netlink socket编程实例

关于Netlink IPC方式的介绍，请参见http://blog.chinaunix.net/space.php?uid=14753126&do=blog&id=2978566
本文通过一个编程实例来更深入地了解netlink。
功能描述：
1. 本实例包含2个部分：内核模块和用户程序
2.用户程序通过netlink socket建立与kernel module连接，并向kernel module发送一条信息。
3.内核在接收到用户的信息后，向用户程序发送一条确认信息。

说明：对于不知如何编译内核模块以及用户程序的童鞋请google

内核模块代码(test_netlink_kmodule.c)：

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/sched.h>
#include <net/sock.h>
#include <linux/netlink.h>

#define NETLINK_TEST 17
struct {
    __u32 pid;
}user_process;

static struct sock *netlinkfd = NULL;

int send_to_user(char *info) //发送到用户空间
{
    int size;
    struct sk_buff *skb;
    unsigned char *old_tail;
    struct nlmsghdr *nlh;  //报文头

    int retval;

    size = NLMSG_SPACE(strlen(info));  //报文大小
    skb = alloc_skb(size, GFP_ATOMIC); //分配一个新的套接字缓存,使用GFP_ATOMIC标志进程不>会被置为睡眠

    //初始化一个netlink消息首部
    nlh = nlmsg_put(skb, 0, 0, 0, NLMSG_SPACE(strlen(info))-sizeof(struct nlmsghdr), 0);
    old_tail = skb->tail;
    memcpy(NLMSG_DATA(nlh), info, strlen(info));  //填充数据区
    nlh->nlmsg_len = skb->tail - old_tail;        //设置消息长度

    //设置控制字段
    NETLINK_CB(skb).pid = 0;
    NETLINK_CB(skb).dst_group = 0;

    printk(KERN_DEBUG "[kernel space] skb->data:%s\n", (char *)NLMSG_DATA((struct nlmsghdr *)skb->data));

    //发送数据
    retval = netlink_unicast(netlinkfd, skb, user_process.pid, MSG_DONTWAIT);
    printk(KERN_DEBUG "[kernel space] netlink_unicast return: %d\n", retval);
    return 0;
}

void kernel_receive(struct sk_buff *__skb) //内核从用户空间接收数据
{
    struct sk_buff *skb;
    struct nlmsghdr *nlh = NULL;

    char *data = "This is eric's test message from kernel";

    printk(KERN_DEBUG "[kernel space] begin kernel_receive\n");
    skb = skb_get(__skb);

    if(skb->len >= sizeof(struct nlmsghdr)){
        nlh = (struct nlmsghdr *)skb->data;
        if((nlh->nlmsg_len >= sizeof(struct nlmsghdr))
            && (__skb->len >= nlh->nlmsg_len)){
            user_process.pid = nlh->nlmsg_pid;
            printk(KERN_DEBUG "[kernel space] data receive from user are:%s\n", (char *)NLMSG_DATA(nlh));
            printk(KERN_DEBUG "[kernel space] user_pid:%d\n", user_process.pid);
            send_to_user(data);
        }
    }else{
        printk(KERN_DEBUG "[kernel space] data receive from user are:%s\n",(char *)NLMSG_DATA(nlmsg_hdr(__skb)));
        send_to_user(data);
    }

    kfree_skb(skb);
}

int __init test_netlink_init(void)
{

    netlinkfd = netlink_kernel_create(&init_net, NETLINK_TEST, 0, kernel_receive, NULL, THIS_MODULE);
    if(!netlinkfd){
        printk(KERN_ERR "can not create a netlink socket\n");
        return -1;
    }
    return 0;
}

void __exit test_netlink_exit(void)
{
    sock_release(netlinkfd->sk_socket);
    printk(KERN_DEBUG "test_netlink_exit!!\n");
}

module_init(test_netlink_init);
module_exit(test_netlink_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("eric.hu");

用户空间程序代码：user_sent.c

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

#define NETLINK_TEST 17
#define MSG_LEN 100

struct msg_to_kernel
{
    struct nlmsghdr hdr;
    char data[MSG_LEN];
};
struct u_packet_info
{
    struct nlmsghdr hdr;
    char msg[MSG_LEN];
};

int main(int argc, char* argv[])
{
    char *data = "This message is from eric's space";
    //初始化
    struct sockaddr_nl local;
    struct sockaddr_nl kpeer;
    int skfd, ret, kpeerlen = sizeof(struct sockaddr_nl);
    struct nlmsghdr *message;
    struct u_packet_info info;
    char *retval;
    message = (struct nlmsghdr *)malloc(1);

    skfd = socket(PF_NETLINK, SOCK_RAW, NETLINK_TEST);
    if(skfd < 0){
        printf("can not create a netlink socket\n");
        return -1;
    }
    memset(&local, 0, sizeof(local));
    local.nl_family = AF_NETLINK;
    local.nl_pid = getpid();
    local.nl_groups = 0;
    if(bind(skfd, (struct sockaddr *)&local, sizeof(local)) != 0){
        printf("bind() error\n");
        return -1;
    }
    memset(&kpeer, 0, sizeof(kpeer));
    kpeer.nl_family = AF_NETLINK;
    kpeer.nl_pid = 0;
    kpeer.nl_groups = 0;

    memset(message, '\0', sizeof(struct nlmsghdr));
    message->nlmsg_len = NLMSG_SPACE(strlen(data));
    message->nlmsg_flags = 0;
    message->nlmsg_type = 0;
    message->nlmsg_seq = 0;
    message->nlmsg_pid = local.nl_pid;

    retval = memcpy(NLMSG_DATA(message), data, strlen(data));

    printf("message sendto kernel are:%s, len:%d\n", (char *)NLMSG_DATA(message), message->nlmsg_len);
    ret = sendto(skfd, message, message->nlmsg_len, 0,(struct sockaddr *)&kpeer, sizeof(kpeer));
    if(!ret){
        perror("send pid:");
        exit(-1);
    }

    //接受内核态确认信息
    ret = recvfrom(skfd, &info, sizeof(struct u_packet_info),0, (struct sockaddr*)&kpeer, &kpeerlen);
    if(!ret){
        perror("recv form kerner:");
        exit(-1);
    }

    printf("message receive from kernel:%s\n",(char *)info.msg);
    //内核和用户进行通信

    close(skfd);
    return 0;
}

运行用户程序，输出结果如下：

[root@FriendlyARM netlink]# ./user_sent
message sendto kernel are:This message is from eric's space, len:52
message receive from kernel:This is eric's test message from kernel