Q:

How to use netlink socket to communicate with a kernel module?

I am trying to write a linux kernel module that communicates with user process using netlink. I am using netlink because the user program I want to communicate to communicates only using sockets and I cant change that to add ioctl() or anything.
Problem is that I cant figure out how to do that. I have googled but all examples I found are for old like this one and no longer valid for current kernel versions. I have also looked at this SO question but the sample here uses libnl for socket operations but I want to stick to standard socket functions (defined by sys/socket.h). So can some one plz guide me here to some tutorial or guide or some thing that can help me understand the interface and usage of netlink. I would highly appreciate a working example, nothing fancy, just a very basic example of how to establish a connection from a socket in user program to a socket in kernel and then send data from user process to kernel and receive back from kernel.
Also please do not tell me to look at kernel code. I am already doing it but it will take a lot of time and I dont have lot of it left.
Update:
After lot of trial and error I have following code which sends message from user program to kernel but the message from kernel to user program i.e using netlink_unicast() is not working. Its not only not working, the call hangs the systems and then I have to restart the machine. Can some one plz take a look and tell me what wrong I am doing. The netlink_unicast() call is commented in the following code. It should be uncommented for kernel to user program message.
User Program
#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 -1;  

    memset(&src_addr, 0, sizeof(src_addr));  
    src_addr.nl_family = AF_NETLINK;  
    src_addr.nl_pid = getpid();  /* self pid */  
    /* interested in group 1<<0 */  
    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);  
}

Kernel Code
#include <linux/module.h>  
#include <linux/kernel.h>  
#include <linux/init.h>  
#include <net/sock.h>  
#include <linux/socket.h>  
#include <linux/net.h>  
#include <asm/types.h>  
#include <linux/netlink.h>  
#include <linux/skbuff.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;  

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

        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 */  
        NETLINK_CB(skb).dst_group = 0; /* not in mcast group */  
        NETLINK_CB(skb).pid = 0;      /* from kernel */  
        //NETLINK_CB(skb).groups = 0; /* not in mcast group */  
        //NETLINK_CB(skb).dst_pid = pid;  
        printk("About to send msg bak:\n");  
        //netlink_unicast(nl_sk,skb,pid,MSG_DONTWAIT);  

}  

static int __init hello_init(void)  
{  

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

A:

After reading kernel source I finally managed to make netlink sockets work for me. Below is an example of Netlink socket basics i.e opening a netlink socket, reading and writing to it and closing it.
Kernel Module
#include <linux/module.h>
#include <net/sock.h> 
#include <linux/netlink.h>
#include <linux/skbuff.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");
}

static int __init hello_init(void)
{

    printk("Entering: %s\n", __FUNCTION__);
    //nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, 0, hello_nl_recv_msg, NULL, THIS_MODULE);
    struct netlink_kernel_cfg cfg = {
        .input = hello_nl_recv_msg,
    };

    nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, &cfg);
    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 Program
#include <linux/netlink.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.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;

int main()
{
    sock_fd = socket(PF_NETLINK, SOCK_RAW, NETLINK_USER);
    if (sock_fd < 0)
        return -1;

    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));
    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);
}

Related thread about the magic constant NETLINK_USER 31: Can I have more than 32 netlink sockets in kernelspace?

A:

Just in case anybody doesn't know how to compile, google "how to compile and load kernel module"
refer to http://www.cyberciti.biz/tips/compiling-linux-kernel-module.html
Grab kernel source code to which you'll compile module against http://kernel.org
Or simply update your headers if you are running intended kernel
# apt-get install kernel-headers-$(uname -r)

Create a makefile, for example
obj-m = hello.o
KVERSION = $(shell uname -r)
all:
        make -C /lib/modules/$(KVERSION)/build M=$(PWD) modules
clean:
        make -C /lib/modules/$(KVERSION)/build M=$(PWD) clean

Make and you'll get bunch of files. *.ko is the one you'll load into your kernel, run
# insmod hello.ko

if you us lsmod to check all loaded modules, you'll find yours, most likely you will see:
hello       12575  0 

In our case, compile and run user code:
gcc hello.c -o hello.o
./hello.o

If everything is OK, you'll get following message using binW's code:
Sending message to kernel
Waiting for message from kernel
 Received message payload: Hello from kernel

Finally, remove the module using:
# rmmod hello

A:

It works for me with kernel 3.2. For kernel 3.6 & above, it needs a bit of a change at the netlink_kernel_create function.
 struct netlink_kernel_cfg cfg = {
                .groups = 1,
                .input = hello_nl_recv_msg,
        };
        printk("Entering: %s\n", __FUNCTION__);
        nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, &cfg);

