seg6: add support for SRv6 H.L2Encaps.Red behavior

The SRv6 H.L2Encaps.Red behavior described in [1] is an optimization of
the SRv6 H.L2Encaps behavior [2].

H.L2Encaps.Red reduces the length of the SRH by excluding the first
segment (SID) in the SRH of the pushed IPv6 header. The first SID is
only placed in the IPv6 Destination Address field of the pushed IPv6
header.
When the SRv6 Policy only contains one SID the SRH is omitted, unless
there is an HMAC TLV to be carried.

[1] - https://datatracker.ietf.org/doc/html/rfc8986#section-5.4
[2] - https://datatracker.ietf.org/doc/html/rfc8986#section-5.3

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Signed-off-by: Anton Makarov <anton.makarov11235@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/seg6_iptunnel.h

@@ -36,6 +36,7 @@ enum {
 	SEG6_IPTUN_MODE_ENCAP,
 	SEG6_IPTUN_MODE_L2ENCAP,
 	SEG6_IPTUN_MODE_ENCAP_RED,
+	SEG6_IPTUN_MODE_L2ENCAP_RED,
 };
 
 #endif

net/ipv6/seg6_iptunnel.c

@@ -40,6 +40,7 @@ static size_t seg6_lwt_headroom(struct seg6_iptunnel_encap *tuninfo)
 		head = sizeof(struct ipv6hdr);
 		break;
 	case SEG6_IPTUN_MODE_L2ENCAP:
+	case SEG6_IPTUN_MODE_L2ENCAP_RED:
 		return 0;
 	}
 
@@ -413,6 +414,7 @@ static int seg6_do_srh(struct sk_buff *skb)
 		skb->protocol = htons(ETH_P_IPV6);
 		break;
 	case SEG6_IPTUN_MODE_L2ENCAP:
+	case SEG6_IPTUN_MODE_L2ENCAP_RED:
 		if (!skb_mac_header_was_set(skb))
 			return -EINVAL;
 
@@ -422,7 +424,13 @@ static int seg6_do_srh(struct sk_buff *skb)
 		skb_mac_header_rebuild(skb);
 		skb_push(skb, skb->mac_len);
 
-		err = seg6_do_srh_encap(skb, tinfo->srh, IPPROTO_ETHERNET);
+		if (tinfo->mode == SEG6_IPTUN_MODE_L2ENCAP)
+			err = seg6_do_srh_encap(skb, tinfo->srh,
+						IPPROTO_ETHERNET);
+		else
+			err = seg6_do_srh_encap_red(skb, tinfo->srh,
+						    IPPROTO_ETHERNET);
+
 		if (err)
 			return err;
 
@@ -643,6 +651,8 @@ static int seg6_build_state(struct net *net, struct nlattr *nla,
 		break;
 	case SEG6_IPTUN_MODE_ENCAP_RED:
 		break;
+	case SEG6_IPTUN_MODE_L2ENCAP_RED:
+		break;
 	default:
 		return -EINVAL;
 	}