[PATCH] fs/ext2.c: fix handling of missing sparse extent leafs

[Krzysztof Nowicki]

When a file on ext4 is stored as sparse the data belonging to
zero-filled blocks is not written to storage and the extent map is
missing entries for these blocks. Such case can happen both for depth
0 extents (leafs) as well as higher-level tables.

Consider a scenario of a file which has a zero-filled
beginning (e.g. ISO image). In such case real data starts at block
8. If such a file is stored using 2-level extent structure the extent
list in the inode will be depth 1 and will have an entry to a depth
0 (leaf) extent header for blocks 8-n.

Unfortunately existing GRUB2 ext2 driver is only able to handle
missing entries in leaf extent tables, for which the
grub_ext2_read_block() function returns 0. In case the whole leaf
extent list is missing for a block the function fails with "invalid
extent" error.

The fix for this problem relies on the grub_ext4_find_leaf() helper
function to distinguish two error cases: missing extent and error
walking through the extent tree. The existing error message is raised
only for the latter case, while for the missing leaf extent zero is
returned from grub_ext2_read_block() indicating a sparse block.

Signed-off-by: Krzysztof Nowicki <krzysztof.nowicki@nokia.com>
---
 grub-core/fs/ext2.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/grub-core/fs/ext2.c b/grub-core/fs/ext2.c
index ac33bcd68..884ffaa97 100644
--- a/grub-core/fs/ext2.c
+++ b/grub-core/fs/ext2.c
@@ -407,13 +407,15 @@ grub_ext2_blockgroup (struct grub_ext2_data *data, 
grub_uint64_t group,
                         sizeof (struct grub_ext2_block_group), blkgrp);
 }
 
-static struct grub_ext4_extent_header *
+static grub_err_t
 grub_ext4_find_leaf (struct grub_ext2_data *data,
                      struct grub_ext4_extent_header *ext_block,
-                     grub_uint32_t fileblock)
+                     grub_uint32_t fileblock,
+                     struct grub_ext4_extent_header **leaf)
 {
   struct grub_ext4_extent_idx *index;
   void *buf = NULL;
+  *leaf = NULL;
 
   while (1)
     {
@@ -426,7 +428,10 @@ grub_ext4_find_leaf (struct grub_ext2_data *data,
        goto fail;
 
       if (ext_block->depth == 0)
-        return ext_block;
+        {
+          *leaf = ext_block;
+          return GRUB_ERR_NONE;
+        }
 
       for (i = 0; i < grub_le_to_cpu16 (ext_block->entries); i++)
         {
@@ -435,7 +440,10 @@ grub_ext4_find_leaf (struct grub_ext2_data *data,
         }
 
       if (--i < 0)
-       goto fail;
+        {
+          grub_free (buf);
+          return GRUB_ERR_NONE;
+        }
 
       block = grub_le_to_cpu16 (index[i].leaf_hi);
       block = (block << 32) | grub_le_to_cpu32 (index[i].leaf);
@@ -452,7 +460,7 @@ grub_ext4_find_leaf (struct grub_ext2_data *data,
     }
  fail:
   grub_free (buf);
-  return 0;
+  return GRUB_ERR_BAD_FS;
 }
 
 static grub_disk_addr_t
@@ -474,13 +482,16 @@ grub_ext2_read_block (grub_fshelp_node_t node, 
grub_disk_addr_t fileblock)
       int i;
       grub_disk_addr_t ret;
 
-      leaf = grub_ext4_find_leaf (data, (struct grub_ext4_extent_header *) 
inode->blocks.dir_blocks, fileblock);
-      if (! leaf)
+      if (grub_ext4_find_leaf (data, (struct grub_ext4_extent_header *) 
inode->blocks.dir_blocks, fileblock, &leaf))
         {
           grub_error (GRUB_ERR_BAD_FS, "invalid extent");
           return -1;
         }
 
+      if (!leaf)
+        /* Leaf for the given block is absent (i.e. sparse) */
+        return 0;
+
       ext = (struct grub_ext4_extent *) (leaf + 1);
       for (i = 0; i < grub_le_to_cpu16 (leaf->entries); i++)
         {
-- 
2.31.1