Merge pull request #1991 from kkopachev/png-crc-error-ignore

Ability to skip crc checks for ancillary chunks

PIL/PngImagePlugin.py

@@ -138,6 +138,12 @@ class ChunkStream(object):
     def crc(self, cid, data):
         "Read and verify checksum"
 
+        # Skip CRC checks for ancillary chunks if allowed to load truncated images
+        # 5th byte of first char is 1 [specs, section 5.4]
+        if ImageFile.LOAD_TRUNCATED_IMAGES and (i8(cid[0]) >> 5 & 1):
+            self.crc_skip(cid, data)
+            return
+
         try:
             crc1 = Image.core.crc32(data, Image.core.crc32(cid))
             crc2 = i16(self.fp.read(2)), i16(self.fp.read(2))

Tests/test_file_png.py

@@ -3,6 +3,7 @@ from helper import unittest, PillowTestCase, hopper
 from io import BytesIO
 
 from PIL import Image
+from PIL import ImageFile
 from PIL import PngImagePlugin
 import zlib
 
@@ -319,6 +320,33 @@ class TestFilePng(PillowTestCase):
             self.assertTrue(im.fp is not None)
             self.assertRaises((IOError, SyntaxError), im.verify)
 
+    def test_verify_ignores_crc_error(self):
+        # check ignores crc errors in ancillary chunks
+
+        chunk_data = chunk(b'tEXt', b'spam')
+        broken_crc_chunk_data = chunk_data[:-1] + b'q'  # break CRC
+
+        image_data = HEAD + broken_crc_chunk_data + TAIL
+        self.assertRaises(SyntaxError, PngImagePlugin.PngImageFile, BytesIO(image_data))
+
+        ImageFile.LOAD_TRUNCATED_IMAGES = True
+        try:
+            im = load(image_data)
+            self.assertTrue(im is not None)
+        finally:
+            ImageFile.LOAD_TRUNCATED_IMAGES = False
+
+    def test_verify_not_ignores_crc_error_in_required_chunk(self):
+        # check does not ignore crc errors in required chunks
+
+        image_data = MAGIC + IHDR[:-1] + b'q' + TAIL
+
+        ImageFile.LOAD_TRUNCATED_IMAGES = True
+        try:
+            self.assertRaises(SyntaxError, PngImagePlugin.PngImageFile, BytesIO(image_data))
+        finally:
+            ImageFile.LOAD_TRUNCATED_IMAGES = False
+
 
     def test_roundtrip_dpi(self):
         # Check dpi roundtripping