Sending UDP packet in Linux Kernel

Question

Hi Guys,

For a project, I'm trying to send UDP packets from Linux kernel-space. I'm currently 'hard-coding' my code into the kernel (which I appreciate isn't the best/neatest way) but I'm trying to get a simple test to work (sending "TEST"). It should be mentioned I'm a newbie to kernel hacking - I'm not that clued up on many principles and techniques!

Every time my code gets run the system hangs and I have to reboot - no mouse/keyboard response and the scroll and caps lock key lights flash together - I'm not sure what this means, but I'm assuming it's a kernel panic?

The repeat_send code is unnecessary for this test code, yet when it's working I want to send large messages that may require multiple 'send's - I'm not sure that if could be a cause of my issues?

N.B. This code is being inserted into neighbour.c of linux-source/net/core/ origin, hence the use of NEIGH_PRINTK1, it's just a macro wrapper round printk.

I'm really banging my head against a brick wall here, I can't spot anything obvious, can anyone point me in the right direction (or spot that blindingly obvious error!)?

Here's what I have so far:

void mymethod()
{
    struct socket sock;
    struct sockaddr_in addr_in;
    int ret_val;
    unsigned short port = htons(2048);
    unsigned int host = in_aton("192.168.1.254");
    unsigned int length = 5;
    char *buf = "TEST\0";
    struct msghdr msg;
    struct iovec iov;
    int len = 0, written = 0, left = length;
    mm_segment_t oldmm;

    NEIGH_PRINTK1("forwarding sk_buff at: %p.\n", skb);

    if ((ret_val = sock_create(PF_INET, SOCK_DGRAM, IPPROTO_UDP, &sock)) < 0) {
        NEIGH_PRINTK1("Error during creation of socket; terminating. code: %d\n", ret_val);
        return;
    }

    memset(&addr_in, 0, sizeof(struct sockaddr_in));
    addr_in.sin_family=AF_INET;
    addr_in.sin_port = port;
    addr_in.sin_addr.s_addr = host;

    if((ret_val = sock.ops->bind(&sock, (struct sockaddr *)&addr_in, sizeof(struct sockaddr_in))) < 0) {
    NEIGH_PRINTK1("Error trying to bind socket. code: %d\n", ret_val);
    goto close;
    }

    memset(&msg, 0, sizeof(struct msghdr));
    msg.msg_flags = 0;
    msg.msg_name = &addr_in;
    msg.msg_namelen = sizeof(struct sockaddr_in);
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;
    msg.msg_control = NULL;
    msg.msg_controllen = 0;

repeat_send:
    msg.msg_iov->iov_len = left;
    msg.msg_iov->iov_base = (char *)buf + written;

    oldmm = get_fs(); 
    set_fs(KERNEL_DS);
    len = sock_sendmsg(&sock, &msg, left);
    set_fs(oldmm);

    if (len == -ERESTARTSYS)
        goto repeat_send;
    if (len > 0) {
        written += len;
        left -= len;
        if (left)
            goto repeat_send;
    }

close:
    sock_release(&sock);
}

Any help would be hugely appreciated, thanks!

Answer 1

Run your code when you're in a text mode console (i.e. press Ctrl+Alt+F1 to go to the text console). This way a kernel panic will print out the stack trace and any extra information about what went wrong.

If that doesn't help you, update your question with the stack trace.

Answer 2

I'm not much of a Linux Kernel developer, but can you throw some printk's in there and watch dmesg before it goes down? Or have you thought about hooking up with a kernel debugger?

Answer 3

You may find it easier to use the netpoll API for UDP. Take a look at netconsole for an example of how it's used. The APIs you're using are more intended for userspace (you should never have to play with segment descriptors to send network data!)

Answer 4

I think you should try to put all variables outside mymethod() function and make them static. Remember, that the size of kernel stack is limited do 8KiB, so to much of/too big local variables may cause stack overflow and system hangup.