net: bridge: mcast: export multicast router presence adjacent to a port

To properly support routable multicast addresses in batman-adv in a
group-aware way, a batman-adv node needs to know if it serves multicast
routers.

This adds a function to the bridge to export this so that batman-adv
can then make full use of the Multicast Router Discovery capability of
the bridge.
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_bridge.h

@@ -67,6 +67,7 @@ int br_multicast_list_adjacent(struct net_device *dev,
 			       struct list_head *br_ip_list);
 bool br_multicast_has_querier_anywhere(struct net_device *dev, int proto);
 bool br_multicast_has_querier_adjacent(struct net_device *dev, int proto);
+bool br_multicast_has_router_adjacent(struct net_device *dev, int proto);
 bool br_multicast_enabled(const struct net_device *dev);
 bool br_multicast_router(const struct net_device *dev);
 int br_mdb_replay(struct net_device *br_dev, struct net_device *dev,
@@ -87,6 +88,13 @@ static inline bool br_multicast_has_querier_adjacent(struct net_device *dev,
 {
 	return false;
 }
+
+static inline bool br_multicast_has_router_adjacent(struct net_device *dev,
+						    int proto)
+{
+	return true;
+}
+
 static inline bool br_multicast_enabled(const struct net_device *dev)
 {
 	return false;

net/bridge/br_multicast.c

@@ -4054,6 +4054,61 @@ bool br_multicast_has_querier_adjacent(struct net_device *dev, int proto)
 }
 EXPORT_SYMBOL_GPL(br_multicast_has_querier_adjacent);
 
+/**
+ * br_multicast_has_router_adjacent - Checks for a router behind a bridge port
+ * @dev: The bridge port adjacent to which to check for a multicast router
+ * @proto: The protocol family to check for: IGMP -> ETH_P_IP, MLD -> ETH_P_IPV6
+ *
+ * Checks whether the given interface has a bridge on top and if so returns
+ * true if a multicast router is behind one of the other ports of this
+ * bridge. Otherwise returns false.
+ */
+bool br_multicast_has_router_adjacent(struct net_device *dev, int proto)
+{
+	struct net_bridge_port *port, *p;
+	bool ret = false;
+
+	rcu_read_lock();
+	port = br_port_get_check_rcu(dev);
+	if (!port)
+		goto unlock;
+
+	switch (proto) {
+	case ETH_P_IP:
+		hlist_for_each_entry_rcu(p, &port->br->ip4_mc_router_list,
+					 ip4_rlist) {
+			if (p == port)
+				continue;
+
+			ret = true;
+			goto unlock;
+		}
+		break;
+#if IS_ENABLED(CONFIG_IPV6)
+	case ETH_P_IPV6:
+		hlist_for_each_entry_rcu(p, &port->br->ip6_mc_router_list,
+					 ip6_rlist) {
+			if (p == port)
+				continue;
+
+			ret = true;
+			goto unlock;
+		}
+		break;
+#endif
+	default:
+		/* when compiled without IPv6 support, be conservative and
+		 * always assume presence of an IPv6 multicast router
+		 */
+		ret = true;
+	}
+
+unlock:
+	rcu_read_unlock();
+	return ret;
+}
+EXPORT_SYMBOL_GPL(br_multicast_has_router_adjacent);
+
 static void br_mcast_stats_add(struct bridge_mcast_stats __percpu *stats,
 			       const struct sk_buff *skb, u8 type, u8 dir)
 {