linux-kernel - Re: [PATCH] vfs: new O

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Message-Id: <E1Mr9ke-0005uJ-KD@pomaz-ex.szeredi.hu>
Date:	Fri, 25 Sep 2009 14:18:16 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	"Dr. David Alan Gilbert" <linux@...blig.org>
CC:	miklos@...redi.hu, agruen@...e.de, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfs: new O_NODE open flag

On Fri, 25 Sep 2009, Dr. David Alan Gilbert wrote:
> * Miklos Szeredi (miklos@...redi.hu) wrote:
> > On Fri, 25 Sep 2009, Andreas Gruenbacher wrote:
> > > On Thursday, 24 September 2009 16:51:58 Miklos Szeredi wrote:
> > > > This patch adds a new open flag, O_NODE.  This flag means: open just
> > > > the filesystem node instead of the object referenced by the node.
> > > 
> > > What is the intended use for O_NODE?
> > 
> > It lets userspace file descriptors reference a inode without actually
> > "dereferencing" it to get the underlying object.  This allows for a
> > couple of new things:
> > 
> >  - opening a special file (device/socket/fifo) without side effects
> > 
> >  - opening a symlink
> > 
> >  - opening any type of file without any permission is also possible
> >    (of course the resuling file descriptor may not be read or written)
> > 
> > The above allows fstat(), fchmod(), ioctl(), etc to be used for files
> > previously not possible.
> 
> Given an fd opened in this way is it possible to reopen it normally and
> be guarenteed to get the same object?

For directories it should be possible with:

  fd = open(path, O_NOACCESS | O_NODE);
  fd2 = openat(fd, ".", O_RDWR);

We could implement something similar for non-directories as well, by
treating a NULL path parameter to openat() to mean - reopen:

  fd2 = openat(fd, NULL, O_RDWR);

Comments?

Thanks,
Miklos
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