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Date: Fri, 5 Apr 2013 14:19:41 +0200
From: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20130405121941.GB12305@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] ide: refuse WIN_READ_NATIVE_MAX on empty
 device
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To: Markus Armbruster <armbru@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org

On Fri, Apr 05, 2013 at 12:56:07PM +0200, Markus Armbruster wrote:
> Stefan Hajnoczi <stefanha@redhat.com> writes:
> 
> > What is the highest addressable sector on an empty CD-ROM?  Nothing is
> > addressable so produce an error.
> >
> > This patch prevents a divide-by-zero in ide_set_sector() since
> > s->sectors and s->heads would be 0.  Not to mention that a sector=-1
> > argument would be nonsense.
> >
> > Note that WIN_READ_NATIVE_MAX can be triggered using hdparm -N 1024
> > /dev/cdrom.  The LBA bit will be set to 1 though, so the only easy way
> > to go down the ide_set_sector() CHS code path which divides by zero is
> > to comment out the s->select & 0x40 case for testing.
> 
> Suggests you did that.
> 
> Have you tried the reproducer with a physical drive?  Does it fail the
> command when empty, too?

Believe it or not, I don't have access to an ATAPI CD-ROM drive.  Would
you be able to try out hdparm -N 1024 /dev/cdrom?

Note that READ NATIVE MAX is optional, real drives may not implement it
since it seems geared towards the Host Protected Area feature which
makes no sense on CD-ROMs.  (The idea is a reserved area on the disk
where system data can be stored and the OS will not touch it.)

Stefan