Re: ISO installer image: GPT versus MBR partitions

["pelzflorian (Florian Pelz)" <pelzflorian@pelzflorian.de>]

On Sun, Apr 21, 2019 at 11:35:49AM +0200, Thomas Schmitt wrote:
> Ok, now for the last experiment. Just for curiosity, as i believe it
> is not a good solution (and mformat -k isn't either):
> Does start LBA 1 of partition 1 prevent the endless loop, too ?
> 
> So please write to byte 446 (dec) to 461 (dec) of the FAT image:
> 
>   80  00  02  00  01  01  12  4f  01  00  00  00  3f  0b  00  00
> 
> In the FAT image treated by the "dd" above, these bytes are all 00.
> In the original FAT image they are:
> 
>   80  00  01  00  01  01  12  4f  00  00  00  00  40  0b  00  00
>           ==                      ==              ==  ==
> 

Why is 0b underlined?

OK, so I just wrote the Guix git master with ludo’s reproducibility
patches to a USB drive (boot gets stuck again) and then did:

sudo dd if=/dev/sdc2 of=mysdc2.img

When I open it with ghex, I find at position 446:

    80  00  00  04  01  24  4f  00  00  00  00  80  16  00  00  00

Strange.  I write the guixsd-install-0.16.0.x86_64-linux.iso to
/dev/sdc (boot gets stuck) and repeat

sudo dd if=/dev/sdc2 of=mysdc2.img

I find:

    80  00  01  00  01  01  12  4f  00  00  00  00  40  0b  00  00

This matches what you wrote.  I change it to

    80  00  02  00  01  01  12  4f  01  00  00  00  3f  0b  00  00

I dd the changed mysdc2.img back to /dev/sdc2.

Now it boots. :)  Strangely, I now have only one entry “GRUB 2.02” in
the boot selection, but “EFI Boot” (or what it was called) is gone.



> Nevertheless, start LBA 1 is less desirable than no partition table
> entry at all, because LBA 1 causes a partition that is not mountable.
> So an option for mformat would be of interest, which writes an MBR
> (other than with -k) but does not create a partition entry.
> 

This raises the question what should be done in Guix.  Is it a good
idea to add -k to mformat in grub-mkrescue for the upcoming Guix 1.0
release (even though you don’t like it)?  Could it break install on
other machines?  Or should mformat be patched instead?  Could any of
this be upstreamed?  What about your MBR repacking?  I’m sure Ludo has
an opinion on these things too.  I just doing what I’m told, but I
don’t quite understand what I’m doing here.

Regards,
Florian