linux-kernel - Re: [RFC PATCH] fs: Add user_file_or_path

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Date:	Wed, 28 Aug 2013 00:08:27 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Willy Tarreau <w@....eu>, Ingo Molnar <mingo@...nel.org>,
	"security@...nel.org" <security@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Brad Spengler <spender@...ecurity.net>
Subject: Re: [RFC PATCH] fs: Add user_file_or_path_at and use it for truncate

On Tue, Aug 27, 2013 at 12:16:34PM -0700, Andy Lutomirski wrote:
> This is an experiment to see if we can get nice semantics for all syscalls
> that either follow symlinks or allow AT_EMPTY_PATH without jumping through
> enormous hoops.  This converts truncate (although you can't tell using
> truncate from coreutils, because it actually uses open + ftruncate).
> 
> The basic idea is that there's a new helper function
> user_file_or_path_at.  It takes an fd and a path and, depending on
> flags, the emptiness of the path, and whether path is a magic /proc
> symlink (or a symlink to a magic /proc/symlink), it returns either a
> struct path or a struct file *.

No.  

> +	path_get(&nd->path);
> +	if (nd->flags & LOOKUP_FILE) {
> +		if (nd->last_symlink_file)
> +			fput(nd->last_symlink_file);
> +		nd->last_symlink_file = file;

This is ugly (and costs quite a bit of overhead)

> -static int proc_cwd_link(struct dentry *dentry, struct path *path)
> +static int proc_cwd_link(struct dentry *dentry, struct file_or_path *link)

... and this is even more vile.  Vetoed, for being too ugly to live.

I think I've a saner approach, not involving anything that ugly; I'll post
a writeup later tonight.
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