[PATCH] Use full btrfs bootloader area

[Michael Chang]

Up to now grub can only embed to the first 64 KiB before primary
superblock of btrfs, effectively limiting the size that could
consequently pose restrictions to feature enablement like advancing zstd
compression.

This patch attempts to utilize full unused area reserved by btrfs for
bootloader outlined in the document [1] where this paragraph quoted.

"The first 1MiB on each device is unused with the exception of primary
superblock that is on the offset 64KiB and spans 4KiB."

Apart from that, adjacent sectors to superblock and first block group
are not used for embedding in case of overflow and it's tracing.

This patch has been tested to provide out of the box support to btrfs
zstd compression with which grub has been installed to the partition.

[1] https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs(5)#BOOTLOADER_SUPPORT

Signed-off-by: Michael Chang <mchang@suse.com>
---
 grub-core/fs/btrfs.c | 80 +++++++++++++++++++++++++++++++++++++-------
 1 file changed, 67 insertions(+), 13 deletions(-)

diff --git a/grub-core/fs/btrfs.c b/grub-core/fs/btrfs.c
index 63203034d..dcd4635f1 100644
--- a/grub-core/fs/btrfs.c
+++ b/grub-core/fs/btrfs.c
@@ -2159,6 +2159,35 @@ grub_btrfs_label (grub_device_t device, char **label)
 }
 
 #ifdef GRUB_UTIL
+
+struct embed_region {
+  unsigned int offset;
+  unsigned int len;
+};
+
+#define KB_TO_SECTOR(x) ((x) << 1)
+
+/*
+ * https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs(5)#BOOTLOADER_SUPPORT
+ * The first 1MiB on each device is unused with the exception of primary
+ * superblock that is on the offset 64KiB and spans 4KiB.
+ */
+
+static const struct {
+  struct embed_region available;
+  struct embed_region used[6];
+} area = {
+  .available = {0, KB_TO_SECTOR(1024)}, /* The first 1MiB */
+  .used = {
+    {0, 1},                              /* boot.S */
+    {KB_TO_SECTOR(64) - 1, 1},           /* overflow guard */
+    {KB_TO_SECTOR(64), KB_TO_SECTOR(4)}, /* 4KiB superblock */
+    {KB_TO_SECTOR(68), 1},               /* overflow guard */
+    {KB_TO_SECTOR(1024) - 1, 1},         /* overflow guard */
+    {0, 0}                               /* array terminator */
+  }
+};
+
 static grub_err_t
 grub_btrfs_embed (grub_device_t device __attribute__ ((unused)),
                  unsigned int *nsectors,
@@ -2166,25 +2195,50 @@ grub_btrfs_embed (grub_device_t device __attribute__ 
((unused)),
                  grub_embed_type_t embed_type,
                  grub_disk_addr_t **sectors)
 {
-  unsigned i;
+  unsigned int i, j;
+  const struct embed_region *u;
+  grub_disk_addr_t *map;
+  unsigned int n = 0;
 
   if (embed_type != GRUB_EMBED_PCBIOS)
     return grub_error (GRUB_ERR_NOT_IMPLEMENTED_YET,
                       "BtrFS currently supports only PC-BIOS embedding");
 
-  if (64 * 2 - 1 < *nsectors)
-    return grub_error (GRUB_ERR_OUT_OF_RANGE,
-                      N_("your core.img is unusually large.  "
-                         "It won't fit in the embedding area"));
-
-  *nsectors = 64 * 2 - 1;
-  if (*nsectors > max_nsectors)
-    *nsectors = max_nsectors;
-  *sectors = grub_calloc (*nsectors, sizeof (**sectors));
-  if (!*sectors)
+  map = grub_calloc (area.available.len, sizeof (*map));
+  if (!map)
     return grub_errno;
-  for (i = 0; i < *nsectors; i++)
-    (*sectors)[i] = i + 1;
+
+  for (u = area.used; u->len; ++u)
+    {
+      unsigned int end = u->offset + u->len;
+
+      if (end > area.available.len)
+        end = area.available.len;
+      for (i = u->offset; i < end; ++i)
+        map[i] = 1;
+    }
+
+  for (i = 0; i < area.available.len; ++i)
+    if (map[i] == 0)
+      n++;
+
+  if (n < *nsectors)
+    {
+      grub_free (map);
+      return grub_error (GRUB_ERR_OUT_OF_RANGE,
+                        N_("your core.img is unusually large.  "
+                           "It won't fit in the embedding area"));
+    }
+
+  if (n > max_nsectors)
+    n = max_nsectors;
+
+  for (i = 0, j = 0; i < area.available.len && j < n; ++i)
+    if (map[i] == 0)
+      map[j++] = area.available.offset + i;
+
+  *nsectors = n;
+  *sectors = map;
 
   return GRUB_ERR_NONE;
 }
-- 
2.31.1