diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index d0ee6ae..d09f60c 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -25,6 +25,7 @@
 import re
 import shlex
 import shutil
+import string
 import subprocess
 import sys
 import tempfile
@@ -34,6 +35,7 @@
 from hashlib import sha1, sha256
 
 import blockimgdiff
+import sparse_img
 
 class Options(object):
   def __init__(self):
@@ -124,6 +126,11 @@
   return subprocess.Popen(args, **kwargs)
 
 
+def RoundUpTo4K(value):
+  rounded_up = value + 4095
+  return rounded_up - (rounded_up % 4096)
+
+
 def CloseInheritedPipes():
   """ Gmake in MAC OS has file descriptor (PIPE) leak. We close those fds
   before doing other work."""
@@ -618,6 +625,56 @@
   return tmp, zipfile.ZipFile(filename, "r")
 
 
+def GetSparseImage(which, tmpdir, input_zip):
+  """Returns a SparseImage object suitable for passing to BlockImageDiff.
+
+  This function loads the specified sparse image from the given path, and
+  performs additional processing for OTA purpose. For example, it always adds
+  block 0 to clobbered blocks list. It also detects files that cannot be
+  reconstructed from the block list, for whom we should avoid applying imgdiff.
+
+  Args:
+    which: The partition name, which must be "system" or "vendor".
+    tmpdir: The directory that contains the prebuilt image and block map file.
+    input_zip: The target-files ZIP archive.
+
+  Returns:
+    A SparseImage object, with file_map info loaded.
+  """
+  assert which in ("system", "vendor")
+
+  path = os.path.join(tmpdir, "IMAGES", which + ".img")
+  mappath = os.path.join(tmpdir, "IMAGES", which + ".map")
+
+  # The image and map files must have been created prior to calling
+  # ota_from_target_files.py (since LMP).
+  assert os.path.exists(path) and os.path.exists(mappath)
+
+  # In ext4 filesystems, block 0 might be changed even being mounted R/O. We add
+  # it to clobbered_blocks so that it will be written to the target
+  # unconditionally. Note that they are still part of care_map. (Bug: 20939131)
+  clobbered_blocks = "0"
+
+  image = sparse_img.SparseImage(path, mappath, clobbered_blocks)
+
+  # block.map may contain less blocks, because mke2fs may skip allocating blocks
+  # if they contain all zeros. We can't reconstruct such a file from its block
+  # list. Tag such entries accordingly. (Bug: 65213616)
+  for entry in image.file_map:
+    # "/system/framework/am.jar" => "SYSTEM/framework/am.jar".
+    arcname = string.replace(entry, which, which.upper(), 1)[1:]
+    # Skip artificial names, such as "__ZERO", "__NONZERO-1".
+    if arcname not in input_zip.namelist():
+      continue
+
+    info = input_zip.getinfo(arcname)
+    ranges = image.file_map[entry]
+    if RoundUpTo4K(info.file_size) > ranges.size() * 4096:
+      ranges.extra['incomplete'] = True
+
+  return image
+
+
 def GetKeyPasswords(keylist):
   """Given a list of keys, prompt the user to enter passwords for
   those which require them.  Return a {key: password} dict.  password
diff --git a/tools/releasetools/ota_from_target_files.py b/tools/releasetools/ota_from_target_files.py
index 95b7303..0044a87 100755
--- a/tools/releasetools/ota_from_target_files.py
+++ b/tools/releasetools/ota_from_target_files.py
@@ -140,7 +140,6 @@
 
 import common
 import edify_generator
-import sparse_img
 
 if sys.hexversion < 0x02070000:
   print("Python 2.7 or newer is required.", file=sys.stderr)
@@ -452,31 +451,6 @@
         source_info.GetBuildProp("ro.build.thumbprint"))
 
 
-def GetImage(which, tmpdir):
-  """Returns an image object suitable for passing to BlockImageDiff.
-
-  'which' partition must be "system" or "vendor". A prebuilt image and file
-  map must already exist in tmpdir.
-  """
-
-  assert which in ("system", "vendor")
-
-  path = os.path.join(tmpdir, "IMAGES", which + ".img")
-  mappath = os.path.join(tmpdir, "IMAGES", which + ".map")
-
-  # The image and map files must have been created prior to calling
-  # ota_from_target_files.py (since LMP).
-  assert os.path.exists(path) and os.path.exists(mappath)
-
-  # Bug: http://b/20939131
-  # In ext4 filesystems, block 0 might be changed even being mounted
-  # R/O. We add it to clobbered_blocks so that it will be written to the
-  # target unconditionally. Note that they are still part of care_map.
-  clobbered_blocks = "0"
-
-  return sparse_img.SparseImage(path, mappath, clobbered_blocks)
-
-
 def AddCompatibilityArchiveIfTrebleEnabled(target_zip, output_zip, target_info,
                                            source_info=None):
   """Adds compatibility info into the output zip if it's Treble-enabled target.
@@ -662,7 +636,7 @@
   # has the effect of writing new data from the package to the entire
   # partition, but lets us reuse the updater code that writes incrementals to
   # do it.
-  system_tgt = GetImage("system", OPTIONS.input_tmp)
+  system_tgt = common.GetSparseImage("system", OPTIONS.input_tmp, input_zip)
   system_tgt.ResetFileMap()
   system_diff = common.BlockDifference("system", system_tgt, src=None)
   system_diff.WriteScript(script, output_zip)
@@ -673,7 +647,7 @@
   if HasVendorPartition(input_zip):
     script.ShowProgress(0.1, 0)
 
-    vendor_tgt = GetImage("vendor", OPTIONS.input_tmp)
+    vendor_tgt = common.GetSparseImage("vendor", OPTIONS.input_tmp, input_zip)
     vendor_tgt.ResetFileMap()
     vendor_diff = common.BlockDifference("vendor", vendor_tgt)
     vendor_diff.WriteScript(script, output_zip)
@@ -846,8 +820,8 @@
   target_recovery = common.GetBootableImage(
       "/tmp/recovery.img", "recovery.img", OPTIONS.target_tmp, "RECOVERY")
 
-  system_src = GetImage("system", OPTIONS.source_tmp)
-  system_tgt = GetImage("system", OPTIONS.target_tmp)
+  system_src = common.GetSparseImage("system", OPTIONS.source_tmp, source_zip)
+  system_tgt = common.GetSparseImage("system", OPTIONS.target_tmp, target_zip)
 
   blockimgdiff_version = max(
       int(i) for i in target_info.get("blockimgdiff_versions", "1").split(","))
@@ -872,8 +846,8 @@
   if HasVendorPartition(target_zip):
     if not HasVendorPartition(source_zip):
       raise RuntimeError("can't generate incremental that adds /vendor")
-    vendor_src = GetImage("vendor", OPTIONS.source_tmp)
-    vendor_tgt = GetImage("vendor", OPTIONS.target_tmp)
+    vendor_src = common.GetSparseImage("vendor", OPTIONS.source_tmp, source_zip)
+    vendor_tgt = common.GetSparseImage("vendor", OPTIONS.target_tmp, target_zip)
 
     # Check first block of vendor partition for remount R/W only if
     # disk type is ext4
diff --git a/tools/releasetools/rangelib.py b/tools/releasetools/rangelib.py
index 87380a5..8af61c3 100644
--- a/tools/releasetools/rangelib.py
+++ b/tools/releasetools/rangelib.py
@@ -25,6 +25,7 @@
 
   def __init__(self, data=None):
     self.monotonic = False
+    self._extra = {}
     if isinstance(data, str):
       self._parse_internal(data)
     elif data:
@@ -56,6 +57,10 @@
   def __repr__(self):
     return '<RangeSet("' + self.to_string() + '")>'
 
+  @property
+  def extra(self):
+    return self._extra
+
   @classmethod
   def parse(cls, text):
     """Parse a text string consisting of a space-separated list of
diff --git a/tools/releasetools/validate_target_files.py b/tools/releasetools/validate_target_files.py
index b590392..1b3eb73 100755
--- a/tools/releasetools/validate_target_files.py
+++ b/tools/releasetools/validate_target_files.py
@@ -29,35 +29,17 @@
 import sys
 
 import common
-import sparse_img
-
-
-def _GetImage(which, tmpdir):
-  assert which in ('system', 'vendor')
-
-  path = os.path.join(tmpdir, 'IMAGES', which + '.img')
-  mappath = os.path.join(tmpdir, 'IMAGES', which + '.map')
-
-  # Map file must exist (allowed to be empty).
-  assert os.path.exists(path) and os.path.exists(mappath)
-
-  clobbered_blocks = '0'
-  return sparse_img.SparseImage(path, mappath, clobbered_blocks)
 
 
 def _ReadFile(file_name, unpacked_name, round_up=False):
   """Constructs and returns a File object. Rounds up its size if needed."""
 
-  def RoundUpTo4K(value):
-    rounded_up = value + 4095
-    return rounded_up - (rounded_up % 4096)
-
   assert os.path.exists(unpacked_name)
   with open(unpacked_name, 'r') as f:
     file_data = f.read()
   file_size = len(file_data)
   if round_up:
-    file_size_rounded_up = RoundUpTo4K(file_size)
+    file_size_rounded_up = common.RoundUpTo4K(file_size)
     file_data += '\0' * (file_size_rounded_up - file_size)
   return common.File(file_name, file_data)
 
@@ -79,33 +61,28 @@
 
   def CheckAllFiles(which):
     logging.info('Checking %s image.', which)
-    image = _GetImage(which, input_tmp)
+    image = common.GetSparseImage(which, input_tmp, input_zip)
     prefix = '/' + which
     for entry in image.file_map:
+      # Skip entries like '__NONZERO-0'.
       if not entry.startswith(prefix):
         continue
 
       # Read the blocks that the file resides. Note that it will contain the
       # bytes past the file length, which is expected to be padded with '\0's.
       ranges = image.file_map[entry]
+
+      incomplete = ranges.extra.get('incomplete', False)
+      if incomplete:
+        logging.warning('Skipping %s that has incomplete block list', entry)
+        continue
+
       blocks_sha1 = image.RangeSha1(ranges)
 
       # The filename under unpacked directory, such as SYSTEM/bin/sh.
       unpacked_name = os.path.join(
           input_tmp, which.upper(), entry[(len(prefix) + 1):])
       unpacked_file = _ReadFile(entry, unpacked_name, True)
-      file_size = unpacked_file.size
-
-      # block.map may contain less blocks, because mke2fs may skip allocating
-      # blocks if they contain all zeros. We can't reconstruct such a file from
-      # its block list. (Bug: 65213616)
-      if file_size > ranges.size() * 4096:
-        logging.warning(
-            'Skipping %s that has less blocks: file size %d-byte,'
-            ' ranges %s (%d-byte)', entry, file_size, ranges,
-            ranges.size() * 4096)
-        continue
-
       file_sha1 = unpacked_file.sha1
       assert blocks_sha1 == file_sha1, \
           'file: %s, range: %s, blocks_sha1: %s, file_sha1: %s' % (