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

[Vladimir 'phcoder' Serbinenko]

On Fri, Mar 31, 2017, 06:43 Andrei Borzenkov <address@hidden> wrote:

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

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

Looks good. Did you check all instances sizeof, so we don't have sizeof of pointer?

Did you check return paths for free'ing on all of them?

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)

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