file descriptor abuses

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sun, May 03, 2009 at 08:01:36PM +0100, Al Viro wrote:

> General rules:
> 	* descriptor you've generated is fit only for return to userland;
> 	* descriptor you've got from userland is fit only for *single*
> fget() or equivalent, unless you are one of the core syscalls manipulating
> the descriptor table itself (dup2, etc.)
> 	* once file is installed in descriptor table, you'd better be past
> the last failure exit; sys_close() on cleanup path is not acceptable.
> That's what reserving descriptors is for.
> 
> IOW, the sane solution would be to export something that returns your
> struct file *.  And stop playing with fd completely.

Speaking of which, quick look through fget() callers shows this turd:

static int p9_socket_open(struct p9_client *client, struct socket *csocket)
{
	int fd, ret;

	fd = sock_map_fd(csocket, 0);
	.....

	ret = p9_fd_open(client, fd, fd);
	if (ret < 0) {
		P9_EPRINTK(KERN_ERR, "p9_socket_open: failed to open fd\n");
		sockfd_put(csocket);
		return ret;
	}
	.....
	return 0;
}

where p9_fd_open() calls fget() on its 2nd and 3rd arguments.  Which does
worse than just a leak, AFAICT - on failure exit it leaves a dangling
pointer from descriptor table.

On the almost unrelated note, we have (in drivers/sneak_in^Wstaging/usbip)
sockfd_to_socket(), with all callers leaking struct file, AFAICS.