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

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Message-ID: <20090623134640.GA13831@infradead.org>
Date:	Tue, 23 Jun 2009 09:46:40 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, viro@...IV.linux.org.uk,
	adilger@....com, dhowells@...hat.com, alan@...rguk.ukuu.org.uk,
	akpm@...ux-foundation.org
Subject: Re: [RFC] O_NOACC: open without any access

On Thu, Jun 18, 2009 at 08:55:37AM +0200, Miklos Szeredi wrote:
> Implement a new open flag that doesn't require any access on the file.
> It works on any file type including symlinks.
> 
> The sole purpose is to help race free "userspace lookup" type
> operations.  So fstat, fch*, *at work but nothing else.  Filesystem's
> ->open() is not called and f_op is set to NULL.

we guarantee that f_op is never NULL, so you'll need to assign a
file operations structure that is empty to it to avoid crashed in
various places.

> It would be logical to reuse the open_flag=3 value, but that has
> historically been used with different semantics so I'm afraid of
> touching it.

I think the historical semantics are exactly that you can open it
an issue ioctls + stat / etc on it ut not actually read/write it.

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