net: mctp: Add some detail on the key allocation implementation

We could do with a little more comment on where MCTP_ADDR_ANY will match
in the key allocations.
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/mctp/route.c

@@ -73,6 +73,40 @@ static struct mctp_sock *mctp_lookup_bind(struct net *net, struct sk_buff *skb)
 	return NULL;
 }
 
+/* A note on the key allocations.
+ *
+ * struct net->mctp.keys contains our set of currently-allocated keys for
+ * MCTP tag management. The lookup tuple for these is the peer EID,
+ * local EID and MCTP tag.
+ *
+ * In some cases, the peer EID may be MCTP_EID_ANY: for example, when a
+ * broadcast message is sent, we may receive responses from any peer EID.
+ * Because the broadcast dest address is equivalent to ANY, we create
+ * a key with (local = local-eid, peer = ANY). This allows a match on the
+ * incoming broadcast responses from any peer.
+ *
+ * We perform lookups when packets are received, and when tags are allocated
+ * in two scenarios:
+ *
+ *  - when a packet is sent, with a locally-owned tag: we need to find an
+ *    unused tag value for the (local, peer) EID pair.
+ *
+ *  - when a tag is manually allocated: we need to find an unused tag value
+ *    for the peer EID, but don't have a specific local EID at that stage.
+ *
+ * in the latter case, on successful allocation, we end up with a tag with
+ * (local = ANY, peer = peer-eid).
+ *
+ * So, the key set allows both a local EID of ANY, as well as a peer EID of
+ * ANY in the lookup tuple. Both may be ANY if we prealloc for a broadcast.
+ * The matching (in mctp_key_match()) during lookup allows the match value to
+ * be ANY in either the dest or source addresses.
+ *
+ * When allocating (+ inserting) a tag, we need to check for conflicts amongst
+ * the existing tag set. This requires macthing either exactly on the local
+ * and peer addresses, or either being ANY.
+ */
+
 static bool mctp_key_match(struct mctp_sk_key *key, mctp_eid_t local,
 			   mctp_eid_t peer, u8 tag)
 {
@@ -368,6 +402,9 @@ static int mctp_route_input(struct mctp_route *route, struct sk_buff *skb)
 			 * key lookup to find the socket, but don't use this
 			 * key for reassembly - we'll create a more specific
 			 * one for future packets if required (ie, !EOM).
+			 *
+			 * this lookup requires key->peer to be MCTP_ADDR_ANY,
+			 * it doesn't match just any key->peer.
 			 */
 			any_key = mctp_lookup_key(net, skb, MCTP_ADDR_ANY, &f);
 			if (any_key) {