genl: add a monitor subcommand

usage:
$ genl monitor <family> <multicats group>

this subcommand allows to monitor the message from a multicast group
of a given family when received.

If it knows how to parse the messages received it will dump the decoded
version, otherwise it will just inform a new message has been received

So far it only knows how to parse nlctrl notify messages, but the plan
to allow to make the parsing extensible via lua scripts

Differential Revision:	https://reviews.freebsd.org/D40372

usr.bin/genl/genl.1

@@ -24,23 +24,44 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd May 20, 2023
+.Dd Oct 5, 2023
 .Dt GENL 1
 .Os
 .Sh NAME
 .Nm genl
-.Nd "generic netlink list"
+.Nd "generic netlink"
 .Sh SYNOPSIS
 .Nm
+.Pp
+.Nm Cm list
+.Pp
+.Nm Cm monitor Ao family Ac Ao multicast group Ac
 .Sh DESCRIPTION
+The
 .Nm
-lists all available generic netlink protocols, and presents its details:
+utility is design to provide access to the user to generic netlink
+protocols.
+.Pp
+The following commands are available:
+.Bl -tag -ident
+.It Cm list Po default Pc
+Lists all available generic netlink protocols, and presents its details:
 .Bl -tag -width "multicast groups"
 .It operations
 Id of the operation if any and associated capabilities
 .It multicast groups
 If of the available multicast group if any and it associated name
 .El
+.It Cm monitor Ao family Ac Ao multicast group Ac
+Connect to the
+.Ar family
+protocol and subscribe to the
+.Ar mulicast group
+then print the received messages in a readable format if the protocol is known.
+So far only
+.Qq nlctrl
+is known.
+.El
 .Sh SEE ALSO
 .Xr genetlink 4 ,
 .Xr netlink 4

usr.bin/genl/genl.c

@@ -27,18 +27,41 @@
 
 #include <sys/param.h>
 #include <sys/module.h>
+#include <sys/socket.h>
 
 #include <stdint.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <err.h>
 #include <stdio.h>
+#include <poll.h>
 
 #include <netlink/netlink.h>
 #include <netlink/netlink_generic.h>
 #include <netlink/netlink_snl.h>
 #include <netlink/netlink_snl_generic.h>
 
+static int monitor_mcast(int argc, char **argv);
+static int list_families(int argc, char **argv);
+static void parser_nlctrl_notify(struct snl_state *ss, struct nlmsghdr *hdr);
+static void parser_fallback(struct snl_state *ss, struct nlmsghdr *hdr);
+
+static struct commands {
+	const char *name;
+	const char *usage;
+	int (*cmd)(int argc, char **argv);
+} cmds[] = {
+	{ "monitor", "monitor <family> <multicast group>", monitor_mcast },
+	{ "list", "list", list_families },
+};
+
+static struct mcast_parsers {
+	const char *family;
+	void (*parser)(struct snl_state *ss, struct nlmsghdr *hdr);
+} mcast_parsers [] = {
+	{ "nlctrl", parser_nlctrl_notify },
+};
+
 struct genl_ctrl_op {
 	uint32_t id;
 	uint32_t flags;
@@ -131,6 +154,13 @@ dump_mcast_groups( struct snl_genl_ctrl_mcast_groups *mcast_groups)
 		    mcast_groups->groups[i]->mcast_grp_name);
 }
 
+static void
+usage(void)
+{
+	fprintf(stderr, "Usage: %s\n", getprogname());
+	for (size_t i = 0; i < nitems(cmds); i++)
+		fprintf(stderr, "       %s %s\n", getprogname(), cmds[i].usage);
+}
 
 static void
 dump_family(struct genl_family *family)
@@ -143,8 +173,87 @@ dump_family(struct genl_family *family)
 	dump_mcast_groups(&family->mcast_groups);
 }
 
+void
+parser_nlctrl_notify(struct snl_state *ss, struct nlmsghdr *hdr)
+{
+	struct genl_family family = {};
+
+	if (snl_parse_nlmsg(ss, hdr, &genl_family_parser,
+				&family))
+		dump_family(&family);
+}
+
+void
+parser_fallback(struct snl_state *ss __unused, struct nlmsghdr *hdr __unused)
+{
+	printf("New unknown message\n");
+}
+
 int
-main(int argc, char **argv __unused)
+monitor_mcast(int argc __unused, char **argv)
+{
+	struct snl_state ss;
+	struct nlmsghdr *hdr;
+	struct _getfamily_attrs attrs;
+	struct pollfd pfd;
+	bool found = false;
+	void (*parser)(struct snl_state *ss, struct nlmsghdr *hdr);
+
+	parser = parser_fallback;
+
+	if (!snl_init(&ss, NETLINK_GENERIC))
+		err(EXIT_FAILURE, "snl_init()");
+
+	if (argc != 2) {
+		usage();
+		return (EXIT_FAILURE);
+	}
+	if (!snl_get_genl_family_info(&ss, argv[0], &attrs))
+		errx(EXIT_FAILURE, "Unknown family '%s'", argv[0]);
+	for (uint32_t i = 0; i < attrs.mcast_groups.num_groups; i++) {
+		if (strcmp(attrs.mcast_groups.groups[i]->mcast_grp_name,
+		    argv[1]) == 0) {
+			found = true;
+			if (setsockopt(ss.fd, SOL_NETLINK,
+			    NETLINK_ADD_MEMBERSHIP,
+			    (void *)&attrs.mcast_groups.groups[i]->mcast_grp_id,
+			    sizeof(attrs.mcast_groups.groups[i]->mcast_grp_id))
+			    == -1)
+				err(EXIT_FAILURE, "Cannot subscribe to command "
+				    "notify");
+			break;
+		}
+	}
+	if (!found)
+		errx(EXIT_FAILURE, "No such multicat group '%s'"
+		    " in family '%s'", argv[1], argv[0]);
+	for (size_t i= 0; i < nitems(mcast_parsers); i++) {
+		if (strcmp(mcast_parsers[i].family, argv[0]) == 0) {
+			parser = mcast_parsers[i].parser;
+			break;
+		}
+	}
+	memset(&pfd, 0, sizeof(pfd));
+	pfd.fd = ss.fd;
+	pfd.events = POLLIN | POLLERR;
+	while (true) {
+		pfd.revents = 0;
+		if (poll(&pfd, 1, -1) == -1) {
+			if (errno == EINTR)
+				continue;
+			err(EXIT_FAILURE, "poll()");
+		}
+		hdr = snl_read_message(&ss);
+		if (hdr != NULL && hdr->nlmsg_type != NLMSG_ERROR)
+			parser(&ss, hdr);
+
+	}
+
+	return (EXIT_SUCCESS);
+}
+
+int
+list_families(int argc, char **argv __unused)
 {
 	struct snl_state ss;
 	struct snl_writer nw;
@@ -152,16 +261,16 @@ main(int argc, char **argv __unused)
 	struct snl_errmsg_data e = {};
 	uint32_t seq_id;
 
-	if (argc > 1)
-		errx(EXIT_FAILURE, "usage: genl does not accept any argument");
-	if (modfind("netlink") == -1)
-		err(EXIT_FAILURE, "require netlink module to be loaded");
-
+	if (argc != 0) {
+		usage();
+		return (EXIT_FAILURE);
+	}
 	if (!snl_init(&ss, NETLINK_GENERIC))
 		err(EXIT_FAILURE, "snl_init()");
 
 	snl_init_writer(&ss, &nw);
-	hdr = snl_create_genl_msg_request(&nw, GENL_ID_CTRL, CTRL_CMD_GETFAMILY);
+	hdr = snl_create_genl_msg_request(&nw, GENL_ID_CTRL,
+	    CTRL_CMD_GETFAMILY);
 	if ((hdr = snl_finalize_msg(&nw)) == NULL)
 		err(EXIT_FAILURE, "snl_finalize_msg");
 	seq_id = hdr->nlmsg_seq;
@@ -179,3 +288,21 @@ main(int argc, char **argv __unused)
 
 	return (EXIT_SUCCESS);
 }
+
+int
+main(int argc, char **argv)
+{
+	if (modfind("netlink") == -1)
+		err(EXIT_FAILURE, "require netlink module to be loaded");
+
+	if (argc == 1)
+		return (list_families(0, NULL));
+
+	for (size_t i = 0; i < nitems(cmds); i++) {
+		if (strcmp(argv[1], cmds[i].name) == 0)
+			return (cmds[i].cmd(argc - 2, argv + 2));
+	}
+	usage();
+
+	return (EXIT_FAILURE);
+}