Re: reading grub.cfg

[Daniel Kiper]

Hey,

On Wed, Apr 12, 2023 at 01:47:56PM +0200, Laszlo Ersek wrote:
> Hi,
>
> sorry for the beginner questions in advance.
>
> How does grub2 locate and read "grub.cfg"?
>
> My (very superficial) understanding is this:
>
> - grub2 relies on its own filesystem drivers; for example, it does not
> attempt to consume EFI_SIMPLE_FILE_SYSTEM_PROTOCOL directly on a UEFI
> system -- it presumably consumes the BlockIo and/or DiskIo protocols, as
> a dependency of its own filesystem drivers

Yep, here is the driver: grub-core/disk/efi/efidisk.c

> - in one of those detected filesystems, it locates "grub.cfg", and reads it.

It works in a bit different way. Please take a look at [1], [2], [3],
grub-core/kern/main.c:grub_load_normal_mode() and grub-core/normal/main.c.

> Is this more or less correct?
>
> What about the following dependency problem:
> - some filesystem driver is built as a grub2 module,
> - grub.cfg contains a matching "insmod" grub2 command, loading said module,
> - but "grub.cfg" is placed on a filesystem of that particular type.
>
> Is this a realistic problem? If so, how is it solved? Is there something
> obvious I'm missing?

Required filesystem drivers (modules) are embedded into GRUB core.img
and loaded automatically [4] (the picture mentions "stages" which are
legacy thing and should not be used when you speak about GRUB2; though
otherwise it is OK).

> Another question: assuming an EFI_SIMPLE_FILE_SYSTEM_PROTOCOL-based fs
> driver existed in grub2, a particular filesystem might then appear
> *twice* to grub2, once via its own "native" driver, and another time via
> the UEFI SimpleFs-backed driver.

Yes, if both drivers do not communicate between each other to remove duplicates
then a given filesystem can be seen twice through different paths.

> Would this be a problem?

Due to lack of write support in the GRUB, except an environment file,
it should not be big deal. Though I think it would be better if GRUB
uses only EFI_SIMPLE_FILE_SYSTEM_PROTOCOL and ignores "own drivers"
where possible.

I hope that helps.

Daniel

[1] https://www.gnu.org/software/grub/manual/grub/grub.html#prefix
[2] https://www.gnu.org/software/grub/manual/grub/grub.html#root
[3] https://www.gnu.org/software/grub/manual/grub/grub.html#normal
[4] 
https://en.wikipedia.org/wiki/GNU_GRUB#/media/File:GNU_GRUB_on_MBR_partitioned_hard_disk_drives.svg