How to get sockfd from kernel space?

Question

Is there a way to get the sockfd from a struct sock or any other way that would allow me to uniquely identify the socket / connection I'm working with in kernel space?

I need this piece of information in the context of a device driver for a network adapter.

Answer 1

I thought it was impossible but actually there is a way, at least for simple cases where we have no duplicate file descriptors for a single socket. I'm answering my own question, hoping it'll help people out there.

int get_sockfd(struct sock *sk)
{
    int sockfd;
    unsigned int i;
    struct files_struct *current_files;
    struct fdtable *files;
    struct socket *sock;
    struct file *sock_filp;

    sockfd = -1;

    sock = sk->sk_socket;
    sock_filp = sock->file;

    current_files = current->files;
    files = files_fdtable(current_files);

    for (i = 0; files->fd[i] != NULL; i++) {
            if (sock_filp == files->fd[i]) {
                    sockfd = i;
                    break;
            }
    }

    return sockfd;
}

You would of course want to check for NULL pointers, starting with struct sock *sk passed in param.

So, basically, the idea is that the numerical value of a file descriptor (a sockfd is just a regular file descriptor, after all) corresponds to the index of its corresponding entry in a process open files table. All we have to do when given a struct sock *sk pointer is loop over the open files table of the current process until the addres pointed to by sk->sk_socket->file matches an entry in the table.