Several GNU projects wondering about the reason for mformat partition table

[Thomas Schmitt]

Hi,

in the course of a problem search between GRUB, Guix, and xorriso we found
the trigger of a problem with some old Macbook EFI firmware in a gesture
of mformat.
Line 1375 of
  
http://svn.savannah.gnu.org/viewvc/mtools/trunk/mformat.c?revision=506&view=markup
has

  /* install fake partition table pointing to itself */

Now we are wondering for which use case there is this MBR partition table
entry starting with LBA 0 and claiming the whole device.

The SVN at savannah shows that this gesture was introduced with revision 4
in may 2002 by "aknaff". My hope is that this was you and that you can tell
grub-devel (Cc'ed) more about the motivation.

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In case you are curious about the motivation of this mail:

Our particular use case is to create a FAT filesystem image for use as EFI
system partition in ISO 9660 images on USB sticks.
grub-mkrescue runs mformat to create this image and then uses mcopy to
populate it with EFI start programs. See
  http://git.savannah.gnu.org/cgit/grub.git/tree/util/grub-mkrescue.c#n812

The partition entry with start LBA 0 caused the EFI firmware to get stuck,
probably looping endlessly. Experiments with dd reveiled that the partition
entry was to blame, and in it the start LBA 0.

This is clearly a firmware bug and obviously fixed in later models of
Macbooks. Nevertheless we care for old oddball hardware.

So the question is which of these alternatives to choose:

- Keep the partition entry because its removal could break some other
  EFI firmware's boot process.

- Overwrite the partition table in bytes 446 to 509 of the mformat result
  by zeros before populating it with files.

- Use mformat option -k to avoid production of the partition table.

I personally have scruples to omit the other fields which get written
if option -k is not present. Is there anything written with (!keepBoot)
which we need for a vanilla FAT filesystem to be recognizable or usable ?

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Have a nice day :)

Thomas