Hi,
my assembler reading skills are obviously insufficient. It turns out that
the bytes in the MBR partition table range are not significant for booting
the ISO from qemu -hda.
I boldly zeroed them in a copy of the GNU/Hurd ISO and it still boots.
(To make sure that it does not boot by El Torito i copied the
terminating volume descriptor from block 19 to block 17.)
Encouraged by this outcome, i repacked the ISO by these commands:
# Obtain GRUB image file from start of original ISO
dd if=debian-hurd-2017-i386-NETINST-1.iso bs=2048 count=16 of=grub_embed
mount debian-hurd-2017-i386-NETINST-1.iso /mnt/iso
# Most options as learned from file /mnt/iso/.disk/mkisofs
# but adding option --protective-msdos-label to create a partition table.
xorriso -as mkisofs \
-o test.iso \
-J -joliet-long -r \
-V 'Debian 9.0 h-i386 n' \
--embedded-boot grub_embed \
--protective-msdos-label \
-cache-inodes \
-c boot/boot.cat \
-b boot/grub/grub_eltorito \
-no-emul-boot -boot-load-size 4 -boot-info-table -no-pad \
/mnt/iso
The resulting test.iso boots as qemu -hda to the same GRUB menu as
the original ISO.
The partition table is what grub-mkrescue would ask for in case of
i386-pc without any efi platform:
$ sbin/fdisk -lu test.iso
...
Disklabel type: dos
...
Device Boot Start End Sectors Size Id Type
test.iso1 * 1 311555 311555 152.1M cd unknown
This partition is not mountable because of its start offset 1.
But that's how Vladimir wanted it for grub-mkrescue.
------------------------------------------------------------------
Udate after receiving reply on grub-devel:
http://lists.gnu.org/archive/html/grub-devel/2017-06/msg00043.html
Vladimir 'phcoder' Serbinenko wrote:
> Those are only for floppies with old BIOS. If your image is over 2.88 MiB
> and thus never useful on floppies, it's safe to overwrite.
This explains why it looks like somewhat plausible executable code
and why i386-pc/boot.img of Debian 8 is in the state it is in.
------------------------------------------------------------------
So my proposal to Debian GNU/Hurd is to boldly add option
--protective-msdos-label
to the debian-installer run of xorriso -as mkisofs for hurd.
If an empty partition table is desired instead, then at least zeroize
the 64 bytes beginning at offset 446 of file "grub_embed" before it
gets handed over to xorriso.
Have a nice day :)
Thomas
