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

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Wed, Feb 27, 2008 at 01:14:16PM +0000, Ian Jackson wrote:
> Daniel P. Berrange writes ("Re: [Xen-devel] [PATCH] ioemu block device extent 
> checks"):
> > 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.
> 
> That's fine because it's only called like that as a the parent block
> driver.  Perhaps a different name would have been better.
> 
> Out of bounds reads have to be permitted because the block size in the
> header may remain un-updated, so reads of newly-cloned blocks may
> fail.

Which is why I updated the the total_sectors count during writes...

> 
> > 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.
> 
> Well, that seems like makework to me but fine if upstream accept it.

It could help prevent future bugs if some new code is written which is
expecting the total_sectors value to be correct - admitedly the existing
code has incorrect total_sectors count already, but I figure its worth
fixing this to avoid unexpected surprises in the future.

Dan.
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