[PATCH v2 0/9] Add support for external environment block on Btrfs

[Michael Chang via Grub-devel <grub-devel@gnu.org>]

This patch series adds support for storing the GRUB environment block in
a reserved area of the Btrfs header. On copy on write filesystems such
as Btrfs, the normal file based envblk cannot be updated safely at
runtime because block addresses are not stable. The reserved area
provides a fixed location that GRUB can write directly, allowing
commands such as grub-reboot and savedefault to work on Btrfs volumes.

The series proceeds in small chunks to keep each change buildable and
easier to review. The first patches add new data structures and helpers
for creating, opening, and writing an environment block in the reserved
area. Later patches update set_variables, unset_variables, and
list_variables so they can use the external block when it is present. An
entry is added to the Btrfs header to reserve space at 256 KiB for the
environment block. Finally, grub.cfg is modified so that load_env and
save_env use the external block automatically when env_block is defined.

v2:
- Define ENV_BTRFS_OFFSET as 256*1024
- Do not conflate type and variable definitions
- Align typedef with struct declaration to follow coding style
- Use bool as the return type of is_abstraction()
- Add "if (dev->disk != NULL)" check in is_abstraction()
- Refine the loop logic in is_abstraction() tests
- Remove extra indentation and redundant lines in read_envblk_fs
- Use off_t and size_t for offset and size variables, fix similar cases
  throughout
- Use explicit check "(fp == NULL)" instead of "(! fp)", fix similar
  cases throughout
- Use bool for the "found" field in var_lookup_ctx
- Add documentation describing the Btrfs environment block and special
  environment block variables

Michael Chang (9):
  util/grub-editenv: add basic structures and probe call for external
    envblk
  util/grub-editenv: add fs_envblk open helper
  util/grub-editenv: add fs_envblk write helper
  util/grub-editenv: wire set_variables to optional fs_envblk
  util/grub-editenv: wire unset_variables to optional fs_envblk
  util/grub-editenv: wire list_variables to optional fs_envblk
  btrfs: add environment block to reserved header area
  00_header.in: wire grub.cfg to use env_block when present
  docs: add Btrfs env block and special env vars

 docs/grub.texi           |  60 ++++++
 grub-core/fs/btrfs.c     |   3 +-
 include/grub/fs.h        |   2 +
 util/grub-editenv.c      | 393 ++++++++++++++++++++++++++++++++++++++-
 util/grub.d/00_header.in |  26 ++-
 5 files changed, 477 insertions(+), 7 deletions(-)

-- 
2.51.0


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