Re: [PATCH] kpartx: Improve finding loopback device by file

[Martin Wilck <mwilck@suse.com>]

On Mon, 2018-02-05 at 15:41 +0100, Julian Andres Klode wrote:
> On Mon, Feb 05, 2018 at 02:53:07PM +0100, Martin Wilck wrote:
> > On Mon, 2018-02-05 at 11:45 +0100, Julian Andres Klode wrote:
> > > On Mon, Feb 05, 2018 at 11:32:13AM +0100, Martin Wilck wrote:
> > > > On Mon, 2018-02-05 at 09:58 +0100, Julian Andres Klode wrote:
> > > > > C
> > > > >  		close (fd);
> > > > >  
> > > > > -		if (0 == strcmp(filename, loopinfo.lo_name))
> > > > > {
> > > > > +		if (0 == strcmp(filename, loopinfo.lo_name)
> > > > > ||
> > > > > +		    0 == strcmp(rfilename, loopinfo.lo_name)
> > > > > ||
> > > > > +		    (realpath(loopinfo.lo_name,
> > > > > rloopfilename)
> > > > > &&
> > > > > +		     0 == strcmp(rfilename, rloopfilename)))
> > > > > {
> > > > >  			found = realpath(path, NULL);
> > > > >  			break;
> > > > >  		}
> > > > 
> > > > That "if x matches y or x matches y' or x' matches y" feels
> > > > like
> > > > guesswork. Can't we simply call realpath() on both
> > > > loopinfo.lo_name
> > > > and
> > > > filename, and compare the two? 
> > > 
> > > Probably yes. That said there might be some corner cases:
> > > 
> > > (1) What happens if the backing file of a loopback is deleted -
> > > realpath
> > >     can fail with ENOENT.
> > > (2) A path could be longer than PATH_MAX and it would fail with
> > > ENAMETOOLONG
> > >     - this is a "problem" with the initial realpath as well, but
> > > less
> > > so, because
> > >     the user can control that.
> > 
> > Both are scenarios in which kpartx would have good reason to fail
> > (with
> > a meaningful error message). That's better than guessing, IMO.
> 
> (1) Definitely not. Just because one of your (potentially other)
> loopback devices
> has a deleted file you should not fail.

Sorry for being imprecise. You're right, aborting here while scanning
for a matching device would of course be wrong. And deleting such a
broken loop device can't be a big mistake.

>  (2) I don't really know.
> 
> But the problem here is that you are looking at loopback devices
> created by
> stuff other than kpartx, and they might not be in a "good" state. But
> you
> don't want to abort just because some _other_ loopback device is
> broken.

Can we agree on the following:

 1 if realpath (filename) results in error, abort
 2 if realpath(lo_name) results in ENODEV and filename matches lo_name,
remove loop device
 3 if realpath(lo_name) results in another error code, skip it
 4 remove if realpath(filename) matches realpath(lo_name)

???

Martin

-- 
Dr. Martin Wilck <mwilck@suse.com>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)

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