1 files changed, 178 insertions, 0 deletions
diff --git a/us/genltest.c b/us/genltest.c
new file mode 100644
index 0000000..caf44f5
--- /dev/null
+++ b/us/genltest.c
@@ -0,0 +1,178 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Generic Netlink libnl example program
+ *
+ * An example on how to use the libnl library for communicating over Generic
+ * Netlink with kernel modules, with unicast and multicast group messages.
+ *
+ *  Copyright (c) 2022 Yaroslav de la Peña Smirnov <yps@yaroslavps.com>
+ */
+#include <stdio.h>
+#include <errno.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <netlink/socket.h>
+#include <netlink/netlink.h>
+#include <netlink/genl/ctrl.h>
+#include <netlink/genl/genl.h>
+#include <netlink/genl/family.h>
+
+#include "../ks/genltest.h"
+
+#define prerr(...) fprintf(stderr, "error: " __VA_ARGS__)
+
+/*
+ * libnl docs and API: https://www.infradead.org/~tgr/libnl/
+ * Current libnl repo: https://github.com/thom311/libnl
+ */
+
+/*
+ * Handler for all received messages from our Generic Netlink family, both
+ * unicast and multicast.
+ */
+static int echo_reply_handler(struct nl_msg *msg, void *arg)
+{
+	int		   err	   = 0;
+	struct genlmsghdr *genlhdr = nlmsg_data(nlmsg_hdr(msg));
+	struct nlattr	  *tb[GENLTEST_A_MAX + 1];
+
+	/* Parse the attributes */
+	err = nla_parse(tb, GENLTEST_A_MAX, genlmsg_attrdata(genlhdr, 0),
+			genlmsg_attrlen(genlhdr, 0), NULL);
+	if (err) {
+		prerr("unable to parse message: %s\n", strerror(-err));
+		return NL_SKIP;
+	}
+	/* Check that there's actually a payload */
+	if (!tb[GENLTEST_A_MSG]) {
+		prerr("msg attribute missing from message\n");
+		return NL_SKIP;
+	}
+
+	/* Print it! */
+	printf("message received: %s\n", nla_get_string(tb[GENLTEST_A_MSG]));
+
+	return NL_OK;
+}
+
+/* Send (unicast) GENLTEST_CMD_ECHO request message */
+static int send_echo_msg(struct nl_sock *sk, int fam)
+{
+	int	       err = 0;
+	struct nl_msg *msg = nlmsg_alloc();
+	if (!msg) {
+		return -ENOMEM;
+	}
+
+	/* Put the genl header inside message buffer */
+	void *hdr = genlmsg_put(msg, NL_AUTO_PORT, NL_AUTO_SEQ, fam, 0, 0,
+				GENLTEST_CMD_ECHO, GENLTEST_GENL_VERSION);
+	if (!hdr) {
+		return -EMSGSIZE;
+	}
+
+	/* Put the string inside the message. */
+	err = nla_put_string(msg, GENLTEST_A_MSG,
+			     "Hello from User Space, Netlink!");
+	if (err < 0) {
+		return -err;
+	}
+	printf("message sent\n");
+
+	/* Send the message. */
+	err = nl_send_auto(sk, msg);
+	err = err >= 0 ? 0 : err;
+
+	nlmsg_free(msg);
+
+	return err;
+}
+
+/* Allocate netlink socket and connect to generic netlink */
+static int conn(struct nl_sock **sk)
+{
+	*sk = nl_socket_alloc();
+	if (!sk) {
+		return -ENOMEM;
+	}
+
+	return genl_connect(*sk);
+}
+
+/* Disconnect and release socket */
+static void disconn(struct nl_sock *sk)
+{
+	nl_close(sk);
+	nl_socket_free(sk);
+}
+
+/* Modify the callback for replies to handle all received messages */
+static inline int set_cb(struct nl_sock *sk)
+{
+	return nl_socket_modify_cb(sk, NL_CB_VALID, NL_CB_CUSTOM,
+				   echo_reply_handler, NULL);
+}
+
+int main(void)
+{
+	int		ret = 1;
+	struct nl_sock *ucsk, *mcsk;
+
+	/*
+	 * We use one socket to receive asynchronous "notifications" over
+	 * multicast group, and another for ops. We do this so that we don't mix
+	 * up responses from ops with notifications to make handling easier.
+	 */
+	if ((ret = conn(&ucsk)) || (ret = conn(&mcsk))) {
+		prerr("failed to connect to generic netlink\n");
+		goto out;
+	}
+
+	/* Resolve the genl family. One family for both unicast and multicast. */
+	int fam = genl_ctrl_resolve(ucsk, GENLTEST_GENL_NAME);
+	if (fam < 0) {
+		prerr("failed to resolve generic netlink family: %s\n",
+		      strerror(-fam));
+		goto out;
+	}
+
+	/* Disable sequence checks for asynchronous multicast messages. */
+	nl_socket_disable_seq_check(mcsk);
+
+	/* Resolve the multicast group. */
+	int mcgrp = genl_ctrl_resolve_grp(mcsk, GENLTEST_GENL_NAME,
+					  GENLTEST_MC_GRP_NAME);
+	if (mcgrp < 0) {
+		prerr("failed to resolve generic netlink multicast group: %s\n",
+		      strerror(-mcgrp));
+		goto out;
+	}
+	/* Join the multicast group. */
+	if ((ret = nl_socket_add_membership(mcsk, mcgrp) < 0)) {
+		prerr("failed to join multicast group: %s\n", strerror(-ret));
+		goto out;
+	}
+
+	if ((ret = set_cb(ucsk)) || (ret = set_cb(mcsk))) {
+		prerr("failed to set callback: %s\n", strerror(-ret));
+		goto out;
+	}
+
+	/* Send unicast message and listen for response. */
+	if ((ret = send_echo_msg(ucsk, fam))) {
+		prerr("failed to send message: %s\n", strerror(-ret));
+	}
+	printf("listening for messages\n");
+	nl_recvmsgs_default(ucsk);
+
+	/* Listen for "notifications". */
+	while (1) {
+		nl_recvmsgs_default(mcsk);
+	}
+
+	ret = 0;
+out:
+	disconn(ucsk);
+	disconn(mcsk);
+	return ret;
+}