From mboxrd@z Thu Jan  1 00:00:00 1970
From: "J. Bruce Fields" <bfields@fieldses.org>
Subject: Re: [PATCH -V14 0/11] Generic name to handle and open by handle
 syscalls
Date: Wed, 7 Jul 2010 12:39:19 -0400
Message-ID: <20100707163919.GA28815@fieldses.org>
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Cc: Miklos Szeredi <miklos@szeredi.hu>, neilb@suse.de,
	david@fromorbit.com, hch@infradead.org, viro@zeniv.linux.org.uk,
	adilger@sun.com, corbet@lwn.net, serue@us.ibm.com,
	hooanon05@yahoo.co.jp, linux-fsdevel@vger.kernel.org,
	sfrench@us.ibm.com, philippe.deniel@CEA.FR,
	linux-kernel@vger.kernel.org
To: "Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>
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On Wed, Jul 07, 2010 at 10:03:39PM +0530, Aneesh Kumar K. V wrote:
> On Wed, 7 Jul 2010 10:45:11 -0400, "J. Bruce Fields" <bfields@fieldses.org> wrote:
> > On Wed, Jul 07, 2010 at 03:35:50PM +0200, Miklos Szeredi wrote:
> > > On Wed, 7 Jul 2010, J. Bruce Fields wrote:
> > > > > > If you use sys or proc, is it possible to get the uuid from a file
> > > > > > descriptor or pathname without races?
> > > > > 
> > > > > You can do stat/fstat to find out the device number (which is unique,
> > > > > but not persistent)
> > > > 
> > > > Is it really unique over time? (Can't a given st_dev value map to one
> > > > filesystem now, and another later?) 
> > > 
> > > It's unique at a single point in time.  But if you have a reference
> > > (e.g. open file descriptor) on the mount then that's not a problem.
> > > 
> > >    fd = open(path, ...);
> > >    fstat(fd, &st);
> > >    search st.st_dev in mountinfo
> > >    close(fd)
> > > 
> > > is effectively the same as an getuuid(path) syscall (lazy unmounted
> > > filesystems will not be found in mountinfo, but the reference is still
> > > there so st_dev will not be reused for other filesystems).
> > 
> > OK, cool.
> > 
> > That still leaves the problem that there isn't always an underlying
> > block device, and/or when there is it doesn't always uniquely specify
> > the filesystem.
> > 
> 
> And for this reason we would need this as a syscall right ?

That's the only solution I see.  (Or use an xattr?)

--b.