c - Netlink 导致内核 panic

Question

我试过this program 。该程序向内核发送“hello”，内核再次向用户空间回复“hello”一次。

我的要求:用户必须发送“hello”并且必须每2秒接收一次消息。

我已经尝试过了。

用户应用:添加了发送和接收循环

#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));
    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;
    while(1) {           // <--- loop here. only works once.
        sleep(2);
        printf("Sending message to kernel\n");
        sendmsg(sock_fd,&msg,0);

        recvmsg(sock_fd, &msg, 0);
        printf("Received message payload: %s\n", (char *)NLMSG_DATA(nlh));
    }

    close(sock_fd);
}

内核模块:没有改变任何东西。

#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)
{
    struct netlink_kernel_cfg cfg = {
        .input = hello_nl_recv_msg
    };

    nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, &cfg);

    printk("Entering: %s\n",__FUNCTION__);

    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);

此代码仅适用于第一个循环。下次内核崩溃并在黑屏中出现内核 panic 错误时。每次我重新启动时。为什么它会引起内核 panic ？我需要修改哪里？我认为问题出在内核模块中。
内核:3.13.0-24-generic

Answer 1

嗯，我认为问题出在内核模块中。但问题出在用户模块上。我花了三天时间才发现这个问题。问题出在

while(1) {           
    sleep(2);
    printf("Sending message to kernel\n");
    sendmsg(sock_fd,&msg,0);     // In 2nd iteration msg value chaged

    recvmsg(sock_fd, &msg, 0);  // <--- msg will update
    printf("Received message payload: %s\n", (char *)NLMSG_DATA(nlh));
}

此循环仅有效一次。那是真实的。使用 recvmsg 接收消息后，msg 变量被更新，我使用该变量再次发送。这是因为代码过于智能，(源和目标)都引用相同的指针。

通过为 sendmsg 和 recvmsg 编写不同的变量将解决该问题。

int main()
{
    int seq_no = 1;
    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;
                                            /* receiver parameters */
    nlh_in = (struct nlmsghdr *)malloc(NLMSG_SPACE(MAX_PAYLOAD));
    iov_in.iov_base = (void *)nlh_in;
    iov_in.iov_len = nlh->nlmsg_len;
    msg_in.msg_iov = &iov_in;
    msg_in.msg_iovlen = 1;                  /* end */

    while(1) {
        printf("sending message to kernel\n");
        sendmsg(sock_fd,&msg,0);

        recvmsg(sock_fd, &msg_in, 0);
        printf("Received message payload: %s\n", (char *)NLMSG_DATA(nlh_in));
        sleep(1);
    }
    free(nlh);
    close(sock_fd);
}