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netlink-socket

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Linux Kernel Version : 2.6.27.55

netlink 是一种用于内核态和用户态进程之间进行数据传输的特殊的IPC机制。

特点：

  1. 用户态采用socket风格的API
  2. 除了预定义的协议类型之外，支持自定义协议类型
  3. 异步通讯
  4. 支持消息组播
  5. 全双工（特别是支持内核主动发起会话）

数据结构

include/linux/netlink.h
include/linux/rtnetlink.h

netlink地址结构

struct sockaddr_nl                                                            
{                                                                             
    sa_family_t nl_family;  /* AF_NETLINK   */                                
    unsigned short  nl_pad; /* zero     */                                
    __u32       nl_pid;     /* port ID  */                                    
    __u32       nl_groups;  /* multicast groups mask */                   
};

netlink消息结构

nlmsghdr + pad + payload + pad + nlmsghdr + pad + payload + pad …

可以看出来，netlink消息在顶层呈现数组形式平行排列，也就是说，多条netlink消息可以以数组形式一次性传输

netlink消息头结构

struct nlmsghdr                                                                                                                    
{                                                                             
    __u32       nlmsg_len;  /* Length of message including header */          
    __u16       nlmsg_type; /* Message content */                             
    __u16       nlmsg_flags;    /* Additional flags */                        
    __u32       nlmsg_seq;  /* Sequence number */                             
    __u32       nlmsg_pid;  /* Sending process port ID */                     
};

Example-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__);
    //nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, 0, hello_nl_recv_msg, NULL, THIS_MODULE);
    struct netlink_kernel_cfg cfg = {
        .input = hello_nl_recv_msg,
    };

    nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, &cfg);
    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");
#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 */

    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);
}

Example-2

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

内核代码

#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");

用户空间代码

#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;
}

uevent message payload size

lib/kobject_uevent.c kobject_uevent_env

#define UEVENT_BUFFER_SIZE      2048
struct kobj_uevent_env {                                                                                                           
    char *envp[UEVENT_NUM_ENVP];                                              
    int envp_idx;                                                             
    char buf[UEVENT_BUFFER_SIZE];                                             
    int buflen;                                                               
}; 
struct kobj_uevent_env *env;
env = kzalloc(sizeof(struct kobj_uevent_env), GFP_KERNEL);

/* default keys */                                                        
retval = add_uevent_var(env, "ACTION=%s", action_string);                                                                      
if (retval)                                                               
    goto exit;                                                            
retval = add_uevent_var(env, "DEVPATH=%s", devpath);                      
if (retval)                                                               
    goto exit;                                                            
retval = add_uevent_var(env, "SUBSYSTEM=%s", subsystem);                  
if (retval)                                                               
    goto exit;

   ...
/* send netlink message */

NETLINK_CB(skb).dst_group = 1;
netlink_broadcast(uevent_sock, skb, 0, 1, GFP_KERNEL);

Ref

  1. netlink socket编程实例
  2. netlink socket 编程之 why & how
  3. Linux 系统内核空间与用户空间通信的实现与分析
  4. linux-kernel – 如何使用netlink socket与内核模块通信？
  5. linux的netlink接口详解上