linux-kernel - Re: [RFC] O_NOACC: open without any access

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Date:	Tue, 23 Jun 2009 10:34:49 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Howells <dhowells@...hat.com>
cc:	Miklos Szeredi <miklos@...redi.hu>, alan@...rguk.ukuu.org.uk,
	hch@...radead.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, viro@...IV.linux.org.uk,
	adilger@....com, akpm@...ux-foundation.org
Subject: Re: [RFC] O_NOACC: open without any access

On Tue, 23 Jun 2009, David Howells wrote:

> Miklos Szeredi <miklos@...redi.hu> wrote:
> 
> > Define O_NOACC as 3.  On open(..., O_FILESYSTEM | O_NOACC) require no
> > privileges on the file.
> 
> It must also work with O_NOFOLLOW, which I think your suggestion will.

This does sound like a fairly natural extension of what we already do.

We essentially already have O_NOACCESS (3), and use it exactly because we 
need to do operations on a file descriptor without "real" accesses 
(notably things like accessing /dev/cdrom without waiting/checking for the 
disk being present etc).

O_FILESYSTEM I don't like as a name (to me, it doesn't say _what_ it is 
doing - of course an open works on a filesystem!), but the concept of 
saying "don't follow device nodes - just open the node itself" makes 
perfect sense. Together with O_NOFOLLOW it also fairly naturally means 
"give me the actual symlink _node_, don't return error or follow it".

And we can trivially test at a higher level that O_FILESYSTEM (with a 
better name, please), is always paired with O_NOACCESS (not O_NOACC: we do 
not try to save three letters, there is no shortage). Because the raw node 
obviously must never really be "accessed" (ie you can't do read/write etc 
on it).

That said, I do _not_ like the notion of

	> Add a new inode->i_filesystem_fop pointer

regardless of whether it's in inode->i_op or wherever. I think we should 
just handle this in the regular "inode->f_op->open" routine, the same way 
we handle FMODE_EXCLUSIVE (O_EXCL), FMODE_NDELAY (O_NONBLOCK) and lack of 
access rights (O_NOACCESS) in the driver open routines that currently 
handle those specially (O_NDELAY is spe

Al?

			Linus
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