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 10:45:11 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	neilb@...e.de, david@...morbit.com,
	aneesh.kumar@...ux.vnet.ibm.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 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.

--b.
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