net: Account for all vlan headers in skb_mac_gso_segment

skb_network_protocol() already accounts for multiple vlan
headers that may be present in the skb. However, skb_mac_gso_segment()
doesn't know anything about it and assumes that skb->mac_len
is set correctly to skip all mac headers. That may not
always be the case. If we are simply forwarding the packet (via
bridge or macvtap), all vlan headers may not be accounted for.

A simple solution is to allow skb_network_protocol to return
the vlan depth it has calculated. This way skb_mac_gso_segment
will correctly skip all mac headers.

Signed-off-by: Vlad Yasevich <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/netdevice.h

···
 {
 	return __skb_gso_segment(skb, features, true);
 }
-__be16 skb_network_protocol(struct sk_buff *skb);
+__be16 skb_network_protocol(struct sk_buff *skb, int *depth);
 
 static inline bool can_checksum_protocol(netdev_features_t features,
 					 __be16 protocol)

net/core/dev.c

···
 }
 EXPORT_SYMBOL(skb_checksum_help);
 
-__be16 skb_network_protocol(struct sk_buff *skb)
+__be16 skb_network_protocol(struct sk_buff *skb, int *depth)
 {
 	__be16 type = skb->protocol;
 	int vlan_depth = ETH_HLEN;
···
 		vlan_depth += VLAN_HLEN;
 	}
 
+	*depth = vlan_depth;
+
 	return type;
 }
 
···
 {
 	struct sk_buff *segs = ERR_PTR(-EPROTONOSUPPORT);
 	struct packet_offload *ptype;
-	__be16 type = skb_network_protocol(skb);
+	int vlan_depth = skb->mac_len;
+	__be16 type = skb_network_protocol(skb, &vlan_depth);
 
 	if (unlikely(!type))
 		return ERR_PTR(-EINVAL);
 
-	__skb_pull(skb, skb->mac_len);
+	__skb_pull(skb, vlan_depth);
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(ptype, &offload_base, list) {
···
 					    const struct net_device *dev,
 					    netdev_features_t features)
 {
+	int tmp;
+
 	if (skb->ip_summed != CHECKSUM_NONE &&
-	    !can_checksum_protocol(features, skb_network_protocol(skb))) {
+	    !can_checksum_protocol(features, skb_network_protocol(skb, &tmp))) {
 		features &= ~NETIF_F_ALL_CSUM;
 	} else if (illegal_highdma(dev, skb)) {
 		features &= ~NETIF_F_SG;

net/core/skbuff.c

···
 	int err = -ENOMEM;
 	int i = 0;
 	int pos;
+	int dummy;
 
-	proto = skb_network_protocol(head_skb);
+	proto = skb_network_protocol(head_skb, &dummy);
 	if (unlikely(!proto))
 		return ERR_PTR(-EINVAL);