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fs/btrfs: Add environment block to reserved header area

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This patch reserves space for the GRUB environment block inside the
Btrfs header. The block is placed at an offset of GRUB_ENV_BTRFS_OFFSET,
256 KiB from the start of the device, and occupies one sector. To
protect the space, overflow guard sectors are placed before and after
the reserved block.

The Btrfs header already defines regions for bootloader use. By adding
this entry, GRUB gains a fixed and safe location to store the environment
block without conflicting with other structures in the header.

Add Btrfs and its reserved area information to the fs_envblk_spec table.
With the groundworks done in previous patches, the function is now
complete and working in grub-editenv.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
This commit is contained in:
Michael Chang 2025-10-17 17:01:40 +08:00 committed by Daniel Kiper
parent d6525f0e65
commit e4e1776613
3 changed files with 19 additions and 2 deletions
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10
grub-core/fs/btrfs.c
View File

@ -2337,11 +2337,16 @@ struct embed_region {
* https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs(5)#BOOTLOADER_SUPPORT
* The first 1 MiB on each device is unused with the exception of primary
* superblock that is on the offset 64 KiB and spans 4 KiB.
*
* Note: If this table is modified, also update
* util/grub-editenv.c::fs_envblk_spec, which describes the file-system
* specific layout of reserved raw blocks used as environment blocks so that
* both stay consistent.
*/
static const struct {
struct embed_region available;
struct embed_region used[6];
struct embed_region used[9];
} btrfs_head = {
.available = {0, GRUB_DISK_KiB_TO_SECTORS (1024)}, /* The first 1 MiB. */
.used = {
@ -2349,6 +2354,9 @@ static const struct {
{GRUB_DISK_KiB_TO_SECTORS (64) - 1, 1}, /* Overflow guard. */
{GRUB_DISK_KiB_TO_SECTORS (64), GRUB_DISK_KiB_TO_SECTORS (4)}, /* 4 KiB superblock. */
{GRUB_DISK_KiB_TO_SECTORS (68), 1}, /* Overflow guard. */
{(GRUB_ENV_BTRFS_OFFSET >> GRUB_DISK_SECTOR_BITS) - 1, 1}, /* Overflow guard. */
{(GRUB_ENV_BTRFS_OFFSET >> GRUB_DISK_SECTOR_BITS), 1}, /* Environment Block. */
{(GRUB_ENV_BTRFS_OFFSET >> GRUB_DISK_SECTOR_BITS) + 1, 1}, /* Overflow guard. */
{GRUB_DISK_KiB_TO_SECTORS (1024) - 1, 1}, /* Overflow guard. */
{0, 0} /* Array terminator. */
}

2
include/grub/fs.h
View File

@ -132,4 +132,6 @@ grub_fs_unregister (grub_fs_t fs)
grub_fs_t EXPORT_FUNC(grub_fs_probe) (grub_device_t device);
#define GRUB_ENV_BTRFS_OFFSET (256 * 1024)
#endif /* ! GRUB_FS_HEADER */

9
util/grub-editenv.c
View File

@ -153,9 +153,16 @@ static fs_envblk_ops_t fs_envblk_ops = {
/*
* fs_envblk_spec describes the file-system specific layout of reserved raw
* blocks used as environment blocks.
* blocks used as environment blocks. At present only Btrfs is supported. Other
* file-systems may be added if they provide a similar facility and avoid the
* limitation of writing to COW.
*
* Note: If this table is modified, also update
* grub-core/fs/btrfs.c::btrfs_head, which defines the layout in the Btrfs
* header and exports GRUB_ENV_BTRFS_OFFSET, so that both stay consistent.
*/
static fs_envblk_spec_t fs_envblk_spec[] = {
{ "btrfs", GRUB_ENV_BTRFS_OFFSET, GRUB_DISK_SECTOR_SIZE },
{ NULL, 0, 0 }
};