ext2.py: ValueError: requested invalid -8 amount of bytes

[milahu]

ext2 parser is broken

repro:

dd if=/dev/zero of=ext2.bin bs=1024 count=1000
mkfs.ext2 ext2.bin
mkdir mnt
sudo mount -o loop ext2.bin mnt/
echo hello world | sudo tee mnt/hello.txt
sudo umount mnt/

kaitai-struct-compiler -t python filesystem/ext2.ksy

python -c '
import ext2
e = ext2.Ext2.from_file("ext2.bin")
print(e.root_dir)
print([ n.name for n in e.root_dir.entries ])
'

  File "ext2.py", line 217, in _read
    self.entries.append(Ext2.DirEntry(self._io, self, self._root))
  File "ext2.py", line 120, in __init__
    self._read()
  File "ext2.py", line 128, in _read
    self.padding = self._io.read_bytes(((self.rec_len - self.name_len) - 8))
  File "kaitaistruct.py", line 298, in read_bytes
    raise ValueError(
ValueError: requested invalid -8 amount of bytes

fix?

--- a/filesystem/ext2.ksy
+++ b/filesystem/ext2.ksy
@@ -254,10 +254,14 @@ types:
         type: str
         encoding: UTF-8
       - id: padding
-        size: rec_len - name_len - 8
+        size: 'padding_size_raw > 0 ? padding_size_raw : 0'
     instances:
       inode:
         value: '_root.bg1.block_groups[(inode_ptr - 1) / _root.bg1.super_block.inodes_per_group].inodes[(inode_ptr - 1) % _root.bg1.super_block.inodes_per_group]'
+      # workaround: kaitai_struct_compiler has no max(a, b) function
+      # padding_size = max(0, (rec_len - name_len - 8))
+      padding_size_raw:
+        value: 'rec_len - name_len - 8'
     enums:
       # https://www.nongnu.org/ext2-doc/ext2.html#IFDIR-FILE-TYPE
       file_type_enum:

still looks broken, as i dont see the hello.txt file

>>> [ n.name for n in e.root_dir.entries ]
['', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '']

todo: use the array method max (docs)

padding_size = max(0, (rec_len - name_len - 8))

[0, (rec_len - name_len - 8)].max

[armijnhemel]

The ext2 parser is, in my opinion, only useful to parse the super block. There are many features (such as the sparse superblock ( https://www.nongnu.org/ext2-doc/ext2.html#def-superblock ) that are not properly supported.