与系统调用，/proc，sysfs等类似，netlink也是一种用于用户进程与内核通信的机制，它是基于BSD套接字协议，使用AF_NETLINK地址簇。与系统调用，proc，sysfs文件系统等方式相比，netlink具有简单，支持双向通信的特点，并支持消息多播机制。当我们编写内核驱动并需要与用户进程通信时，我们便能利用netlink来实现这个通信机制。hostapd（一个无线 AP的dameon）中就是采用netlink接口（nl80211）与内核进行通信，下文将通过一个实例来说明如何在自己的内核模块中支持netlink通信。

1. 编写内核模块

首先编写内核模块文件：netlink_hello_mod.c，并增加相应的makefile，代码如下：

netlink_hello_mod.c:

#include <linux/module.h>
#include <net/sock.h>
#include <linux/netlink.h>
#include <linux/skbuff.h>

#define MY_NETLINK_TYPE 31 //max is 32, see netlink.h

struct sock *my_nl_sock = 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 receive msg: %s\n", (char *)nlmsg_data(nlh));

    pid = nlh->nlmsg_pid;
    skb_out = nlmsg_new(msg_size, 0);
    if (!skb_out) {
        printk(KERN_INFO "alloc nlmsg failed\n");
        return;
    }
    nlh = nlmsg_put(skb_out, 0, 0, NLMSG_DONE, msg_size, 0);
    NETLINK_CB(skb_out).dst_group = 0;
    strncpy(nlmsg_data(nlh), msg, msg_size);

    res = nlmsg_unicast(my_nl_sock, skb_out, pid);
    if (res < 0)
        printk(KERN_INFO "nlmsg unicast failed\n");
    return;
}

static int __init hello_init(void)
{
    struct netlink_kernel_cfg cfg = {
            .groups = 0,
            .input = hello_nl_recv_msg
    };

    printk("Entering: %s:\n", __FUNCTION__);
    my_nl_sock = netlink_kernel_create(&init_net, MY_NETLINK_TYPE, &cfg);

    if (!my_nl_sock) {
        printk(KERN_ALERT "netlink create sock failed\n");
        return -10;
    }
    return 0;
}

static void __exit hello_exit(void)
{
    printk(KERN_INFO "Exiting hello module\n");
    netlink_kernel_release(my_nl_sock);
    my_nl_sock = NULL;
}

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("Yangjin");
MODULE_DESCRIPTION("netlink hello module");

Makefile:

obj-m = netlink_hello_mod.o
KVERSION = $(shell uname -r)
PWD = $(shell pwd)
all:
        make -C /lib/modules/$(KVERSION)/build M=$(PWD) modules
clean:
        make -C /lib/modules/$(KVERSION)/build M=$(PWD) clean

2. 编写用户进程代码

用户进程主要使用netlink的socket与内核进行通信，首先向内核发送一条消息，然后再接收内核的消息，代码如下:

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

#define MY_NETLINK_TYPE 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(int argc, char *argv[])
{
    sock_fd = socket(PF_NETLINK, SOCK_DGRAM, MY_NETLINK_TYPE);
    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", NLMSG_DATA(nlh));
    close(sock_fd);
    return 0;
}

3. 测试结果

先make一下编译生成我们的内核模块文件：netlink_helloc_mod.ko，然后加载该模块：

: insmod netlink_hello_mod.ko

加载后可用lsmod查看是否加载成功，然后编译并运行用户进程hello_user，可在终端中看到如下结果：

Sending message to kernel
Waiting for message from kernel
Received message payload: Hello from kernel

上面是用户进程的输出，可用dmesg命令查看内核模块的输出（或者直接查看/var/log/kern）.

Living@Greatwall

如何在linux内核模块中加入netlink通信接口

1. 编写内核模块

2. 编写用户进程代码

3. 测试结果

: insmod netlink_hello_mod.ko