batman-adv: prevent duplication of ARP replies when DAT is used

If none of the backbone gateways in a bla setup has already knowledge of
the mac address searched for in an incoming ARP request from the backbone
an address resolution via the DHT of DAT is started. The gateway can send
several ARP requests to different DHT nodes and therefore can get several
replies. This patch assures that not all of the possible ARP replies are
returned to the backbone by checking the local DAT cache of the gateway.
If there is an entry in the local cache the gateway has already learned
the requested address and there is no need to forward the additional reply
to the backbone.
Furthermore it is checked if this gateway has claimed the source of the ARP
reply and only forwards it to the backbone if it has claimed the source or
if there is no claim at all.

Signed-off-by: Andreas Pape <apape@phoenixcontact.com>
Acked-by: Simon Wunderlich <sw@simonwunderlich.de>
[sven@narfation.org: fix conflicts with current version]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/distributed-arp-table.c

@@ -1203,6 +1203,7 @@ void batadv_dat_snoop_outgoing_arp_reply(struct batadv_priv *bat_priv,
 bool batadv_dat_snoop_incoming_arp_reply(struct batadv_priv *bat_priv,
 					 struct sk_buff *skb, int hdr_size)
 {
+	struct batadv_dat_entry *dat_entry = NULL;
 	u16 type;
 	__be32 ip_src, ip_dst;
 	u8 *hw_src, *hw_dst;
@@ -1225,12 +1226,41 @@ bool batadv_dat_snoop_incoming_arp_reply(struct batadv_priv *bat_priv,
 	hw_dst = batadv_arp_hw_dst(skb, hdr_size);
 	ip_dst = batadv_arp_ip_dst(skb, hdr_size);
 
+	/* If ip_dst is already in cache and has the right mac address,
+	 * drop this frame if this ARP reply is destined for us because it's
+	 * most probably an ARP reply generated by another node of the DHT.
+	 * We have most probably received already a reply earlier. Delivering
+	 * this frame would lead to doubled receive of an ARP reply.
+	 */
+	dat_entry = batadv_dat_entry_hash_find(bat_priv, ip_src, vid);
+	if (dat_entry && batadv_compare_eth(hw_src, dat_entry->mac_addr)) {
+		batadv_dbg(BATADV_DBG_DAT, bat_priv, "Doubled ARP reply removed: ARP MSG = [src: %pM-%pI4 dst: %pM-%pI4]; dat_entry: %pM-%pI4\n",
+			   hw_src, &ip_src, hw_dst, &ip_dst,
+			   dat_entry->mac_addr,	&dat_entry->ip);
+		dropped = true;
+		goto out;
+	}
+
 	/* Update our internal cache with both the IP addresses the node got
 	 * within the ARP reply
 	 */
 	batadv_dat_entry_add(bat_priv, ip_src, hw_src, vid);
 	batadv_dat_entry_add(bat_priv, ip_dst, hw_dst, vid);
 
+	/* If BLA is enabled, only forward ARP replies if we have claimed the
+	 * source of the ARP reply or if no one else of the same backbone has
+	 * already claimed that client. This prevents that different gateways
+	 * to the same backbone all forward the ARP reply leading to multiple
+	 * replies in the backbone.
+	 */
+	if (!batadv_bla_check_claim(bat_priv, hw_src, vid)) {
+		batadv_dbg(BATADV_DBG_DAT, bat_priv,
+			   "Device %pM claimed by another backbone gw. Drop ARP reply.\n",
+			   hw_src);
+		dropped = true;
+		goto out;
+	}
+
 	/* if this REPLY is directed to a client of mine, let's deliver the
 	 * packet to the interface
 	 */
@@ -1243,6 +1273,8 @@ bool batadv_dat_snoop_incoming_arp_reply(struct batadv_priv *bat_priv,
 out:
 	if (dropped)
 		kfree_skb(skb);
+	if (dat_entry)
+		batadv_dat_entry_put(dat_entry);
 	/* if dropped == false -> deliver to the interface */
 	return dropped;
 }