netlink example code in c

Instructions for running netlink example code in c

1. Execute make first. This will result in a netlinkKernel.ko output among many others.
2. Execute $ gcc netlinkUser.c -o netlinkUser
3. Insert kernel module by :$ sudo insmod netlinkKernel.ko      
4. Run ./netlinkUser to see message and run dmesg to see debug messages
5. Remove module by : $ sudo rmmod netlinkKernel          
6. Finally make clean to remove output files.

Makefile

KDIR := /lib/modules/$(shell uname -r)/build

obj-m += netlinkKernel.o

all:
	$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

clean:
	rm -rf *.o *.ko *.mod.* *.cmd .module* modules* Module* .*.cmd .tmp*
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

netlinkKernel.c

//Taken from https://stackoverflow.com/questions/15215865/netlink-sockets-in-c-using-the-3-x-linux-kernel?lq=1

#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__);
//This is for 3.6 kernels and above.
struct netlink_kernel_cfg cfg = {
    .input = hello_nl_recv_msg,
};

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");

netlinkUser.c

//Taken from https://stackoverflow.com/questions/15215865/netlink-sockets-in-c-using-the-3-x-linux-kernel?lq=1

#include <sys/socket.h>
#include <linux/netlink.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.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));
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", (char *)NLMSG_DATA(nlh));
close(sock_fd);
}

What a beautiful code format!

Function hello_nl_recv_msg in netlinkKernel.c is incomplete (missing })

Also, the following changes should be made:
See also Does “sendmsg” free memory of the buffer or msg?

*** netlinkUser.c
--- netlinkUser.c
*************** int main()
*** 50,55 ****
--- 50,56 ----
  
  	printf("Sending message to kernel\n");
  	sendmsg(sock_fd,&msg,0);
+ 	free(nlh);
  	printf("Waiting for message from kernel\n");
  
  	/* Read message from kernel */

@sachinites nlmsg_unicast() will automagically free skb_out under the hood for us.
See also kernel crash when trying to free the skb with nlmsg_free(skb_out)