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Re: Several GNU projects wondering about the reason for mformat partitio

From: Thomas Schmitt
Subject: Re: Several GNU projects wondering about the reason for mformat partition table
Date: Sun, 02 Jun 2019 20:34:05 +0200


thanks a lot for the background info.

Alain Knaff wrote:
> A platform that expects the disk to be unpartitioned would just ignore
> this mini partition table.

The EFI platform encounters the mformat-produced image in a partition.
Nearly all implementations indeed ignore the MBR-ish table in the
partition. But one wanted to be smart, obviously.
(The disk device, a USB stick, was partitioned by GPT.)

If you see any other potential problem with using a mformat image
as content of a partition, then please tell.

> In the next release I'll provide a new option to just skip this
> partition table [...]
> If all goes well, I expect to make this new release by the end of the month

Good move.


I assume that grub-mkrescue will have to wait a while before it can
unconditionally use a new option of mformat, or that it will need a
function like check_xorriso()
which is used to look for a five year old option in the help text of xorriso:

The three alternatives for grub-mkrescue now look like this:

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

No objections are known yet against partition table removal. Its purpose
has nothing to do with the use case as EFI System Partition.
I am out of ideas whom else to ask for an opinion.

So i think grub-mkrescue should dare to choose one of the other two:

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

If i am not mistaken, then this is equivalent to the new option of mformat.
grub-mkrescue could anticipate this option by own postprocessing after the
mformat run.

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

Alain Knaff wrote about the new option probably in contrast to -k:
> [...] but still initialize the other fields (jump at
> beginning of boot sector, mformat "banner", 0xaa55 boot signature,
> physdrive, mtools boot code, zeroing out unused bytes)

None of the items in this list looks like it would be necessary for a
partition image which gets created as new file. I assume that no non-zero
bytes can sneak in, if the image file does not exist before it gets
written by mformat.

Have a nice day :)


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