Re: RFC: A partition for grubenv, etc.

[Chris Murphy]

On Wed, Jul 28, 2021 at 6:23 AM Daniel Kiper <dkiper@net-space.pl> wrote:
>
> At this point I am not fully convinced the /boot/grub should be on
> a separate filesystem. Though after going through this thread it seems
> to me we should add full write support to the GRUB for a simple file
> system. The FAT looks like a good candidate. Does not it? This way we
> will be able to support writable grubenv without any limitations and have
> flexibility to add other writable files without bigger hassle if needed.

On UEFI, the firmware's FAT driver should be write capable, but I
don't know if GRUB can make use of it directly rather than using
GRUB's own FAT driver?

On non-UEFI systems, we still need GRUB to have its own read-write
capable driver. I think parity between BIOS and UEFI is important, but
I'd also say if there's a faster/easier/more maintainable path with
some other file system for other bootloaders, that'd be even more
preferred. I don't know of such a file system. But UDF is in the same
ballpark, long since supported in the kernel, supports optical,
removable, hard drive use cases.

> Additionally, it is worth mentioning the [1] work which adds a save_env
> redirection layer. It allows us to choose target for grubenv. However,
> if we want make it more generic then we could redirect any writes to any
> target which can be potentially useful.

Btrfs has two fairly large areas for exclusive bootloader usage,
~64KiB from partition start to the superblock. And ~1-2 MiB after the
first superblock and before the first block group. But no other file
system I'm aware of has such a large reservation for the bootloader.
XFS has none. ext4 might have 512 bytes. I think it's valid to ask for
an explicit reservation for bootloader usage, but this is just more of
the same issue as grubenv and BIOS Boot. There is no file system in
such a location so in effect it's up to GRUB to internally implement
something, in which case it might as well just have a dedicated
partition for grubfs or whatever it'd be.

> Last but not least, I think we do not need to change anything in current
> partitions usage. In particular it seems to me the BIOS Boot Partition
> usage should stay as is.

I don't mind just retasking BIOS Boot into a  general "bootloader"
partition that bootloaders can use. Or even make it explicitly a GRUB
bootloader partition type GUID. It's better if each bootloader has its
own? Originally BIOS Boot wasn't "owned" by GRUB, any bootloader can
use it, but insofar as I'm aware it's the only bootloader that uses
this partition type GUID.





--
Chris Murphy