cfg80211/mac80211: add mesh_param "mesh_nolearn" to skip path discovery

Currently, before being able to forward a packet between two 802.11s
nodes, both a PLINK handshake is performed upon receiving a beacon and
then later a PREQ/PREP exchange for path discovery is performed on
demand upon receiving a data frame to forward.

When running a mesh protocol on top of an 802.11s interface, like
batman-adv, we do not need the multi-hop mesh routing capabilities of
802.11s and usually set mesh_fwding=0. However, even with mesh_fwding=0
the PREQ/PREP path discovery is still performed on demand. Even though
in this scenario the next hop PREQ/PREP will determine is always the
direct 11s neighbor node.

The new mesh_nolearn parameter allows to skip the PREQ/PREP exchange in
this scenario, leading to a reduced delay, reduced packet buffering and
simplifies HWMP in general.

mesh_nolearn is still rather conservative in that if the packet destination
is not a direct 11s neighbor, it will fall back to PREQ/PREP path
discovery.

For normal, multi-hop 802.11s mesh routing it is usually not advisable
to enable mesh_nolearn as a transmission to a direct but distant neighbor
might be worse than reaching that same node via a more robust /
higher throughput etc. multi-hop path.

Cc: Sven Eckelmann <sven@narfation.org>
Cc: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Linus Lüssing <ll@simonwunderlich.de>
Link: https://lore.kernel.org/r/20200617073034.26149-1-linus.luessing@c0d3.blue
[fix nl80211 policy to range 0/1 only]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 39 insertions, 0 deletions

diff --git a/net/mac80211/mesh_hwmp.c b/net/mac80211/mesh_hwmp.c
index bae3a3e15b88..bec23d2eee7a 100644
--- a/net/mac80211/mesh_hwmp.c
+++ b/net/mac80211/mesh_hwmp.c
@@ -1173,6 +1173,40 @@ int mesh_nexthop_resolve(struct ieee80211_sub_if_data *sdata,
 }
 
 /**
+ * mesh_nexthop_lookup_nolearn - try to set next hop without path discovery
+ * @skb: 802.11 frame to be sent
+ * @sdata: network subif the frame will be sent through
+ *
+ * Check if the meshDA (addr3) of a unicast frame is a direct neighbor.
+ * And if so, set the RA (addr1) to it to transmit to this node directly,
+ * avoiding PREQ/PREP path discovery.
+ *
+ * Returns: 0 if the next hop was found and -ENOENT otherwise.
+ */
+static int mesh_nexthop_lookup_nolearn(struct ieee80211_sub_if_data *sdata,
+				       struct sk_buff *skb)
+{
+	struct ieee80211_hdr *hdr = (struct ieee80211_hdr *)skb->data;
+	struct sta_info *sta;
+
+	if (is_multicast_ether_addr(hdr->addr1))
+		return -ENOENT;
+
+	rcu_read_lock();
+	sta = sta_info_get(sdata, hdr->addr3);
+
+	if (!sta || sta->mesh->plink_state != NL80211_PLINK_ESTAB) {
+		rcu_read_unlock();
+		return -ENOENT;
+	}
+	rcu_read_unlock();
+
+	memcpy(hdr->addr1, hdr->addr3, ETH_ALEN);
+	memcpy(hdr->addr2, sdata->vif.addr, ETH_ALEN);
+	return 0;
+}
+
+/**
  * mesh_nexthop_lookup - put the appropriate next hop on a mesh frame. Calling
  * this function is considered "using" the associated mpath, so preempt a path
  * refresh if this mpath expires soon.
@@ -1185,11 +1219,16 @@ int mesh_nexthop_resolve(struct ieee80211_sub_if_data *sdata,
 int mesh_nexthop_lookup(struct ieee80211_sub_if_data *sdata,
 			struct sk_buff *skb)
 {
+	struct ieee80211_if_mesh *ifmsh = &sdata->u.mesh;
 	struct mesh_path *mpath;
 	struct sta_info *next_hop;
 	struct ieee80211_hdr *hdr = (struct ieee80211_hdr *) skb->data;
 	u8 *target_addr = hdr->addr3;
 
+	if (ifmsh->mshcfg.dot11MeshNolearn &&
+	    !mesh_nexthop_lookup_nolearn(sdata, skb))
+		return 0;
+
 	mpath = mesh_path_lookup(sdata, target_addr);
 	if (!mpath || !(mpath->flags & MESH_PATH_ACTIVE))
 		return -ENOENT;