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Re: [Xen-devel] [PATCH] ioemu block device extent checks

On Wed, Feb 27, 2008 at 11:28:05AM +0000, Ian Jackson wrote:
Content-Description: message body text
> Daniel P. Berrange writes ("Re: [Xen-devel] [PATCH] ioemu block device extent 
> checks"):
> >    The qcow driver though calls back into
> > the raw driver for performing I/O on its underlying file. The qcow 
> > driver relies on this file being grow-on-demand for purposes of allocating
> > new qcow sectors. The safety checks cause this allocation to fail and
> > it all goes downhill from there :-(  
> Oh dear.  (I'm a bit surprised that it's taken this long to spot!)
> Here is a patch for xen-unstable which I think will fix it.  Could you
> give it a quick spin, if you have a suitable test setup ?
> Sadly it's rather more intrusive than ideal, since it needs all of the
> drivers which are going to extend files via their parents to announce
> this, and a couple of bits of necessary infrastructure needed adding.

I don't think this is correct - it allows a -ve  size / nb_sectors
value when autoextenable is set, and allows out of bounds reads.

I sent a patch to qemu-devel yuesterday which also uses the auto-extend
flag, but has separate checks for read vs writes. When doing a write that
would extend the device it increases the total_sectors count so that the
subsequent reads can be validated to be within the written bounds.


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