Kernel To Userspace Application Communication

I am trying to make Kernel (Android, kernel 4.9.59) communicate with userspace applications. I found a solution using Netlink sockets: https://stackoverflow.com/a/25071310/4190159

Solution 1:

1. Let's check netlink_kernel_create function in linux kernel:

staticinlinestructsock *
netlink_kernel_create(struct net *net, int unit, struct netlink_kernel_cfg *cfg)
{
    return __netlink_kernel_create(net, unit, THIS_MODULE, cfg);
}

from here https://elixir.bootlin.com/linux/v4.9.59/source/include/linux/netlink.h#L60 Notice, that this function takes only 3 arguments ( instead of 6 in your code )

This function have been changed in kernel 3.6 ( from 6 parameters to 4 ) https://elixir.bootlin.com/linux/v3.6/source/include/linux/netlink.h#L185

And then renamed to __netlink_kernel_create in kernel 3.7

netlink_kernel_create function from 3.7 accepts 3 arguments https://elixir.bootlin.com/linux/v3.7/source/include/linux/netlink.h#L48

Try change this

socket = netlink_kernel_create(&init_net, NETLINK_USERSOCK, 1, nl_receive_callback, NULL, THIS_MODULE);

to this

struct netlink_kernel_cfg cfg = {
    .input  = nl_receive_callback,
    .groups = 1,
};
socket = netlink_kernel_create(&init_net, NETLINK_USERSOCK, &cfg);

2. Now you can send data in direction "kernel => application".

When you start your application, it will bind socket with user_sockaddr.nl_groups = MY_GROUP; bind(sock_fd, (struct sockaddr*)&user_sockaddr, sizeof(user_sockaddr));.

After this you can send data from kernel to application with NETLINK_CB(socket_buff).dst_group = MY_GROUP; nlmsg_multicast(socket, socket_buff, 0, MY_GROUP, GFP_KERNEL); and it will be received by application with recvmsg(sock_fd, &msghdr, 0);

How can I call kernel_send_nl_msg() function to actually communicate with the userspace?

You can call it from kernel module, which you write, compile and insmod into kernel. Or you can call it directly from kernel, but for this you will need to rebuild whole kernel.

If you want to send data in direction "application = > kernel", then you need to do things in reverse: bind new socket with another MY_GROUP2 in kernel and send data from application with nlmsg_multicast