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:

voidnetlink_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!