xfrm: Add mode handlers for IPsec on layer 2

This patch adds a gso_segment and xmit callback for the
xfrm_mode and implement these functions for tunnel and
transport mode.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/net/ipv4/xfrm4_mode_tunnel.c b/net/ipv4/xfrm4_mode_tunnel.c
index 35feda6..d3f2434 100644
--- a/net/ipv4/xfrm4_mode_tunnel.c
+++ b/net/ipv4/xfrm4_mode_tunnel.c
@@ -96,11 +96,36 @@ static int xfrm4_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
 	return err;
 }
 
+static struct sk_buff *xfrm4_mode_tunnel_gso_segment(struct xfrm_state *x,
+						     struct sk_buff *skb,
+						     netdev_features_t features)
+{
+	__skb_push(skb, skb->mac_len);
+	return skb_mac_gso_segment(skb, features);
+
+}
+
+static void xfrm4_mode_tunnel_xmit(struct xfrm_state *x, struct sk_buff *skb)
+{
+	struct xfrm_offload *xo = xfrm_offload(skb);
+
+	if (xo->flags & XFRM_GSO_SEGMENT) {
+		skb->network_header = skb->network_header - x->props.header_len;
+		skb->transport_header = skb->network_header +
+					sizeof(struct iphdr);
+	}
+
+	skb_reset_mac_len(skb);
+	pskb_pull(skb, skb->mac_len + x->props.header_len);
+}
+
 static struct xfrm_mode xfrm4_tunnel_mode = {
 	.input2 = xfrm4_mode_tunnel_input,
 	.input = xfrm_prepare_input,
 	.output2 = xfrm4_mode_tunnel_output,
 	.output = xfrm4_prepare_output,
+	.gso_segment = xfrm4_mode_tunnel_gso_segment,
+	.xmit = xfrm4_mode_tunnel_xmit,
 	.owner = THIS_MODULE,
 	.encap = XFRM_MODE_TUNNEL,
 	.flags = XFRM_MODE_FLAG_TUNNEL,