netlink

kernel_module/netlink.c

@@ -0,0 +1,78 @@
+/*
+https://en.wikipedia.org/wiki/Netlink
+
+https://stackoverflow.com/questions/3299386/how-to-use-netlink-socket-to-communicate-with-a-kernel-module
+*/
+
+#include <linux/delay.h> /* usleep_range */
+#include <linux/jiffies.h>
+#include <linux/module.h>
+#include <linux/netlink.h>
+#include <linux/skbuff.h>
+#include <net/sock.h>
+
+/* Socket identifier, matches userland. TODO can be anything?
+ * Is there a more scalable way to do it? E.g. ioctl device,
+ * kernel generates one on the fly, then give it back and connect?
+ * https://stackoverflow.com/questions/32898173/can-i-have-more-than-32-netlink-sockets-in-kernelspace */
+#define NETLINK_USER 31
+
+struct sock *nl_sk = NULL;
+
+static void callback(struct sk_buff *skb)
+{
+	char readbuf[1024];
+	size_t readbuflen;
+    int pid;
+    int res;
+    struct nlmsghdr *nlh;
+    struct sk_buff *skb_out;
+
+	/* Read user message. */
+    nlh = (struct nlmsghdr *)skb->data;
+    pr_info("kernel received: %s\n", (char *)nlmsg_data(nlh));
+
+	/* Add an artificial sleep to see what happens when
+	 * multiple requests come in at the same time.
+	 *
+	 * Try this out (it works):
+	 * for i in `seq 16`; do /netlink.out & done */
+	usleep_range(1000000, 1000001);
+
+	/* Reply with jiffies. */
+	readbuflen = snprintf(readbuf, sizeof(readbuf), "%llu", (unsigned long long)jiffies);
+    pid = nlh->nlmsg_pid;
+    skb_out = nlmsg_new(readbuflen, 0);
+    if (!skb_out) {
+        pr_err("nlmsg_new\n");
+        return;
+    }
+    nlh = nlmsg_put(skb_out, 0, 0, NLMSG_DONE, readbuflen, 0);
+    NETLINK_CB(skb_out).dst_group = 0;
+    strncpy(nlmsg_data(nlh), readbuf, readbuflen);
+    res = nlmsg_unicast(nl_sk, skb_out, pid);
+    if (res < 0)
+        pr_info("nlmsg_unicast\n");
+}
+
+static int myinit(void)
+{
+    struct netlink_kernel_cfg cfg = {
+        .input = callback,
+    };
+    nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, &cfg);
+    if (!nl_sk) {
+        pr_err("netlink_kernel_create\n");
+        return -10;
+    }
+    return 0;
+}
+
+static void myexit(void)
+{
+    netlink_kernel_release(nl_sk);
+}
+
+module_init(myinit);
+module_exit(myexit);
+MODULE_LICENSE("GPL");