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

Thomas Schmitt scdbackup at gmx.net
Tue Oct 9 08:03:08 UTC 2012


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

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