[Bug 1062737] Re: hybrid images do not include a valid GPT

[Steve Langasek]

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.

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https://bugs.launchpad.net/bugs/1062737

Title:
  hybrid images do not include a valid GPT

Status in Ubuntu CD image build software:
  Fix Released
Status in “libisofs” package in Ubuntu:
  Triaged

Bug description:
  The insyde UEFI firmware implementation does not see the GPT on the
  hybrid images generated on nusakan (e.g., the 12.10 beta2 image).
  This appears to be consistent with what both gdisk and parted show:

  $ gdisk -l ~/devel/iso/ubuntu-12.10-beta2-desktop-amd64.iso 
  GPT fdisk (gdisk) version 0.8.5

  Partition table scan:
    MBR: MBR only
    BSD: not present
    APM: not present
    GPT: not present

  
  ***************************************************************
  Found invalid GPT and valid MBR; converting MBR to GPT format.
  ***************************************************************

  Disk /home/vorlon/devel/iso/ubuntu-12.10-beta2-desktop-amd64.iso: 1548288 sectors, 756.0 MiB
  Logical sector size: 512 bytes
  Disk identifier (GUID): 1ADDF8C9-5CE1-41A4-9124-2CC80EED991F
  Partition table holds up to 128 entries
  First usable sector is 34, last usable sector is 1548254
  Partitions will be aligned on 2048-sector boundaries
  Total free space is 1548221 sectors (756.0 MiB)

  Number  Start (sector)    End (sector)  Size       Code  Name
  $

  Removing the -partition_offset 16 option from the xorriso commandline
  is sufficient to generate an image with a recognizable partition
  table; but I imagine this causes other problems for hybrid systems.

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