monitor: Lookup existing monitor interface via netlink

On startup, request a list of interfaces via netlink. Find out whether
an interface of type nlmon exists and use it if it has all the needed
flags set. The default name of the interface is 'nlmon', which can be
changed from the command line. If the interface name is in use and is
not an nlmon type, print out an error message and exit.

monitor/main.c

@@ -28,8 +28,10 @@
 #include <ctype.h>
 #include <stdlib.h>
 #include <getopt.h>
+#include <errno.h>
 #include <sys/socket.h>
 #include <linux/genetlink.h>
+#include <linux/rtnetlink.h>
 #include <linux/if_arp.h>
 #include <ell/ell.h>
 
@@ -54,6 +56,16 @@ static const char *writer_path = NULL;
 #define NLA_DATA(nla)		((void*)(((char*)(nla)) + NLA_LENGTH(0)))
 #define NLA_PAYLOAD(nla)	((int)((nla)->nla_len - NLA_LENGTH(0)))
 
+#define NLMON_TYPE "nlmon"
+#define NLMON_LEN  5
+
+struct iwmon_interface {
+	char *ifname;
+	bool exists;
+	struct l_netlink *rtnl;
+	struct l_netlink *genl;
+};
+
 static void genl_parse(uint16_t type, const void *data, uint32_t len,
 							const char *ifname)
 {
@@ -143,6 +155,166 @@ static struct l_netlink *genl_lookup(const char *ifname)
 	return genl;
 }
 
+static bool rta_linkinfo_kind(struct rtattr *rta, unsigned short len,
+			const char* kind)
+{
+	for (; RTA_OK(rta, len); rta = RTA_NEXT(rta, len)) {
+		if (rta->rta_type != IFLA_INFO_KIND)
+			continue;
+
+		if (rta->rta_len < NLMON_LEN)
+			continue;
+
+		if (memcmp(RTA_DATA(rta), kind, strlen(kind)))
+			continue;
+
+		return true;
+	}
+
+	return false;
+}
+
+static struct l_netlink *rtm_interface_send_message(struct l_netlink *rtnl,
+					const char *ifname,
+					uint16_t rtm_msg_type,
+					l_netlink_command_func_t callback,
+					void *user_data,
+					l_netlink_destroy_func_t destroy)
+{
+	size_t bufsize;
+	struct ifinfomsg *rtmmsg;
+	void *rta_buf;
+
+	bufsize = NLMSG_LENGTH(sizeof(struct ifinfomsg));
+
+	rtmmsg = l_malloc(bufsize);
+
+	rtmmsg->ifi_family = AF_UNSPEC;
+	rtmmsg->ifi_change = ~0;
+
+	if (!rtnl)
+		rtnl = l_netlink_new(NETLINK_ROUTE);
+
+	if (!rtnl)
+		return NULL;
+
+	rta_buf = rtmmsg + 1;
+
+	switch (rtm_msg_type) {
+
+	case RTM_GETLINK:
+		l_netlink_send(rtnl, RTM_GETLINK, NLM_F_DUMP, rtmmsg,
+				rta_buf - (void *)rtmmsg, callback, user_data,
+				destroy);
+
+		break;
+
+	default:
+		l_netlink_destroy(rtnl);
+		l_free(rtmmsg);
+		return NULL;
+	}
+
+	l_free(rtmmsg);
+
+	return rtnl;
+}
+
+static void iwmon_interface_lookup_done(void *user_data)
+{
+	struct iwmon_interface *monitor_interface = user_data;
+
+	if (monitor_interface->exists && monitor_interface->ifname) {
+		monitor_interface->genl =
+			genl_lookup(monitor_interface->ifname);
+
+		return;
+	}
+
+	if (!monitor_interface->ifname)
+		monitor_interface->ifname = l_strdup(NLMON_TYPE);
+
+	fprintf(stderr, "Monitor interface %s not found or wrong flags\n",
+		monitor_interface->ifname);
+
+	l_main_quit();
+}
+
+static void iwmon_interface_lookup_callback(int error, uint16_t type,
+						const void *data, uint32_t len,
+						void *user_data)
+{
+	const struct ifinfomsg *rtmmsg = data;
+	struct rtattr *rta;
+	struct iwmon_interface *monitor_interface = user_data;
+	const char *ifname = NULL;
+	unsigned short ifname_len = 0;
+	bool nlmon = false;
+
+	if (error)
+		return;
+
+	if (type != RTM_NEWLINK)
+		return;
+
+	for (rta = (struct rtattr *)(rtmmsg + 1); RTA_OK(rta, len);
+			rta = RTA_NEXT(rta, len)) {
+
+		switch(rta->rta_type) {
+
+		case IFLA_IFNAME:
+			ifname = RTA_DATA(rta);
+			ifname_len = rta->rta_len;
+
+			break;
+
+		case IFLA_LINKINFO:
+			nlmon = rta_linkinfo_kind(RTA_DATA(rta), rta->rta_len,
+							NLMON_TYPE);
+
+			break;
+
+		default:
+			break;
+		}
+	}
+
+	if (!ifname)
+		return;
+
+	if (monitor_interface->ifname &&
+			strncmp(ifname, monitor_interface->ifname, ifname_len))
+		return;
+
+	if (!nlmon) {
+		if (monitor_interface->ifname) {
+			fprintf(stderr, "Interface %s already in use\n",
+				ifname);
+
+			l_main_quit();
+
+		}
+		return;
+	}
+
+	if ((rtmmsg->ifi_flags & (IFF_UP | IFF_ALLMULTI | IFF_NOARP)) !=
+			(IFF_UP | IFF_ALLMULTI | IFF_NOARP))
+		return;
+
+	monitor_interface->ifname = l_strndup(ifname, ifname_len);
+	monitor_interface->exists = true;
+}
+
+static void iwmon_interface_lookup(struct iwmon_interface *monitor_interface)
+{
+	monitor_interface->rtnl =
+		rtm_interface_send_message(monitor_interface->rtnl, NULL,
+						RTM_GETLINK,
+						iwmon_interface_lookup_callback,
+						monitor_interface,
+						iwmon_interface_lookup_done);
+}
+
 static int analyze_pcap(const char *pathname)
 {
 	struct l_queue *genl_list;
@@ -427,10 +599,10 @@ int main(int argc, char *argv[])
 {
 	const char *reader_path = NULL;
 	const char *analyze_path = NULL;
-	const char *ifname = "nlmon";
+	const char *ifname = NULL;
+	struct iwmon_interface monitor_interface = { };
 	uint16_t nl80211_family = GENL_ID_GENERATE;
 	struct l_signal *signal;
-	struct l_netlink *genl;
 	sigset_t mask;
 	int exit_status;
 
@@ -532,11 +704,15 @@ int main(int argc, char *argv[])
 		goto done;
 	}
 
-	genl = genl_lookup(ifname);
+	monitor_interface.ifname = l_strdup(ifname);
+	iwmon_interface_lookup(&monitor_interface);
 
 	l_main_run();
 
-	l_netlink_destroy(genl);
+	l_netlink_destroy(monitor_interface.rtnl);
+	l_netlink_destroy(monitor_interface.genl);
+	l_free(monitor_interface.ifname);
+
 	nlmon_close(nlmon);
 
 	exit_status = EXIT_SUCCESS;