gre: let mac_header point to outer header only when necessary

Commit e271c7b442 ("gre: do not keep the GRE header around in collect
medata mode") did reset the mac_header for the collect_md case. Let's
extend this behaviour to classical gre devices as well.

ipgre_header_parse() seems to be the only case that requires mac_header
to point to the outer header. We can detect this case accurately by
checking ->header_ops. For all other cases, we can reset mac_header.

This allows to push an Ethernet header to ipgre packets and redirect
them to an Ethernet device:

  $ tc filter add dev gre0 ingress matchall          \
      action vlan push_eth dst_mac 00:00:5e:00:53:01 \
                           src_mac 00:00:5e:00:53:00 \
      action mirred egress redirect dev eth0

Before this patch, this worked only for collect_md gre devices.
Now this works for regular gre devices as well. Only the special case
of gre devices that use ipgre_header_ops isn't supported.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -107,6 +107,8 @@ module_param(log_ecn_error, bool, 0644);
 MODULE_PARM_DESC(log_ecn_error, "Log packets received with corrupted ECN");
 
 static struct rtnl_link_ops ipgre_link_ops __read_mostly;
+static const struct header_ops ipgre_header_ops;
+
 static int ipgre_tunnel_init(struct net_device *dev);
 static void erspan_build_header(struct sk_buff *skb,
 				u32 id, u32 index,
@@ -364,7 +366,10 @@ static int __ipgre_rcv(struct sk_buff *skb, const struct tnl_ptk_info *tpi,
 					   raw_proto, false) < 0)
 			goto drop;
 
-		if (tunnel->dev->type != ARPHRD_NONE)
+		/* Special case for ipgre_header_parse(), which expects the
+		 * mac_header to point to the outer IP header.
+		 */
+		if (tunnel->dev->header_ops == &ipgre_header_ops)
 			skb_pop_mac_header(skb);
 		else
 			skb_reset_mac_header(skb);