如何使用netlink套接字与内核模块通信? [英] How to use netlink socket to communicate with a kernel module?

问题描述

我正在尝试编写一个Linux内核模块，该模块使用netlink与用户进程进行通信.我之所以使用netlink，是因为我要通信的用户程序只能使用套接字进行通信，而我无法更改它以添加ioctl()或其他任何内容.

问题是我无法弄清楚该怎么做.我已经在Google上搜索过，但是发现的所有示例都是古老的，例如这一个，并且不再适用于当前的内核版本.我还查看了这个SO问题，但是此处的示例使用 libnl 进行套接字操作，但我想坚持使用标准套接字功能(由sys/socket.h).因此，有人可以指导我在这里找到一些教程或指南，或者可以帮助我理解netlink的界面和用法的东西.我将高度赞赏一个有效的示例，它没有什么花哨的地方，只是一个非常基本的示例，该示例说明了如何建立从用户程序中的套接字到内核中的套接字的连接，然后将用户进程中的数据发送到内核并从内核中接收回来.

也请不要告诉我看内核代码.我已经在做，但是要花很多时间，而且我还剩很多.

更新: 经过大量的试验和错误后，我有以下代码将消息从用户程序发送到内核，但是消息从内核到用户程序，即使用netlink_unicast()无法正常工作.它不仅不起作用，而且调用使系统挂起，然后我必须重新启动计算机.可以请一个人看看并告诉我我在做什么错.在以下代码中注释了netlink_unicast()调用.对于内核到用户程序的消息，应该不加注释.

用户程序

#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);  
}

内核代码

#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);  

解决方案

阅读内核源代码之后，我终于设法使netlink套接字对我有用.以下是Netlink套接字基础知识的示例，即打开一个Netlink套接字，对其进行读写，然后将其关闭.

内核模块

 #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");
 

用户程序

 #include <linux/netlink.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.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));
    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);
}
 

有关魔术常数NETLINK_USER 31的相关主题:内核空间中可以有32个以上的netlink套接字吗?

I am trying to write a linux kernel module that communicates with user process using netlink. I am using netlink because the user program I want to communicate to communicates only using sockets and I cant change that to add ioctl() or anything.

Problem is that I cant figure out how to do that. I have googled but all examples I found are for old like this one and no longer valid for current kernel versions. I have also looked at this SO question but the sample here uses libnl for socket operations but I want to stick to standard socket functions (defined by sys/socket.h). So can some one plz guide me here to some tutorial or guide or some thing that can help me understand the interface and usage of netlink. I would highly appreciate a working example, nothing fancy, just a very basic example of how to establish a connection from a socket in user program to a socket in kernel and then send data from user process to kernel and receive back from kernel.

Also please do not tell me to look at kernel code. I am already doing it but it will take a lot of time and I dont have lot of it left.

Update: After lot of trial and error I have following code which sends message from user program to kernel but the message from kernel to user program i.e using netlink_unicast() is not working. Its not only not working, the call hangs the systems and then I have to restart the machine. Can some one plz take a look and tell me what wrong I am doing. The netlink_unicast() call is commented in the following code. It should be uncommented for kernel to user program message.

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);  

解决方案

After reading kernel source I finally managed to make netlink sockets work for me. Below is an example of Netlink socket basics i.e opening a netlink socket, reading and writing to it and closing it.

Kernel Module

#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");

User Program

#include <linux/netlink.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.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));
    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);
}

Related thread about the magic constant NETLINK_USER 31: Can I have more than 32 netlink sockets in kernelspace?

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