Re: [Xen-devel] [PATCH] Add HybridISO support for PyGrub2

Philipp Hahn
Tue, 25 Oct 2011 10:33:41 -0000

Hello Ian,

Am Dienstag 25 Oktober 2011 11:17:38 schrieb Ian
Campbell:
> On Tue, 2011-10-25 at 09:56 +0100, Philipp Hahn wrote:
> > grub-mkrescue internally uses xorriso, which generates a
> > so-called "hybrid ISO": The ISO images also contains a DOS
partition
> > table, which allows the identical ISO file to be stored on an USB
stick
> > for booting from it. This breaks PyGrub, since it (wrongly) detects
the
> > DOS partition table and uses the first partition instead of the
complete
> > ISO file.
>
> Is the problem here that, having detected a DOS partition, pygrub is
> then unwilling to accept that the partition contains an ISO9660 file
> system?

PyGrub chcks for the DOS partition table by reading the last two bytes of the
first 512 byte sector. Since the HybridISO has a valid partition table, the
first partition starting at offset 512 is used instead of the full image
starting at offset 0.

> Or is it that the DOS partition table covers something other
> than the ISO9660 data?

As far as I understood it, the partition covers the same area of the iso to
protect it from accidently trashing the data on the stick. But since it has
different sector sizes (512 vs 2048) and offsets (512 vs. 0), the data
actually looks like garbage (at least "file" detects nothing)

> Where does the kernel we want to boot actually
> live?

In the ISO starting at offset 0.

> Is there some reference to the layout of an hybridiso?

You might want to take a look at syslinux from hpa; his web-page still seems
to be unaccessable.
<http://forum.avira.com/wbb/index.php?page=Thread&threadID=99493> has
a short
summary.

> i.e. what is the  significance on 0x8006?

It''s what "file" does to detect ISO-Images. At offset 0x8000
the ISO 9660
Primary Volume Descriptor starts. From offset [2..6[ is the "id",
which most
often is "CD001". See
<http://users.telenet.be/it3.consultants.bvba/handouts/ISO9960.html> for a
complete spec.

> What are the chances of false positives?

Existing, but small: 1:2^40.
As fas as I know many Linux distributions use the HybridISO trick. I''ve
read
from Debian, Ubuntu, Arch Linux,Gentoo. But they don''t seem to use
PyGrub
with Xen for PV installations ;-)

Sincerely
Philipp
--
Philipp Hahn           Open Source Software Engineer      hahn@univention.de
Univention GmbH        Linux for Your Business        fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99