rxrpc: Provide a getsockopt call to query what cmsgs types are supported

Provide a getsockopt() call that can query what cmsg types are supported by
AF_RXRPC.

Documentation/networking/rxrpc.txt

@@ -406,6 +406,10 @@ calls, to invoke certain actions and to report certain conditions.  These are:
      future communication to that server and RXRPC_UPGRADE_SERVICE should no
      longer be set.
 
+The symbol RXRPC__SUPPORTED is defined as one more than the highest control
+message type supported.  At run time this can be queried by means of the
+RXRPC_SUPPORTED_CMSG socket option (see below).
+
 
 ==============
 SOCKET OPTIONS
@@ -459,6 +463,11 @@ AF_RXRPC sockets support a few socket options at the SOL_RXRPC level:
      must point to an array of two unsigned short ints.  The first is the
      service ID to upgrade from and the second the service ID to upgrade to.
 
+ (*) RXRPC_SUPPORTED_CMSG
+
+     This is a read-only option that writes an int into the buffer indicating
+     the highest control message type supported.
+
 
 ========
 SECURITY

include/linux/rxrpc.h

@@ -38,23 +38,27 @@ struct sockaddr_rxrpc {
 #define RXRPC_EXCLUSIVE_CONNECTION	3	/* Deprecated; use RXRPC_EXCLUSIVE_CALL instead */
 #define RXRPC_MIN_SECURITY_LEVEL	4	/* minimum security level */
 #define RXRPC_UPGRADEABLE_SERVICE	5	/* Upgrade service[0] -> service[1] */
+#define RXRPC_SUPPORTED_CMSG		6	/* Get highest supported control message type */
 
 /*
  * RxRPC control messages
  * - If neither abort or accept are specified, the message is a data message.
  * - terminal messages mean that a user call ID tag can be recycled
  * - s/r/- indicate whether these are applicable to sendmsg() and/or recvmsg()
  */
-#define RXRPC_USER_CALL_ID	1	/* sr: user call ID specifier */
-#define RXRPC_ABORT		2	/* sr: abort request / notification [terminal] */
-#define RXRPC_ACK		3	/* -r: [Service] RPC op final ACK received [terminal] */
-#define RXRPC_NET_ERROR		5	/* -r: network error received [terminal] */
-#define RXRPC_BUSY		6	/* -r: server busy received [terminal] */
-#define RXRPC_LOCAL_ERROR	7	/* -r: local error generated [terminal] */
-#define RXRPC_NEW_CALL		8	/* -r: [Service] new incoming call notification */
-#define RXRPC_ACCEPT		9	/* s-: [Service] accept request */
-#define RXRPC_EXCLUSIVE_CALL	10	/* s-: Call should be on exclusive connection */
-#define RXRPC_UPGRADE_SERVICE	11	/* s-: Request service upgrade for client call */
+enum rxrpc_cmsg_type {
+	RXRPC_USER_CALL_ID	= 1,	/* sr: user call ID specifier */
+	RXRPC_ABORT		= 2,	/* sr: abort request / notification [terminal] */
+	RXRPC_ACK		= 3,	/* -r: [Service] RPC op final ACK received [terminal] */
+	RXRPC_NET_ERROR		= 5,	/* -r: network error received [terminal] */
+	RXRPC_BUSY		= 6,	/* -r: server busy received [terminal] */
+	RXRPC_LOCAL_ERROR	= 7,	/* -r: local error generated [terminal] */
+	RXRPC_NEW_CALL		= 8,	/* -r: [Service] new incoming call notification */
+	RXRPC_ACCEPT		= 9,	/* s-: [Service] accept request */
+	RXRPC_EXCLUSIVE_CALL	= 10,	/* s-: Call should be on exclusive connection */
+	RXRPC_UPGRADE_SERVICE	= 11,	/* s-: Request service upgrade for client call */
+	RXRPC__SUPPORTED
+};
 
 /*
  * RxRPC security levels

net/rxrpc/af_rxrpc.c

@@ -581,6 +581,34 @@ static int rxrpc_setsockopt(struct socket *sock, int level, int optname,
 	return ret;
 }
 
+/*
+ * Get socket options.
+ */
+static int rxrpc_getsockopt(struct socket *sock, int level, int optname,
+			    char __user *optval, int __user *_optlen)
+{
+	int optlen;
+
+	if (level != SOL_RXRPC)
+		return -EOPNOTSUPP;
+
+	if (get_user(optlen, _optlen))
+		return -EFAULT;
+
+	switch (optname) {
+	case RXRPC_SUPPORTED_CMSG:
+		if (optlen < sizeof(int))
+			return -ETOOSMALL;
+		if (put_user(RXRPC__SUPPORTED - 1, (int __user *)optval) ||
+		    put_user(sizeof(int), _optlen))
+			return -EFAULT;
+		return 0;
+
+	default:
+		return -EOPNOTSUPP;
+	}
+}
+
 /*
  * permit an RxRPC socket to be polled
  */
@@ -784,7 +812,7 @@ static const struct proto_ops rxrpc_rpc_ops = {
 	.listen		= rxrpc_listen,
 	.shutdown	= rxrpc_shutdown,
 	.setsockopt	= rxrpc_setsockopt,
-	.getsockopt	= sock_no_getsockopt,
+	.getsockopt	= rxrpc_getsockopt,
 	.sendmsg	= rxrpc_sendmsg,
 	.recvmsg	= rxrpc_recvmsg,
 	.mmap		= sock_no_mmap,