linux-kernel - netlink socket kernel to user comm

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Date:	Fri, 9 Oct 2009 16:50:11 -0500
From:	Richard Langly <richard.ringo.langly@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: netlink socket kernel to user comm

My user space program isn't getting the msg from my module. Having a
problem making netlink communicate from kernel to user space.

I load my module, then I load my user space program. My module waits
20 secs before sending data so that my user space program calls bind
on an existing socket.

Kernel code
---
static void nl_data_ready(struct sock *sk, int len)
{
    wake_up_interruptible(sk->sk_sleep);
}

// I call this from my module_init function.
int my_init()
{
    struct sk_buff *skb = NULL;
    int err = 0;
    u32 pid = 0;

    my_sk_sk = netlink_kernel_create(&init_net, NETLINK_TEST, 0, nl_data_ready,
        NULL, THIS_MODULE);

    if (my_sk_sk == NULL) {
        printk("my_sk_init: could not create socket\n");

        return -1;
    }

    // Wait 20 seconds before sending msg so that I can start my user space
    // program after my module is loaded so that it can call bind() on
an existing
    // socket.
    unsigned long delay = jiffies + 20 * HZ;

    while (time_before(jiffies, delay));

    size_t len = 0;

    strcpy(skb->data, "SEND_INIT");

    if (len > 0)
        skb->data[len - 1] = '\0';

    len = strlen(skb->data);

    skb = alloc_skb(NLMSG_SPACE(len), GFP_KERNEL);

    if (skb) {
        char *scratch;

        scratch = skb_put(skb, len);
    }

    NETLINK_CB(skb).pid = 0;

    err = netlink_unicast(my_sk_sk, skb, pid, MSG_DONTWAIT);

    kfree_skb(skb);

    sock_release(my_sk_sk->sk_socket);

    return 0;
}

User space code which I call from main()
---
int test_nl()
{
    nl_sock = socket(PF_NETLINK, SOCK_RAW, NETLINK_TEST);

    memset(&src_addr, 0, sizeof(src_addr));

    src_addr.nl_family = AF_NETLINK;
    src_addr.nl_pid = getpid();     /* self pid */
    src_addr.nl_groups = 0;         /* not in mcast groups */
    bind(nl_sock, (struct sockaddr *) &src_addr, sizeof(src_addr));

    nlh = (struct nlmsghdr *) malloc(NLMSG_SPACE(MAX_PAYLOAD));

    memset(&nlh, 0, NLMSG_SPACE(MAX_PAYLOAD));

    iov.iov_base = (void *) nlh;
    iov.iov_len = NLMSG_SPACE(MAX_PAYLOAD);

    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;

    while (1) {
        recvmsg(nl_sock, &msg, 0);
        printf("Received from kernel: %s\n", NLMSG_DATA(nlh));
    }

    close_nl();
}


int main(int argc, char *argv[])
{
    test_nl();

    close_nl();
}
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