net: ipv6: mcast_routing: hop limit handling

While forwarding a multicast packet decrement hop limit in a common net
buffer. Also, packets with hop limit equal to 0 should not be forwarded.

Signed-off-by: Konrad Derda <konrad.derda@nordicsemi.no>

subsys/net/ip/ipv6.c

@@ -411,12 +411,15 @@ static enum net_verdict ipv6_forward_mcast_packet(struct net_pkt *pkt,
 #if defined(CONFIG_NET_ROUTE_MCAST)
 	int routed;
 
-	/* check if routing loop could be created or if the destination is of
-	 * interface local scope or if from link local source
+	/* Continue processing without forwarding if:
+	 *   1. routing loop could be created
+	 *   2. the destination is of interface local scope
+	 *   3. is from link local source
+	 *   4. hop limit is or would become zero
 	 */
 	if (net_ipv6_is_addr_mcast((struct in6_addr *)hdr->src) ||
 	    net_ipv6_is_addr_mcast_iface((struct in6_addr *)hdr->dst) ||
-	       net_ipv6_is_ll_addr((struct in6_addr *)hdr->src)) {
+	    net_ipv6_is_ll_addr((struct in6_addr *)hdr->src) || hdr->hop_limit <= 1) {
 		return NET_CONTINUE;
 	}
 

subsys/net/ip/route.c

@@ -815,11 +815,16 @@ static void propagate_mld_event(struct net_route_entry_mcast *route, bool route_
 #define propagate_mld_event(...)
 #endif /* CONFIG_NET_MCAST_ROUTE_MLD_REPORTS */
 
-int net_route_mcast_forward_packet(struct net_pkt *pkt,
-				   const struct net_ipv6_hdr *hdr)
+int net_route_mcast_forward_packet(struct net_pkt *pkt, struct net_ipv6_hdr *hdr)
 {
 	int ret = 0, err = 0;
 
+	/* At this point, the original pkt has already stored the hop limit in its metadata.
+	 * Change its value in a common buffer so the forwardee has a proper count. As we have
+	 * a direct access to the buffer there is no need to perform read/write operations.
+	 */
+	hdr->hop_limit--;
+
 	ARRAY_FOR_EACH_PTR(route_mcast_entries, route) {
 		struct net_pkt *pkt_cpy = NULL;
 

subsys/net/ip/route.h

@@ -224,7 +224,7 @@ typedef void (*net_route_mcast_cb_t)(struct net_route_entry_mcast *entry,
  * value in case of an error.
  */
 int net_route_mcast_forward_packet(struct net_pkt *pkt,
-				   const struct net_ipv6_hdr *hdr);
+				   struct net_ipv6_hdr *hdr);
 
 /**
  * @brief Go through all the multicast routing entries and call callback