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

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Date:	Tue, 23 Jun 2009 18:05:20 +0100
From:	David Howells <dhowells@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	dhowells@...hat.com, alan@...rguk.ukuu.org.uk, hch@...radead.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, viro@...IV.linux.org.uk,
	adilger@....com, akpm@...ux-foundation.org
Subject: Re: [RFC] O_NOACC: open without any access

Miklos Szeredi <miklos@...redi.hu> wrote:

> So how about the following: provide a new open flag O_FILESYSTEM,
> meaning it opens the file on the underlying filesystem instead of the
> device/socket/symlink/etc...

Yes.  That's what I need.  That's what pioctl() is for: it operates on the
underlying fs, not any special aspects of the various types of file, and since
it doesn't do traditional I/O on those files, it doesn't need to, and
shouldn't, open them (thus avoiding side effects from ->open()), and doesn't
need R/W access to them.

> Add a new inode->i_filesystem_fop pointer

I'd rather not put it there.  That means the inode struct grows.  Perhaps
attach it to the inode_operations table or stick an open_noaccess() op in the
iops table.

> 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.

> AFS would set up i_filesystem_fop with its ->ioctl() function. No
> special handling needed for revoke()...

Sounds reasonable.

> That would work, no?

I think so.

David
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