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

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Date:	Tue, 23 Jun 2009 22:04:10 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	torvalds@...ux-foundation.org
CC:	dhowells@...hat.com, 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, Linus Torvalds wrote:
> 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".

O_NODEV?  It applies just as well to fifos, sockets and symlinks, but
it's hard to express that in a compact way.

> 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 

No, it's a totally different open, one comes from the device/fifo
code, the other from the filesystem.  Yes, the filesystem could
in theory wedge itself between the VFS and device's f_ops.  Not
sure if that's how this should be done, though...

Also how should the default case (filesystem doesn't handle
O_NODEV) be handled.  The nice thing about O_NODEV | O_NOACCESS
would be that it could be implemented totally in generic code in
a secure way and it would even be useful for a variety of
cases.

Thanks,
Miklos
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