batman-adv: mcast: detect, distribute and maintain multicast router presence

To be able to apply our group aware multicast optimizations to packets
with a scope greater than link-local we need to not only keep track of
multicast listeners but also multicast routers.

With this patch a node detects the presence of multicast routers on
its segment by checking if
/proc/sys/net/ipv{4,6}/conf/<bat0|br0(bat)>/mc_forwarding is set for one
thing. This option is enabled by multicast routing daemons and needed
for the kernel's multicast routing tables to receive and route packets.

For another thing if a bridge is configured on top of bat0 then the
presence of an IPv6 multicast router behind this bridge is currently
detected by checking for an IPv6 multicast "All Routers Address"
(ff02::2). This should later be replaced by querying the bridge, which
performs proper, RFC4286 compliant Multicast Router Discovery (our
simplified approach includes more hosts than necessary, most notably
not just multicast routers but also unicast ones and is not applicable
for IPv4).

If no multicast router is detected then this is signalized via the new
BATADV_MCAST_WANT_NO_RTR4 and BATADV_MCAST_WANT_NO_RTR6
multicast tvlv flags.
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

include/uapi/linux/batadv_packet.h

@@ -107,12 +107,20 @@ enum batadv_icmp_packettype {
  * @BATADV_MCAST_WANT_ALL_UNSNOOPABLES: we want all packets destined for
  *  224.0.0.0/24 or ff02::1
  * @BATADV_MCAST_WANT_ALL_IPV4: we want all IPv4 multicast packets
+ *  (both link-local and routable ones)
  * @BATADV_MCAST_WANT_ALL_IPV6: we want all IPv6 multicast packets
+ *  (both link-local and routable ones)
+ * @BATADV_MCAST_WANT_NO_RTR4: we have no IPv4 multicast router and therefore
+ * only need routable IPv4 multicast packets we signed up for explicitly
+ * @BATADV_MCAST_WANT_NO_RTR6: we have no IPv6 multicast router and therefore
+ * only need routable IPv6 multicast packets we signed up for explicitly
  */
 enum batadv_mcast_flags {
 	BATADV_MCAST_WANT_ALL_UNSNOOPABLES	= 1UL << 0,
 	BATADV_MCAST_WANT_ALL_IPV4		= 1UL << 1,
 	BATADV_MCAST_WANT_ALL_IPV6		= 1UL << 2,
+	BATADV_MCAST_WANT_NO_RTR4		= 1UL << 3,
+	BATADV_MCAST_WANT_NO_RTR6		= 1UL << 4,
 };
 
 /* tt data subtypes */

net/batman-adv/originator.c

@@ -27,6 +27,7 @@
 #include <linux/stddef.h>
 #include <linux/workqueue.h>
 #include <net/sock.h>
+#include <uapi/linux/batadv_packet.h>
 #include <uapi/linux/batman_adv.h>
 
 #include "bat_algo.h"
@@ -1043,7 +1044,8 @@ struct batadv_orig_node *batadv_orig_node_new(struct batadv_priv *bat_priv,
 	orig_node->bcast_seqno_reset = reset_time;
 
 #ifdef CONFIG_BATMAN_ADV_MCAST
-	orig_node->mcast_flags = BATADV_NO_FLAGS;
+	orig_node->mcast_flags = BATADV_MCAST_WANT_NO_RTR4;
+	orig_node->mcast_flags |= BATADV_MCAST_WANT_NO_RTR6;
 	INIT_HLIST_NODE(&orig_node->mcast_want_all_unsnoopables_node);
 	INIT_HLIST_NODE(&orig_node->mcast_want_all_ipv4_node);
 	INIT_HLIST_NODE(&orig_node->mcast_want_all_ipv6_node);

net/batman-adv/types.h

@@ -404,6 +404,17 @@ struct batadv_orig_node {
 	 *  list
 	 */
 	struct hlist_node mcast_want_all_ipv6_node;
+
+	/**
+	 * @mcast_want_all_rtr4_node: a list node for the mcast.want_all_rtr4
+	 *  list
+	 */
+	struct hlist_node mcast_want_all_rtr4_node;
+	/**
+	 * @mcast_want_all_rtr6_node: a list node for the mcast.want_all_rtr6
+	 *  list
+	 */
+	struct hlist_node mcast_want_all_rtr6_node;
 #endif
 
 	/** @capabilities: announced capabilities of this originator */
@@ -1218,6 +1229,18 @@ struct batadv_priv_mcast {
 	 */
 	struct hlist_head want_all_ipv6_list;
 
+	/**
+	 * @want_all_rtr4_list: a list of orig_nodes wanting all routable IPv4
+	 *  multicast traffic
+	 */
+	struct hlist_head want_all_rtr4_list;
+
+	/**
+	 * @want_all_rtr6_list: a list of orig_nodes wanting all routable IPv6
+	 *  multicast traffic
+	 */
+	struct hlist_head want_all_rtr6_list;
+
 	/**
 	 * @mla_flags: flags for the querier, bridge and tvlv state
 	 */
@@ -1240,6 +1263,12 @@ struct batadv_priv_mcast {
 	/** @num_want_all_ipv6: counter for items in want_all_ipv6_list */
 	atomic_t num_want_all_ipv6;
 
+	/** @num_want_all_rtr4: counter for items in want_all_rtr4_list */
+	atomic_t num_want_all_rtr4;
+
+	/** @num_want_all_rtr6: counter for items in want_all_rtr6_list */
+	atomic_t num_want_all_rtr6;
+
 	/**
 	 * @want_lists_lock: lock for protecting modifications to mcasts
 	 *  want_all_{unsnoopables,ipv4,ipv6}_list (traversals are rcu-locked)