How to use netlink socket to communicate with a kernel module?*********************8

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);

where ssk is the netlink socket returned by netlink_kernel_create(),
skb->data points to the netlink message to be sent and pid is the receiving application's pid, assuming NLPID Formula 1 is used.
nonblock indicates whether the API should block when the receiving buffer is unavailable or immediately return a failure.

You also can send a multicast message. The following API delivers a netlink message to both the process specified by pid and the multicast groups specified by group:

void
netlink_broadcast(struct sock *ssk, struct sk_buff
*skb, u32 pid, u32 group, int allocation);

group is the ORed bitmasks of all the receiving multicast groups.
allocation is the kernel memory allocation type. Typically, GFP_ATOMIC is used if from interrupt context; GFP_KERNEL if otherwise. This is due to the fact that the API may need to allocate one or many socket buffers to clone the multicast message.

Closing a Netlink Socket from the Kernel
Given the struct sock *nl_sk returned by netlink_kernel_create(), we can call the following kernel API to close the netlink socket in the kernel:

sock_release(nl_sk->socket);

So far, we have shown only the bare minimum code framework to illustrate the concept of netlink programming. We now will use our NETLINK_TEST netlink protocol type and assume it already has been added to the kernel header file. The kernel module code listed
here contains only the netlink-relevant part, so it should be inserted into a complete kernel module skeleton, which you can find from many other reference sources.

Unicast Communication between Kernel and Application
In this example, a user-space process sends a netlink message to the kernel module, and the kernel module echoes the message back to the sending process. Here is the user-space code:

#include <sys/socket.h>
#include <linux/netlink.h>

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

void main() {
sock_fd = socket(PF_NETLINK, SOCK_RAW,NETLINK_TEST);

memset(&src_addr, 0, sizeof(src_addr));
src__addr.nl_family = AF_NETLINK;
src_addr.nl_pid = getpid();  /* self pid */
src_addr.nl_groups = 0;  /* not in mcast groups */
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));
/* Fill the netlink message header */
nlh->nlmsg_len = NLMSG_SPACE(MAX_PAYLOAD);
nlh->nlmsg_pid = getpid();  /* self pid */
nlh->nlmsg_flags = 0;
/* Fill in the netlink message payload */
strcpy(NLMSG_DATA(nlh), "Hello you!");

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;

sendmsg(fd, &msg, 0);

/* Read message from kernel */
memset(nlh, 0, NLMSG_SPACE(MAX_PAYLOAD));
recvmsg(fd, &msg, 0);
printf(" Received message payload: %s\n",
NLMSG_DATA(nlh));

/* Close Netlink Socket */
close(sock_fd);
}

And, here is the kernel code:

struct sock *nl_sk = NULL;

void nl_data_ready (struct sock *sk, int len)
{
wake_up_interruptible(sk->sleep);
}

void netlink_test() {
struct sk_buff *skb = NULL;
struct nlmsghdr *nlh = NULL;
int err;
u32 pid;

nl_sk = netlink_kernel_create(NETLINK_TEST,
nl_data_ready);
/* wait for message coming down from user-space */
skb = skb_recv_datagram(nl_sk, 0, 0, &err);

nlh = (struct nlmsghdr *)skb->data;
printk("%s: received netlink message payload:%s\n",
__FUNCTION__, NLMSG_DATA(nlh));

pid = nlh->nlmsg_pid; /*pid of sending process */
NETLINK_CB(skb).groups = 0; /* not in mcast group */
NETLINK_CB(skb).pid = 0;      /* from kernel */
NETLINK_CB(skb).dst_pid = pid;
NETLINK_CB(skb).dst_groups = 0;  /* unicast */
netlink_unicast(nl_sk, skb, pid, MSG_DONTWAIT);
sock_release(nl_sk->socket);
}

After loading the kernel module that executes the kernel code above, when we run the user-space executable, we should see the following dumped from the user-space program:

Received message payload: Hello you!

And, the following message should appear in the output of dmesg:

netlink_test: received netlink message payload:
Hello you!