Date:	Fri, 17 Apr 2009 11:18:17 +0200
From:	Thierry <chocapiiic.tiery@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Trouble using netlink_broadcast, need help

Hi,

I want to use netlink socket between kernel and userspace in order to
get some information from kernel code.
I use netlink_broadcast function in order to send information to a
specific group.
Everythings works when I use netlink_unicast.

Actually, I get theses errors when using broadcast:
For kernel code: netlink broadcast return -3, No such process.
User code: can't bind: errot -3.

I have put one programs at the end of my mail, who works when using
unicast instead of
broadcast, while GROUP_IB is set to 0, and I fix manually pid of the
user-level program.
When I changed unicast to broadcast, and I set GROUP_IB to an other
value (I have tried many values, 1 24 65), I get the errors.

I have looked into kernel sources, and there is also somethings
strange for me: in source code of netlink_kernel_create:
if (group <32)
  group = 32;
This lines were not present in kernel 2.6.9.
I am also wondering if there isn't somethings wrong with listenners:
there are some tests on *has listenners*.

I use centos5.2, kernel 2.6.18-92.1.22

If someone can help me...

I have already asked help on other mailing list (kernelnewbies) , but
I couln't succed to make netlink_broadcast working.

Regards,

Thierry


----
kernel code:

#include <linux/autoconf.h>
#include <linux/socket.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/netlink.h>
#include <net/sock.h>
#include <net/netlink.h>

#define MAX_PAYLOAD 1024
#define NETLINK_IB 24
#define GROUP_IB 1


static struct sock *nl_sk = NULL;

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

static void netlink_ib_open()
{
       struct sk_buff *skb = NULL;
       struct nlmsghdr *nlh = NULL;
       int err;

       nl_sk = netlink_kernel_create(NETLINK_IB, GROUP_IB, nl_ib_data_ready,
THIS_MODULE);
       if (nl_sk == NULL) {
               printk(KERN_ALERT "Error during netlink_kernel_create");
       }

       skb = alloc_skb(NLMSG_SPACE(MAX_PAYLOAD),GFP_KERNEL);
       nlh = (struct nlmsghdr *)skb->data;
       nlh = skb_put(skb, NLMSG_ALIGN(1024));
       nlh->nlmsg_pid = 0; // Send from kernel
       nlh->nlmsg_flags = 0;

       strcpy(NLMSG_DATA(nlh), "Greeting from kernel!");
       NETLINK_CB(skb).pid = 0;
       NETLINK_CB(skb).dst_pid = 0; //Multicast
       NETLINK_CB(skb).dst_group = GROUP_IB;// 0 if unicast

       printk( KERN_ALERT "Sending data...\n");
       //err = netlink_unicast(nl_sk, skb, 6992, GFP_KERNEL);
       err = netlink_broadcast(nl_sk, skb, 0, GROUP_IB, GFP_KERNEL);
       if (err<0) {
               printk(KERN_ALERT "Error during netlink_broadcast: %i\n",err);
               if (err == -3) {
                       printk(KERN_ALERT "No such process\n");
               }
       }
       goto out;

nlmsg_failure:
       printk(KERN_ALERT "Erreuuuur nlmsg_failure\n");

out:
       sock_release(nl_sk->sk_socket);
}

static int __init netlink_ib_module_init(void)
{
       printk(KERN_INFO "Init netlink_ib module\n");
       netlink_ib_open();
       return 0;
}

static void __exit netlink_ib_module_exit(void)
{
       printk(KERN_INFO "Unloading netlink_ib module\n");
}

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Kernel/User socket for IB support");
module_init(netlink_ib_module_init);
module_exit(netlink_ib_module_exit);


---------
usercode

#include <sys/socket.h>
#include <sys/types.h>
#include <stdio.h>
#include <linux/netlink.h>
#define NETLINK_IB 24
#define GROUP_IB 1
#define MAX_PAYLOAD 1024
struct sockaddr_nl src_addr, dst_addr;
struct nlmsghdr *nlh = NULL;
struct msghdr msg;
struct iovec iov;
int sock_fd;


int main(int argc, char ** argv)
{
       int err;
       sock_fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_IB);
       if (sock_fd<0) {
               char s[BUFSIZ];
               sprintf( s, "%s: can't assign fd for socket", argv[0] );
               perror(s);
               return -1;
       }

       //memset(&src_addr, 0, sizeof(src_addr));
       src_addr.nl_family = AF_NETLINK;
       src_addr.nl_pad = 0;
       src_addr.nl_pid = getpid();
       src_addr.nl_groups = GROUP_IB; // Multicast

       err = bind(sock_fd, (struct sockaddr*)&src_addr, sizeof(src_addr));
       if (err<0) {
               char s[BUFSIZ];
               sprintf( s, "%s: can't bind socket (%d)", argv[0], sock_fd );
               perror(s);
               return -1;
       }

       memset(&dst_addr, 0, sizeof(dst_addr));
       nlh = (struct nlhmsghdr *) 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_name = (void *)&dst_addr;
       msg.msg_namelen = sizeof(dst_addr);
       msg.msg_iov = &iov;
       msg.msg_iovlen = 1;

       printf("Waiting for messages from kernel...\n");
       recvmsg(sock_fd, &msg, 0);
       printf("Message : %s\n", NLMSG_DATA(nlh));
       close(sock_fd);

       return 0;
}
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