fs/btrfs: Add environment block to reserved header area

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>

diff --git a/grub-core/fs/btrfs.c b/grub-core/fs/btrfs.c
index 7bf8d922f6146e16d7a1429d5b6573dc8942d133..fa5d860eb81524afca76fffea67cc4fd06558f74 100644 (file)
--- a/grub-core/fs/btrfs.c
+++ b/grub-core/fs/btrfs.c
@@ -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. */
   }

diff --git a/include/grub/fs.h b/include/grub/fs.h
index df4c93b16f0451c749fd8c1a509eb013af233cbc..89e4d2b9bcadc3e762185140d1b457a4576fb7c3 100644 (file)
--- a/include/grub/fs.h
+++ b/include/grub/fs.h
@@ -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 */

diff --git a/util/grub-editenv.c b/util/grub-editenv.c
index 1b923bf97882f78a1efbb9d4fda7091c49f6accb..41bb867398a0dd3e697f364e7c00ee9df7b6ad73 100644 (file)
--- a/util/grub-editenv.c
+++ b/util/grub-editenv.c
@@ -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 }
 };