[PATCH] btrfs: avoid "used uninitialized" error with GCC7

[Andrei Borzenkov <arvidjaar@gmail.com>]

sblock was local and so considered new variable on every loop
iteration.

While on it, dynamically allocate buffer to reduce stack usage.

Closes: 50597

---
 grub-core/fs/btrfs.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/grub-core/fs/btrfs.c b/grub-core/fs/btrfs.c
index 9cffa91..99e81f9 100644
--- a/grub-core/fs/btrfs.c
+++ b/grub-core/fs/btrfs.c
@@ -229,24 +229,29 @@ read_sblock (grub_disk_t disk, struct grub_btrfs_superblock *sb)
 {
   unsigned i;
   grub_err_t err = GRUB_ERR_NONE;
+  struct grub_btrfs_superblock *sblock;
+
+  sblock = grub_malloc (sizeof (*sblock));
+  if (sblock == NULL)
+    return grub_errno;
+
   for (i = 0; i < ARRAY_SIZE (superblock_sectors); i++)
     {
-      struct grub_btrfs_superblock sblock;
       /* Don't try additional superblocks beyond device size.  */
-      if (i && (grub_le_to_cpu64 (sblock.this_device.size)
+      if (i && (grub_le_to_cpu64 (sblock->this_device.size)
 		>> GRUB_DISK_SECTOR_BITS) <= superblock_sectors[i])
 	break;
       err = grub_disk_read (disk, superblock_sectors[i], 0,
-			    sizeof (sblock), &sblock);
+			    sizeof (*sblock), sblock);
       if (err == GRUB_ERR_OUT_OF_RANGE)
 	break;
 
-      if (grub_memcmp ((char *) sblock.signature, GRUB_BTRFS_SIGNATURE,
+      if (grub_memcmp ((char *) sblock->signature, GRUB_BTRFS_SIGNATURE,
 		       sizeof (GRUB_BTRFS_SIGNATURE) - 1) != 0)
 	break;
-      if (i == 0 || grub_le_to_cpu64 (sblock.generation)
+      if (i == 0 || grub_le_to_cpu64 (sblock->generation)
 	  > grub_le_to_cpu64 (sb->generation))
-	grub_memcpy (sb, &sblock, sizeof (sblock));
+	grub_memcpy (sb, sblock, sizeof (*sblock));
     }
 
   if ((err == GRUB_ERR_OUT_OF_RANGE || !err) && i == 0)
@@ -255,6 +260,7 @@ read_sblock (grub_disk_t disk, struct grub_btrfs_superblock *sb)
   if (err == GRUB_ERR_OUT_OF_RANGE)
     grub_errno = err = GRUB_ERR_NONE;
 
+  grub_free (sblock);
   return err;
 }
 
-- 
tg: (8014b7b..) bug/50597 (depends on: master)