net: Add vxlan_gso_check() helper

Most NICs that report NETIF_F_GSO_UDP_TUNNEL support VXLAN, and not
other UDP-based encapsulation protocols where the format and size of the
header differs. This patch implements a generic ndo_gso_check() for
VXLAN which will only advertise GSO support when the skb looks like it
contains VXLAN (or no UDP tunnelling at all).

Implementation shamelessly stolen from Tom Herbert:
http://thread.gmane.org/gmane.linux.network/332428/focus=333111

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -1571,6 +1571,19 @@ static bool route_shortcircuit(struct net_device *dev, struct sk_buff *skb)
 	return false;
 }
 
+bool vxlan_gso_check(struct sk_buff *skb)
+{
+	if ((skb_shinfo(skb)->gso_type & SKB_GSO_UDP_TUNNEL) &&
+	    (skb->inner_protocol_type != ENCAP_TYPE_ETHER ||
+	     skb->inner_protocol != htons(ETH_P_TEB) ||
+	     (skb_inner_mac_header(skb) - skb_transport_header(skb) !=
+	      sizeof(struct udphdr) + sizeof(struct vxlanhdr))))
+		return false;
+
+	return true;
+}
+EXPORT_SYMBOL_GPL(vxlan_gso_check);
+
 #if IS_ENABLED(CONFIG_IPV6)
 static int vxlan6_xmit_skb(struct vxlan_sock *vs,
 			   struct dst_entry *dst, struct sk_buff *skb,

include/net/vxlan.h

@@ -45,6 +45,8 @@ int vxlan_xmit_skb(struct vxlan_sock *vs,
 		   __be32 src, __be32 dst, __u8 tos, __u8 ttl, __be16 df,
 		   __be16 src_port, __be16 dst_port, __be32 vni, bool xnet);
 
+bool vxlan_gso_check(struct sk_buff *skb);
+
 /* IP header + UDP + VXLAN + Ethernet header */
 #define VXLAN_HEADROOM (20 + 8 + 8 + 14)
 /* IPv6 header + UDP + VXLAN + Ethernet header */