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RFC: A partition for grubenv, etc.

From: Chris Murphy
Subject: RFC: A partition for grubenv, etc.
Date: Tue, 25 May 2021 16:58:23 -0600


It's not possible for GRUB pre-boot environment to write to grubenv
when it's on Btrfs, ZFS, LVM, mdadm raid, or LUKS. Also, at least XFS
upstream is super skeptical of anything except kernel code making any
kind of modification inside the file system region, and I suspect it's
the same concern with ext4 developers too. While there are file system
specific locations for bootloader usage, they're all different and
quite small. XFS has none. ext4 has 512 bytes. Btrfs has maybe 1 or 2
MiB, ZFS (?), mdadm (?) and LVM (?).

Proposal: A new partition type for MBR and GPT, functionally a
replacement for the BIOS Boot partition, but it would be a partition
owned by the bootloader for whatever it wants to use it for. It'd be
up to the bootloader to figure out how to segment it for bootloader
and environmental portions. We definitely need both MBR and GPT
partition types, it should be a partition exclusively reserved for the
bootloader. This effectively deprecates the use of the MBR gap, and
BIOS Boot partition types, and further it deprecates the use of file
systems (all of them, for consistency sake) for the use of grubenv.

Variation: Keep BIOS Boot and repurpose it to include grubenv, while
also specifying an MBR type code for its equivalent.

Use case: For example, Fedora has a "hidden grub menu" feature where
by a variable in grubenv is reset (written to) by grub pre-boot. And
then a systemd unit changes that variable to indicate a successful
boot once some time and/or tests have been met. If grubenv indicates
successful boot, the next boot's grub menu is hidden. If it wasn't
successful the next boot's grub menu is displayed. It's only possible
to achieve this with some reliable bidirectional way of communicating
between the preboot and booted environments, which is the point of
grubenv, but it can't work much of the time due to the above

Chris Murphy

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