Re: Grub on BIOS/GPT

[Andrei Borzenkov]

On 21.02.2022 01:53, David Ventura wrote:
> Hi
> I'm trying to boot grub on a BIOS/GPT combination and am currently stuck.
> I've created a test disk image with the following GPT partition (and
> protective MBR):
> 
> 
> Found valid GPT with protective MBR; using GPT.
> Disk hdd-grub.raw: 131072 sectors, 64.0 MiB
> Sector size (logical): 512 bytes
> Disk identifier (GUID): 488005E7-D3AF-4B71-AEE6-F54B497A63FC
> Partition table holds up to 128 entries
> Main partition table begins at sector 2 and ends at sector 33
> First usable sector is 34, last usable sector is 131038
> Partitions will be aligned on 2048-sector boundaries
> Total free space is 2014 sectors (1007.0 KiB)
> 
> Number  Start (sector)    End (sector)  Size       Code  Name
>    1            2048            4095   1024.0 KiB  EF02  BIOS boot partition
>    2            4096           65535   30.0 MiB    EF00  EFI system partition
>    3           65536          131038   32.0 MiB    8300  Linux filesystem
> 
> On this disk image, I've applied the boot.img MBR like so:
> 
>     dd if=/usr/lib/grub/i386-pc/boot.img of=hdd-grub.raw bs=440
> count=1 conv=notrunc
> which successfully shows a "GRUB" prompt on BIOS.
> 
> The only thing I've done after this, is to apply the core.img on the
> exact BIOS boot partition offset:
> 
>     dd if=core.img of=hdd-grub.raw bs=512 seek=2048 conv=notrunc
> 
> However, nothing different happens (the only thing I see is, still,
> "GRUB" and a flashing cursor).
> To be clear, I am _not_ seeing the emergency shell.
> 
> The core image I've generated with this command:
>     grub-mkimage -O i386-pc -o ./core.img -p '(hd0,gpt3)/boot/grub'
> biosdisk part_msdos xfs part_gpt
> 
> I've tried embedding an early config and actually populating
> (hd0,gpt3)/boot/grub/grub.cfg out of desperation but that did not
> help.
> 
> What am I doing wrong?
> 

You do not use grub-install.

core.img location is stored in both boot.img by grub.install. What you
see is boot.img which attempts to load and execute garbage so system hangs.