ipv6: sr: add support for encapsulation of L2 frames

This patch implements the L2 frame encapsulation mechanism, referred to
as T.Encaps.L2 in the SRv6 specifications [1].

A new type of SRv6 tunnel mode is added (SEG6_IPTUN_MODE_L2ENCAP). It only
accepts packets with an existing MAC header (i.e., it will not work for
locally generated packets). The resulting packet looks like IPv6 -> SRH ->
Ethernet -> original L3 payload. The next header field of the SRH is set to
NEXTHDR_NONE.

[1] https://tools.ietf.org/html/draft-filsfils-spring-srv6-network-programming-01

Signed-off-by: David Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/seg6_iptunnel.h

@@ -33,16 +33,26 @@ struct seg6_iptunnel_encap {
 enum {
 	SEG6_IPTUN_MODE_INLINE,
 	SEG6_IPTUN_MODE_ENCAP,
+	SEG6_IPTUN_MODE_L2ENCAP,
 };
 
 #ifdef __KERNEL__
 
 static inline size_t seg6_lwt_headroom(struct seg6_iptunnel_encap *tuninfo)
 {
-	int encap = (tuninfo->mode == SEG6_IPTUN_MODE_ENCAP);
-
-	return ((tuninfo->srh->hdrlen + 1) << 3) +
-	       (encap * sizeof(struct ipv6hdr));
+	int head = 0;
+
+	switch (tuninfo->mode) {
+	case SEG6_IPTUN_MODE_INLINE:
+		break;
+	case SEG6_IPTUN_MODE_ENCAP:
+		head = sizeof(struct ipv6hdr);
+		break;
+	case SEG6_IPTUN_MODE_L2ENCAP:
+		return 0;
+	}
+
+	return ((tuninfo->srh->hdrlen + 1) << 3) + head;
 }
 
 #endif

net/ipv6/seg6_iptunnel.c

@@ -238,6 +238,22 @@ static int seg6_do_srh(struct sk_buff *skb)
 		if (err)
 			return err;
 
+		skb->protocol = htons(ETH_P_IPV6);
+		break;
+	case SEG6_IPTUN_MODE_L2ENCAP:
+		if (!skb_mac_header_was_set(skb))
+			return -EINVAL;
+
+		if (pskb_expand_head(skb, skb->mac_len, 0, GFP_ATOMIC) < 0)
+			return -ENOMEM;
+
+		skb_mac_header_rebuild(skb);
+		skb_push(skb, skb->mac_len);
+
+		err = seg6_do_srh_encap(skb, tinfo->srh, NEXTHDR_NONE);
+		if (err)
+			return err;
+
 		skb->protocol = htons(ETH_P_IPV6);
 		break;
 	}
@@ -386,6 +402,8 @@ static int seg6_build_state(struct nlattr *nla,
 		break;
 	case SEG6_IPTUN_MODE_ENCAP:
 		break;
+	case SEG6_IPTUN_MODE_L2ENCAP:
+		break;
 	default:
 		return -EINVAL;
 	}
@@ -409,8 +427,11 @@ static int seg6_build_state(struct nlattr *nla,
 	memcpy(&slwt->tuninfo, tuninfo, tuninfo_len);
 
 	newts->type = LWTUNNEL_ENCAP_SEG6;
-	newts->flags |= LWTUNNEL_STATE_OUTPUT_REDIRECT |
-			LWTUNNEL_STATE_INPUT_REDIRECT;
+	newts->flags |= LWTUNNEL_STATE_INPUT_REDIRECT;
+
+	if (tuninfo->mode != SEG6_IPTUN_MODE_L2ENCAP)
+		newts->flags |= LWTUNNEL_STATE_OUTPUT_REDIRECT;
+
 	newts->headroom = seg6_lwt_headroom(tuninfo);
 
 	*ts = newts;