Subject: Re: [PATCH net-next 1/2] socket: Allow external sockets to use socket syscalls

From: Jason Wang <[email-masked]>
Date: Fri, 26 Dec 2014 17:45:53 +0800

On 12/26/2014 02:50 PM, Alex Gartrell wrote:
> Currently the "is-socket" test for a file compares the ops table pointer,
> which is static and local to the socket.c.  Instead, this adds a flag for
> private_data_is_socket.  This is an exceptionally long commit message for a
> two-line patch.
>
> Signed-off-by: Alex Gartrell <[email-masked]>
> ---
>  include/linux/fs.h | 2 +-
>  net/socket.c       | 3 ++-
>  2 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index bb29b02..d162476 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -809,7 +809,7 @@ struct file {
>  #endif
>  	/* needed for tty driver, and maybe others */
>  	void			*private_data;
> -
> +	bool			private_data_is_socket : 1;
>  #ifdef CONFIG_EPOLL
>  	/* Used by fs/eventpoll.c to link all the hooks to this file */
>  	struct list_head	f_ep_links;
> diff --git a/net/socket.c b/net/socket.c
> index 8809afc..cd853be 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -388,6 +388,7 @@ struct file *sock_alloc_file(struct socket *sock, int flags, const char *dname)
>  	sock->file = file;
>  	file->f_flags = O_RDWR | (flags & O_NONBLOCK);
>  	file->private_data = sock;
> +	file->private_data_is_socket = true;

This is only safe if all user of sock_alloc_file() have full support for
each method in proto_ops.
>  	return file;
>  }
>  EXPORT_SYMBOL(sock_alloc_file);
> @@ -411,7 +412,7 @@ static int sock_map_fd(struct socket *sock, int flags)
>  
>  struct socket *sock_from_file(struct file *file, int *err)
>  {
> -	if (file->f_op == &socket_file_ops)
> +	if (file->private_data_is_socket)
>  		return file->private_data;	/* set in sock_map_fd */
>  
>  	*err = -ENOTSOCK;

Not sure it's the best method, how about a dedicated f_op to do this?