Re: [Bug-xorriso] ISO installer image: GPT versus MBR partitions

[Thomas Schmitt]

Hi,

Florian Pelz wrote:
> Sorry, I meant to quote this aspect:

I wrote:
> > > One can read trailing garbage not only from USB sticks but also from
> > > most DVD types.

Florian Pelz wrote:
> > So… GPT is wrong for some optical media too?

I wrote:
> Not in this aspect. GPT partitions must not start at block 0.
> So the overall ISO superblock is not mountable as partition and thus
> free to claim the full image size as filesystem size.

I already understood you correctly ... i think. :))

GPT partitioned ISO are not hampered from telling the full image
size as size of the ISO 9660 filesystem.

  $ /sbin/gdisk -l guixsd-install-0.15.0.i686-linux.iso
  ...
  Number  Start (sector)    End (sector)  Size       Code  Name
     1              64           34107   16.6 MiB    0700  Gap0
     2           34108           39867   2.8 MiB     EF00  EFI boot partition
     3           39868         1815727   867.1 MiB   AF00  HFSPLUS
     4         1815728         1816327   300.0 KiB   0700  Gap1

  $ expr $(/sbin/isosize guixsd-install-0.15.0.i686-linux.iso) / 512
  1816376

The isosize is a bit larger than the end of partition 4, because after
this partition comes the GPT backup.
The isosize matches the image file size:

  $ expr $(ls -l guixsd-install-0.15.0.i686-linux.iso | awk '{print $5}') / 512
  1816376


But an MBR partition which begins by block 0 and is mountable as ISO 9660
filesystem keeps its ISO superblock from telling a size larger than the
partition. So it cannot claim an appended partition 2 as part of its range.

  $ export MKRESCUE_SED_MODE=mbr_hfs
  $ grub-mkrescue --xorriso=./grub-mkrescue-sed.sh -o output.iso minimal \
                  --iso_mbr_part_type 0x83
  ...

  $ /sbin/fdisk -l output.iso
  ...
  Disklabel type: dos
  ...
  Device      Boot Start   End Sectors  Size Id Type
  output.iso1 *        0 28371   28372 13.9M 83 Linux
  output.iso2      28372 34131    5760  2.8M ef EFI (FAT-12/16/32)

  $ expr $(/sbin/isosize output.iso) / 512
  28372

This discrepancy of image file size and isosize is undesirable when the
image file itself cannot say its size any more, because it is on USB
stick or a DVD which delivers more bytes than were written as image.

So i propose partition offset 16 to get the partition superblock away
from being mountable by the base device.

  $ export MKRESCUE_SED_MODE=mbr_hfs
  $ grub-mkrescue --xorriso=./grub-mkrescue-sed.sh -o output.iso minimal \
                  -partition_offset 16 --iso_mbr_part_type 0x83

This yields

  $ /sbin/fdisk -l output.iso
  ...
  Disklabel type: dos
  ...
  Device      Boot Start   End Sectors  Size Id Type
  output.iso1 *       64 28695   28632   14M 83 Linux
  output.iso2      28696 34455    5760  2.8M ef EFI (FAT-12/16/32)

The ISO filesystem of the overall image then claims the image file size

  $ expr $(/sbin/isosize output.iso) / 512
  34456

With the partition's ISO filesystem we only get the partition size:

  $ dd if=output.iso bs=512 skip=64 of=partition1.iso

  $ expr $(/sbin/isosize partition1.iso) / 512
  28632

This is how it should be.


The ISOs of Fedora, Debian, Ubuntu, and others are MBR partitioned and
most of them have no partition offset 16. But despite that fact they can
claim the full image size as ISO 9660 filesystem size.

This is possible because partition 1 covers the whole image and partition 2
is located inside partition 1. Strictly illegal in UEFI specs, unless
the outer partition has MBR partition type 0x00 and thus does not exist
for EFI and some partition editors. (I.e. this is a really dirty hack.)

  $ /sbin/fdisk -l debian-live-9.8.0-amd64-xfce.iso
  ...
  Disklabel type: dos
  ...
  Device                            Boot Start     End Sectors  Size Id Type
  debian-live-9.8.0-amd64-xfce.iso1 *        0 3811391 3811392  1.8G  0 Empty
  debian-live-9.8.0-amd64-xfce.iso2       1432    2263     832  416K ef EFI 
(FAT-1

  $ expr $(/sbin/isosize debian-live-9.8.0-amd64-xfce.iso) / 512
  3811392

Old /sbin/gdisk only reports partition 2:

  $ /sbin/gdisk -l debian-live-9.8.0-amd64-xfce.iso
  ...
  Found valid MBR and GPT. Which do you want to use?
  ...
  Your answer: 1
  ...
  Number  Start (sector)    End (sector)  Size       Code  Name
     2            1432            2263   416.0 KiB   EF00  EFI System



Have a nice day :)

Thomas