Comment 15 for bug 1062737

On Mon, Oct 08, 2012 at 07:53:10PM -0000, Thomas Schmitt wrote:
> > However, as the resultant image still appears to not have a valid MBR
> > partition table, there doesn't seem to be any advantage to adding this
> > option back to our builds for 12.10. So I think we'll leave it as is
> > with -partition_offset 16 disabled for now.

> Do you rather mean "no valid GPT" ?

That's not what I mean, but it's possible that I'm mistaken. UEFI looks for
an EFI system partition, which is supposed to be defined as a GPT partition
of type EF00 containing a FAT filesystem. Since this image now boots under
UEFI, UEFI is satisfied that it's found a system partition - which I was
assuming meant a valid GPT. However, I now see that an MBR partition of
type 'EF' will be up-converted to a GPT partition of type 'EF00', which I
guess is what is happening here.

> The MBR seems to be ok now. After all it made the difference
> between boot and not-boot.

fdisk remains unhappy with the image, but this may be a separate issue:

Warning: /home/vorlon/devel/iso/quantal-desktop-amd64.iso contains GPT
signatures, indicating that it has a GPT table. However, it does not have a
valid fake msdos partition table, as it should. Perhaps it was corrupted --
possibly by a program that doesn't understand GPT partition tables. Or perhaps
you deleted the GPT table, and are now using an msdos partition table. Is
this a GPT partition table?
   y Yes
   n No

gdisk in turn reports that the image is MBR-only.

> The GPT is there but it usually not recognized because the MBR
> is not trivial enough (not a "protective MBR"). This should have
> nothing to do with -partition_offset.
> If it has, then this would be probably another bug.

> I googled a bit and found:
> http://www.rodsbooks.com/gdisk/hybrid.html
> The described "Hybrid MBR" would differ from mjg's mainly by the
> type of the first MBR partition: 0xee rather than 0x00 im mjg's.

> So does it help GPT visibility to set byte 450 of the ISO image
> to the value 0xee ? (Byte count starting at 0)

That doesn't seem to make any difference in whether fdisk/gdisk are happy
with it, at least.