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

[Pascal]

does this new summary, which takes into account the previous feedback, seem
"better" to you ?
Is the second remark² correct ?
is the boot sector of the image directly/readily accessible (eg. editable
with hexedit) in the boot sector ?
regards, lacsaP.

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               ││
      ││                   │││/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 ││...                │││...                ││
      ││                   │││                   ││
      ││                   │││                   ││
      │└───────────────────┘│└───────────────────┘│
      └─────────────────────┴─────────────────────┘

3 steps for GRUB in BIOS mode and 2 steps in UEFI mode.

# in BIOS mode :
  - 1/3 BIOS load and run GRUB boot loader located in the MBR
  - 2/3 GRUB boot loader load and run GRUB core image stored in
POST-MBR-GAP (INTEL/MBR) or on BIOS-BOOT partition (GTP)¹
  - 3/3 GRUB core image find and mount the GRUB folder², load configuration
etc..

# in UEFI mode :
  - 1/2 UEFI³ load and run GRUB boot loader located on ESP⁴
  - 2/2 GRUB boot loader find and mount the GRUB folder², load
configuration etc..

¹ in these two cases, start sector of GRUB core image is hard coded in GRUB
boot loader (during installation of GRUB)
² GRUB folder can be installed/located on any type of partition supported
by GRUB
³ or SHIM previously loaded and ran by UEFI if secured boot is activated
⁴ EFI System Partition formated with FAT file system (0xEF in INTEL/MBR and
C12A7328-F81F-11D2-BA4B-00A0C93EC93B in GPT)

Le ven. 16 déc. 2022 à 13:39, Pascal <patatetom@gmail.com> a écrit :

> thank you for this feedback.
>
> Le ven. 16 déc. 2022 à 12:47, Pascal Hambourg <pascal@plouf.fr.eu.org> a
> écrit :
>
>> 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.
>>
>>