Re: USB key and lost space

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

Pascal Hambourg wrote:
> I did not have trouble with the Apple partition table except when using
> libparted-based partition editors, but could you explain how you remove it ?

By dd-ing the first 32 bytes from file "isohdpfx.bin" of package "isolinux"
onto the first 32 bytes of the ISO image (as data file with conv=notrunc
or as USB stick).

debian-9.3.0-amd64-netinst.iso has in its MBR the APM signature and a
fake block count which happen to be x86 machine code without undesirable
side effects:

00000000 :    45  52  08  00  00  00  90  90  00  00  00  00  00  00  00  00
00000010 :    00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00

The first 432 bytes of amd64 and i386 isohybrid ISOs stem from
  /usr/lib/ISOLINUX/isohdpfx.bin
which has x86 code that is similarly harmless but does not indicate APM:

00000000 :    33  ed  90  90  90  90  90  90  90  90  90  90  90  90  90  90
00000010 :    90  90  90  90  90  90  90  90  90  90  90  90  90  90  90  90

This jackalope was invented by Matthew J. Garrett.
  https://mjg59.dreamwidth.org/11285.html

Of course one may use own useless x86 code to deface APM. Some machine code
knowledge would be precondition, though. Assembler source of isohdpfx.bin
is at
  https://github.com/geneC/syslinux/blob/master/mbr/isohdpfx.S


> (Strangely enough, wipefs does not find the GPT)

The GPT in mjg's layout is not valid, because the partition table of the
MBR does not consist of a single partition of type 0xee which starts at
LBA 1.

I meanwhile doubt that it is of any use. The history of mjg's development
indicates that he first started with valid GPT and then ihad to add a MBR
partition of type 0xef for marking the EFI System Partition / Boot Image,
because some machines did not work with the GPT marked EFI partition.

Partition editors like gparted or gdisk detect the presence of the GPT
header block at LBA 1 and then complain about the wrong MBR partition table.
This can be silenced by zeroing bytes 512 to 1023 of the ISO.


A valid GPT can be found in grub-mkrescue ISOs which were produced
while GRUB binary packages for 32-bit EFI and/or 64-bit EFI were installed.


Have a nice day :)

Thomas