Re: Do I need to act on this warning from apt when installing grub?

[Pascal Hambourg]

On 16/12/2022 at 11:15, Pascal wrote:
> GRUB :
>
>               INTEL/MBR               GPT
>              PARTITIONING         PARTITIONING
>        +---------------------+---------------------+
>        |MBR 1/3              |MBR 1/3              |
>        |POST-MBR-GAP 2/3     |                     |
>        |+-------------------+|+-------------------+|
>        ||PART               |||PART BIOS-BOOT 2/3 ||
>        ||/boot/grub/ 3/3    ||+-------------------+|
>   BIOS ||...                ||+-------------------+|
>   BOOT ||                   |||PART ESP (FAT)     ||
>        ||                   |||/boot/grub/ 3/3    ||
>        ||                   |||...                ||
>        |+-------------------+|+-------------------+|
>        +---------------------+---------------------+
>        |                     |                     |
>        |                     |                     |
>        |+-------------------+|+-------------------+|
>        ||PART ESP (FAT)     |||PART ESP (FAT)     ||
>        ||/boot/efi/ 1/2     |||/boot/efi/ 1/2     ||
>   UEFI ||/boot/grub/ 2/2    |||/boot/grub/ 2/2    ||
>   BOOT ||...                |||...                ||
>        ||                   |||                   ||
>        ||                   |||                   ||
>        |+-------------------+|+-------------------+|
>        +---------------------+---------------------+

1) There is no need for an ESP for BIOS boot.
2) /boot/grub does not have to be in the ESP.

> 3 steps for GRUB in BIOS mode and 2 steps in UEFI mode.
>
> in BIOS mode :
> first, BIOS load MBC (Master Boot Code - step 1/3 of GRUB) from MBR to
> memory and run it
> - with INTEL/MBR partitioning, GRUB will be chained from MBR, then to
> POST-MBR-GAP (step 2/3) and then to /boot/grub/... (step 3/3)

"GRUB will be chained from MBR" makes no sense and does not fit between 
1/3 and 2/3.

> - with GPT partitioning, GRUB will be chained from MBR, then to a special
> partition of type BIOS-BOOT (step 2/3 - can't use POST-MBR-GAP because used
> by GPT) and then /boot/grub/... (step 3/3)

From the GRUB boot loader point of view, both are exactly the same. The 
GRUB boot image in the MBR does not read the partition table nor know 
anything about the BIOS boot partition. The GRUB core image start sector 
is embedded in the boot image in both cases. Only grub-install uses the 
BIOS boot partition.

> in UEFI mode :
> first, UEFI load an run a (GRUB) loader (step 1/2 of GRUB) located on ESP
> (EFI system partition formated with FAT file system)
> then the loader chain to /boot/grub/... (step 2/2)

With secure boot, the UEFI firmware first loads shim which then loads GRUB.