From: "Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>
To: "J. Bruce Fields" <bfields@fieldses.org>,
       Miklos Szeredi <miklos@szeredi.hu>
Cc: 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
Subject: Re: [PATCH -V14 0/11] Generic name to handle and open by handle syscalls
In-Reply-To: <20100707144511.GA24360@fieldses.org>
References: <20100706161002.GD7387@fieldses.org> <87eifgfsez.fsf@linux.vnet.ibm.com> <20100706232351.GD25018@dastard> <20100707093629.10c2feab@notabene.brown> <20100707021150.GF25018@dastard> <20100707125726.3695587a@notabene.brown> <20100707125701.GA19872@fieldses.org> <E1OWUOL-0004DM-Ul@pomaz-ex.szeredi.hu> <20100707131721.GB19872@fieldses.org> <E1OWUn0-0004JI-7F@pomaz-ex.szeredi.hu> <20100707144511.GA24360@fieldses.org>
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Date: Wed, 07 Jul 2010 22:03:39 +0530
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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 ?

-aneesh
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