seg6: add support for IPv4 decapsulation in ipv6_srh_rcv()

As specified in IETF RFC 8754, section 4.3.1.2, if the upper layer
header is IPv4 or IPv6, perform IPv6 decapsulation and resubmit the
decapsulated packet to the IPv4 or IPv6 module.
Only IPv6 decapsulation was implemented. This patch adds support for IPv4
decapsulation.

Link: https://tools.ietf.org/html/rfc8754#section-4.3.1.2Signed-off-by: Julien Massonneau <julien.massonneau@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ipv6.h

@@ -30,6 +30,7 @@
  */
 
 #define NEXTHDR_HOP		0	/* Hop-by-hop option header. */
+#define NEXTHDR_IPV4		4	/* IPv4 in IPv6 */
 #define NEXTHDR_TCP		6	/* TCP segment. */
 #define NEXTHDR_UDP		17	/* UDP message. */
 #define NEXTHDR_IPV6		41	/* IPv6 in IPv6 */

net/ipv6/exthdrs.c

@@ -381,7 +381,7 @@ static int ipv6_srh_rcv(struct sk_buff *skb)
 
 looped_back:
 	if (hdr->segments_left == 0) {
-		if (hdr->nexthdr == NEXTHDR_IPV6) {
+		if (hdr->nexthdr == NEXTHDR_IPV6 || hdr->nexthdr == NEXTHDR_IPV4) {
 			int offset = (hdr->hdrlen + 1) << 3;
 
 			skb_postpull_rcsum(skb, skb_network_header(skb),
@@ -397,7 +397,8 @@ static int ipv6_srh_rcv(struct sk_buff *skb)
 			skb_reset_network_header(skb);
 			skb_reset_transport_header(skb);
 			skb->encapsulation = 0;
-
+			if (hdr->nexthdr == NEXTHDR_IPV4)
+				skb->protocol = htons(ETH_P_IP);
 			__skb_tunnel_rx(skb, skb->dev, net);
 
 			netif_rx(skb);