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 can't change that to add ioctl() or anything.
I can't figure out how to do that, though! I found old examples like this one that are no longer valid for current kernel versions. I have also looked at this SO question but the sample here uses libnl for socket operations, and I want to stick to standard socket functions (defined by sys/socket.h). Can some one please 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.
Please do not tell me to look at kernel code. I am already doing that, but it will take a lot of time and I don't have lot of it left.
After lot of trial and error I have following code which sends a message from a user program to the kernel, but the message from the kernel to the user program (using netlink_unicast()) is not working. It's not only not working, but the call hangs the systems and then I have to restart the machine. Can some one please take a look and tell me what I'm doing wrong? The netlink_unicast() call is commented in the following code. It should be uncommented for the kernel to user program message.
#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);
}
#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);
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.
#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");
#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?