Netlink socket creation returns NULL

I have been trying to implement a pretty simple netlink communication between a kernel module and a process. But it seems I'm stuck on creating the netlink socket on the kernel module. As I have understood from kernel 3.6 and up, the netlink_kernel_create signature is:

static inline struct sock * netlink_kernel_create(struct net *net, int unit, struct netlink_kernel_cfg *cfg)

My kernel is 3.13 and therefore this is the declaration I'm using. But it always returns NULL.

here is the code:

#define NETLINK_LISTEN 17 

struct netlink_kernel_cfg cfg = {
        .input = recieved_message,
};
/* create netlink socket */
netlink = netlink_kernel_create(&init_net, NETLINK_LISTEN , &cfg);

if (!netlink)
{
    << always gets here >> 
    return -1;
} 
else 
{
    return 0;
}

my OS information:

uname -a
Linux Mint 3.13.0-37-generic #64-Ubuntu SMP Mon Sep 22 21:28:38 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

Solution

I have taken code from : How to use netlink socket to communicate with a kernel module?

If you initialise groups like :

struct netlink_kernel_cfg cfg = {
   .groups  = 1,
   .input = hello_nl_recv_msg,
};

Then it works perfectly fine in 3.13 kernel also :

This code worked for me :
Kernel space :

#include <linux/module.h>
#include <net/sock.h>
#include <linux/netlink.h>
#include <linux/skbuff.h>
#include <net/net_namespace.h>

#define NETLINK_USER 31

struct sock *nl_sk = NULL;

static void hello_nl_recv_msg(struct sk_buff *skb)
{

    struct nlmsghdr *nlh;
    int pid;
    struct sk_buff *skb_out;
    int msg_size;
    char *msg = "Hello from kernel";
    int res;

    printk(KERN_INFO "Entering: %s\n", __FUNCTION__);

    msg_size = strlen(msg);

    nlh = (struct nlmsghdr *)skb->data;
    printk(KERN_INFO "Netlink received msg payload: %s\n", (char *)nlmsg_data(nlh));
    pid = nlh->nlmsg_pid; /*pid of sending process */

    skb_out = nlmsg_new(msg_size, 0);

    if (!skb_out)
    {

        printk(KERN_ERR "Failed to allocate new skb\n");
        return;

    }
    nlh = nlmsg_put(skb_out, 0, 0, NLMSG_DONE, msg_size, 0);
    NETLINK_CB(skb_out).dst_group = 0; /* not in mcast group */
        strncpy(nlmsg_data(nlh), msg, msg_size);

    res = nlmsg_unicast(nl_sk, skb_out, pid);

    if (res < 0)
        printk(KERN_INFO "Error while sending bak to user\n");
}

struct netlink_kernel_cfg cfg = {
   .groups  = 1,
   .input = hello_nl_recv_msg,
};

static int __init hello_init(void)
{

    printk("Entering: %s\n", __FUNCTION__);
    nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, &cfg);

    // nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, 0, hello_nl_recv_msg,
    //                              NULL, THIS_MODULE);
    if (!nl_sk)
    {
        printk(KERN_ALERT "Error creating socket.\n");
        return -10;
    }

    return 0;
}

static void __exit hello_exit(void)
{

    printk(KERN_INFO "exiting hello module\n");
    netlink_kernel_release(nl_sk);
}
module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");

User Space :

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <sys/socket.h>
#include <linux/netlink.h>

#define NETLINK_USER 31

#define MAX_PAYLOAD 1024 /* maximum payload size*/
struct sockaddr_nl src_addr, dest_addr;
struct nlmsghdr *nlh = NULL;
struct iovec iov;
int sock_fd;
struct msghdr msg;

void main()
{
    sock_fd = socket(PF_NETLINK, SOCK_RAW, NETLINK_USER);
    if (sock_fd < 0)
        return;

    memset(&src_addr, 0, sizeof(src_addr));
    src_addr.nl_family = AF_NETLINK;
    src_addr.nl_pid = getpid(); /* self pid */

    bind(sock_fd, (struct sockaddr *)&src_addr, sizeof(src_addr));

    memset(&dest_addr, 0, sizeof(dest_addr));
    memset(&dest_addr, 0, sizeof(dest_addr));
    dest_addr.nl_family = AF_NETLINK;
    dest_addr.nl_pid = 0; /* For Linux Kernel */
    dest_addr.nl_groups = 0; /* unicast */

    nlh = (struct nlmsghdr *)malloc(NLMSG_SPACE(MAX_PAYLOAD));
    memset(nlh, 0, NLMSG_SPACE(MAX_PAYLOAD));
    nlh->nlmsg_len = NLMSG_SPACE(MAX_PAYLOAD);
    nlh->nlmsg_pid = getpid();
    nlh->nlmsg_flags = 0;

    strcpy(NLMSG_DATA(nlh), "Hello");

    iov.iov_base = (void *)nlh;
    iov.iov_len = nlh->nlmsg_len;
    msg.msg_name = (void *)&dest_addr;
    msg.msg_namelen = sizeof(dest_addr);
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;

    printf("Sending message to kernel\n");
    sendmsg(sock_fd, &msg, 0);
    printf("Waiting for message from kernel\n");

    /* Read message from kernel */
    recvmsg(sock_fd, &msg, 0);
    printf("Received message payload: %s\n", NLMSG_DATA(nlh));
    close(sock_fd);
}

Insert the kernel module first and then run the user space executable.