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

[Thomas Schmitt]

Hi,

> 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.
 
At leat the existence of that MBR partition is the difference
between libisofs with and witout the bug fix. GPT did not change
by this.

 
> 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.

Yes. That is most likely the reason why you need the MBR partition.
It is a feature of mjg's layout.
http://mjg59.dreamwidth.org/11285.html states
"it turns out that there are some systems that refuse to BIOS-boot
 off GPT media. So we need an MBR partition map. The first entry
 covers the entire disk and is of type 0. This is important, because
 some EFI implementations are strict about MBR parsing - if there's
 an MBR with overlapping partitions, the entire MBR will be ignored.
"
"Why have a GPT at all? The EFI spec says that machines should boot
 fine from an MBR partition. Sadly, not all seem to.
"

> > 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.

Pity. Did you check that the manipulation really changed
only byte 450(dec) from 0x00 to 0xee ?
I had hoped that at least gdisk would accept this. The description
hybrid.html obviously stems from gdisk documentation.

I assume you do not see a valid GPT without -partition_offset
either.
If you see a GPT, then please correct me.
In that case, the partition start at 64 blocks could indeed
be the culprit that deceives gdisk.

The normal way to make GPT visible would be to reduce the
MBR partition table to form a "protective MBR".
To achieve a protective MBR you would have to zeroize the bytes
462(dec) to 509 in the ISO image (made without -partition_offset).
Then you would have to set byte 450(dec) to 0xee (again).

This would possibly not boot on all systems but should at
least make the GPT recognizable.

Have a nice day :)

Thomas

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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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