
+	skb->encap_hdr_csum = need_csum;
+
 	features &= skb->dev->hw_enc_features;
 
 	/* segment inner packet. */
 	segs = skb_mac_gso_segment(skb, features);
 	if (IS_ERR_OR_NULL(segs)) {
-		skb_gso_error_unwind(skb, protocol, ghl, mac_offset, mac_len);
+		skb_gso_error_unwind(skb, protocol, tnl_hlen, mac_offset,
+				     mac_len);
 		goto out;
 	}
 
+	outer_hlen = skb_tnl_header_len(skb);
+	gre_offset = outer_hlen - tnl_hlen;
 	skb = segs;
-	tnl_hlen = skb_tnl_header_len(skb);
 	do {
-		__skb_push(skb, ghl);
-		if (csum) {
-			__be32 *pcsum;
-
-			skb_reset_transport_header(skb);
-
-			greh = (struct gre_base_hdr *)
-			    skb_transport_header(skb);
-			pcsum = (__be32 *)(greh + 1);
-			*pcsum = 0;
-			*(__sum16 *)pcsum = gso_make_checksum(skb, 0);
-		}
-		__skb_push(skb, tnl_hlen - ghl);
+		__be32 *pcsum;
 
 		skb_reset_inner_headers(skb);
 		skb->encapsulation = 1;
 
-		skb_reset_mac_header(skb);
-		skb_set_network_header(skb, mac_len);
 		skb->mac_len = mac_len;
 		skb->protocol = protocol;
+
+		__skb_push(skb, outer_hlen);
+		skb_reset_mac_header(skb);
+		skb_set_network_header(skb, mac_len);
+		skb_set_transport_header(skb, gre_offset);
+
+		if (!need_csum)
+			continue;
+
+		greh = (struct gre_base_hdr *)skb_transport_header(skb);
+		pcsum = (__be32 *)(greh + 1);
+
+		*pcsum = 0;
+		*(__sum16 *)pcsum = gso_make_checksum(skb, 0);
 	} while ((skb = skb->next));
 out:
 	return segs;