linux-kernel - Re: [PATCH -V14 0/11] Generic name to handle and open by handle syscalls

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Date:	Wed, 7 Jul 2010 12:39:19 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>, neilb@...e.de,
	david@...morbit.com, hch@...radead.org, viro@...iv.linux.org.uk,
	adilger@....com, corbet@....net, serue@...ibm.com,
	hooanon05@...oo.co.jp, linux-fsdevel@...r.kernel.org,
	sfrench@...ibm.com, philippe.deniel@....FR,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V14 0/11] Generic name to handle and open by handle
 syscalls

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@...ldses.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.
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