Re: [Bug-xorriso] Creating a data partition after writing iso-hybrid image

[Thomas Schmitt]

Hi,

i made this successful experiment with a Debian amd64 ISO:

Obtain an MBR template file from the existing MBR. Ubuntu uses local
disk file /usr/lib/syslinux/isohdpfx.bin instead. Both are supposed
to yield the same result after -isohybrid-mbr.

  dd if=debian-8.4.0-amd64-netinst.iso bs=512 count=1 \
     of=debian-8.4.0-amd64-netinst.mbr

Mount the ISO for copying its files

  mount debian-8.4.0-amd64-netinst.iso /mnt/iso

Mix original ISO bootability options (learned from
-report_system_area "as_mkisofs") with options used by the grub-mkrescue
wrapper in mode "mbr_only" and with -partition_offset 16 from the Ubuntu
ISO generation:

  xorriso -as mkisofs \
    -o test.iso \
    -partition_offset 16 \
    -V 'Debian 8.4.0 amd64 1' \
    -isohybrid-mbr debian-8.4.0-amd64-netinst.mbr \
    -no-pad \
    -append_partition 2 0xef /mnt/iso/boot/grub/efi.img \
    -c /isolinux/boot.cat \
    -b /isolinux/isolinux.bin \
       -no-emul-boot -boot-load-size 4 -boot-info-table \
    -eltorito-alt-boot \
    -e --interval:appended_partition_2:all:: \
       -no-emul-boot \
    /mnt/iso

--------------------------------------------------------------------------

Inspecting the result by

  xorriso -indev test.iso -report_system_area plain -report_el_torito plain

yields:

  Volume id    : 'Debian 8.4.0 amd64 1'
  System area options: 0x00000102
  System area summary: MBR isohybrid cyl-align-on
  ISO image size/512 : 569344
  Partition offset   : 16
  MBR heads per cyl  : 64
  MBR secs per head  : 32
  MBR partition table:   N Status  Type        Start       Blocks
  MBR partition      :   1   0x80  0x17           64       569280
  MBR partition      :   2   0x00  0xef       569344          832
  El Torito catalog  : 1039  1
  El Torito cat path : /isolinux/boot.cat
  El Torito images   :   N  Pltf  B   Emul  Ld_seg  Hdpt  Ldsiz         LBA
  El Torito boot img :   1  BIOS  y   none  0x0000  0x00      4        1040
  El Torito boot img :   2  UEFI  y   none  0x0000  0x00    832      142336
  El Torito img path :   1  /isolinux/isolinux.bin
  El Torito img opts :   1  boot-info-table isohybrid-suitable
  El Torito img blks :   2  208

There is still a copy of the EFI boot image in the ISO, because i did not
remove it from the input tree. This file /boot/grub/efi.img is not used
for booting, though, but rather its copy in partition 2 outside the ISO
filesystem is published by El Torito and MBR partition table.

  /sbin/fdisk -l test.iso

reports

  Disk /dvdbuffer/test.iso: 278.4 MiB, 291930112 bytes, 570176 sectors
  Units: sectors of 1 * 512 = 512 bytes
  Sector size (logical/physical): 512 bytes / 512 bytes
  I/O size (minimum/optimal): 512 bytes / 512 bytes
  Disklabel type: dos
  Disk identifier: 0x7c277999

  Device               Boot  Start    End Sectors  Size Id Type
  /dvdbuffer/test.iso1 *        64 569343  569280  278M 17 Hidden HPFS/NTFS
  /dvdbuffer/test.iso2      569344 570175     832  416K ef EFI (FAT-12/16/32)

There have been reports that some EFI firmware does not boot if there is
no partition with boot flag. But there is also the EFI prescription that
the EFI System Partition shall not be the one marked by this flag.
So we can leave it where isohybrid prescribes it to be.
(Afaik, it is obeyed only by certain Windows-ish boot loaders which
 then chainload the bootloader in the marked partition.)

Putting the ISO onto an USB stick, i get a mountable partition 1:

  mount /dev/sdc1 /mnt/iso2

  diff -r /mnt/iso /mnt/iso2

yields only these lines

  diff: /mnt/iso/debian: recursive directory loop
  Binary files /mnt/iso/isolinux/boot.cat and /mnt/iso2/isolinux/boot.cat differ
  Binary files /mnt/iso/isolinux/isolinux.bin and 
/mnt/iso2/isolinux/isolinux.bin differ

The files differ because they contain the block address of the BIOS boot
image, which differs between both ISOs.

It boots as BIOS -hda on qemu of Debian 8

  qemu-system-x86_64 -enable-kvm -m 512 -hda test.iso

as EFI -hda

  qemu-system-x86_64 -bios /usr/share/ovmf/OVMF.fd -enable-kvm -m 512 -hda 
test.iso

and with both virtual firmwares as -cdrom instead of -hda.

--------------------------------------------------------------------------

Now i am curious whether this suits better your intentions and needs.

One disadvantage is that the ISO filesystem size does not cover the
full size of the image file. If you have copying instructions or verification
instructions, which depend on the result of program "isosize", then you
would have to modify them to rather use the size of the image file.


Have a nice day :)

Thomas