Fix VDI reads across allocation blocks

dissect/hypervisor/disk/vdi.py

@@ -118,7 +118,7 @@ def _read(self, offset: int, length: int) -> bytes:
 
         block_idx, offset_in_block = divmod(offset, self.block_size)
         while length > 0:
-            read_len = min(length, max(length, self.block_size - offset_in_block))
+            read_len = min(length, self.block_size - offset_in_block)
 
             block = self.map[block_idx]
             if block == VDI_IMAGE_BLOCK_FREE:

tests/disk/test_vdi.py

@@ -1,12 +1,15 @@
 from __future__ import annotations
 
 import gzip
+import struct
+from io import BytesIO
 from pathlib import Path
+from types import SimpleNamespace
 from typing import BinaryIO
 from unittest.mock import patch
 
 from dissect.hypervisor.disk.c_vdi import c_vdi
-from dissect.hypervisor.disk.vdi import VDI
+from dissect.hypervisor.disk.vdi import VDI, VDIStream
 from tests._util import absolute_path
 
 
@@ -34,6 +37,27 @@ def test_vdi() -> None:
         assert stream.read() == b""
 
 
+def test_vdi_read_across_non_contiguous_blocks() -> None:
+    """Test a single read spanning non-contiguous VDI allocation-map blocks."""
+    block_size = 4
+    allocation_map = struct.pack("<ii", 1, 0)
+    fh = BytesIO(allocation_map + b"AAAABBBBCCCC")
+
+    vdi = SimpleNamespace(
+        fh=fh,
+        blocks_offset=0,
+        number_of_blocks=2,
+        block_size=block_size,
+        size=2 * block_size,
+        data_offset=len(allocation_map),
+        parent=None,
+    )
+
+    stream = VDIStream(vdi)
+
+    assert stream.read(vdi.size) == b"BBBBAAAA"
+
+
 def test_vdi_context_manager() -> None:
     """Test VDI context manager."""
     with patch.object(Path, "open", gzip.open), VDI(absolute_path("_data/disk/vdi/basic.vdi.gz")) as vdi: