Re: [Bug-xorriso] Mountable ISO and GRUB2

[Thomas Schmitt]

Hi,

Andrei Borzenkov wrote:
> As example, openSUSE image provides access to ISO content via partition
> with second file table

Yep. They make two ISOs, cut off the head of the second one, and graft it
on the skullcap of the first ISO. Partition 1 marks the EFI part of the head.
Partition 2 marks the first ISO. It lacks a hidden file of the size of the
gap between first and second ISO superblock. So both superblocks and
directory trees can use the same file content blocks.
I think it's done here
  https://github.com/openSUSE/kiwi/blob/master/modules/KIWIIsoLinux.pm
about file "glump".

This relies on a reproducible sequence of data file content in the ISO.
Older xorriso might not be eager enough to keep that sequence. Meanwhile
it is supposed to be clean of random influences except some automatic
timestamps which may be overridden explicitely by SOURCE_DATE_EPOCH.


> Or is having the same ISO both via full device and partition a problem?

Not to my knowledge. ISOLINUX isohybrid does this regularly.

The OpenSUSE ISO is a bit exotic in the ISOLINUX world by not following
the mjg layout. It rather has an MBR partition table with disjoint partitions
and no (useless) GPT added, just like my favorite "mbr_only". So "mbr_only"
has some foreign trail blazer experience to rely on.

A main difference towards my "mbr_only" layout plus -partition_offset 16
is that xorriso delivers the child already with two heads, whereas KIWI
has to act as doctor Frankenstein.

An advantage of Franken-ISO is that the first superblock claims the whole
image size.
I will have to think about letting my first superblock claim the blocks up
to the end of all appended partitions, if this first superblock is not
inside a partition. In this case it simply does not exist for UEFI and
thus cannot break the specs by spanning over several partitions.


Have a nice day :)

Thomas