sctp: introduce SCTP_FUTURE/CURRENT/ALL_ASSOC

This patch is to add 3 constants SCTP_FUTURE_ASSOC,
SCTP_CURRENT_ASSOC and SCTP_ALL_ASSOC for reserved
assoc_ids, as defined in rfc6458#section-7.2.

And add the process for them when doing lookup and
inserting in sctp_id2assoc and sctp_assoc_set_id.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/sctp.h

@@ -59,6 +59,10 @@
 
 typedef __s32 sctp_assoc_t;
 
+#define SCTP_FUTURE_ASSOC	0
+#define SCTP_CURRENT_ASSOC	1
+#define SCTP_ALL_ASSOC		2
+
 /* The following symbols come from the Sockets API Extensions for
  * SCTP <draft-ietf-tsvwg-sctpsocket-07.txt>.
  */

net/sctp/associola.c

@@ -1651,8 +1651,11 @@ int sctp_assoc_set_id(struct sctp_association *asoc, gfp_t gfp)
 	if (preload)
 		idr_preload(gfp);
 	spin_lock_bh(&sctp_assocs_id_lock);
-	/* 0 is not a valid assoc_id, must be >= 1 */
-	ret = idr_alloc_cyclic(&sctp_assocs_id, asoc, 1, 0, GFP_NOWAIT);
+	/* 0, 1, 2 are used as SCTP_FUTURE_ASSOC, SCTP_CURRENT_ASSOC and
+	 * SCTP_ALL_ASSOC, so an available id must be > SCTP_ALL_ASSOC.
+	 */
+	ret = idr_alloc_cyclic(&sctp_assocs_id, asoc, SCTP_ALL_ASSOC + 1, 0,
+			       GFP_NOWAIT);
 	spin_unlock_bh(&sctp_assocs_id_lock);
 	if (preload)
 		idr_preload_end();

net/sctp/socket.c

@@ -248,7 +248,7 @@ struct sctp_association *sctp_id2assoc(struct sock *sk, sctp_assoc_t id)
 	}
 
 	/* Otherwise this is a UDP-style socket. */
-	if (!id || (id == (sctp_assoc_t)-1))
+	if (id <= SCTP_ALL_ASSOC)
 		return NULL;
 
 	spin_lock_bh(&sctp_assocs_id_lock);