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

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Message-ID: <20090624105336.GO8633@ZenIV.linux.org.uk>
Date:	Wed, 24 Jun 2009 11:53:36 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	David Howells <dhowells@...hat.com>,
	Miklos Szeredi <miklos@...redi.hu>, alan@...rguk.ukuu.org.uk,
	hch@...radead.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, adilger@....com,
	akpm@...ux-foundation.org
Subject: Re: [RFC] O_NOACC: open without any access

On Tue, Jun 23, 2009 at 10:34:49AM -0700, Linus Torvalds wrote:

> 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?

i_filesystem_fop is certainly bogus, but why do we want to bother with
file_operations at all?

Unless you really insist on unlimited use of ioctl(2) on such beasts (and
any users will be non-portable for obvious reasons anyway), there's no need
to go anywhere near ->open() *or* ->f_op in general.

Just add new methods to ->i_op (and we already have that coming from
fs code) and teach do_filp_open() to
	* call permission() with new flag (MAY_TALK_TO_FS_NODE) for such
open()
	* do not die with -ELOOP on symlinks if we have O_NOFOLLOW + your flag
	* do not call ->f_op->open() at all for such open()
and we are all set.  Hell, we can even teach sys_ioctl() that given set
of ioctls maps to calls of our new methods.  Taken from ->i_op...

If we want full-blown ->ioctl() coming from the fs code on such opens, we
will need distinct file_operations, no matter what we do with ->open().
It's also doable (we'd need ->i_fop pointing to new foofs_special_file_ops
and its ->open() to be a boilerplate that would replace file->f_op with
the normal one in case of normal open()), but it's more boilerplate patches
and I really don't see what would it buy...

Comments?
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