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Ability to skip crc checks for ancillary chunks #1991

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merged 3 commits into from
Jun 30, 2016
Merged

Ability to skip crc checks for ancillary chunks #1991

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c3b970f
Ability to skip crc checks for ancillary chunks
kkopachev Jun 29, 2016
885297d
Fix python 3 bytes to string concat error
kkopachev Jun 29, 2016
5f2fe60
CR feedback: don't use lambda
kkopachev Jun 30, 2016
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6 changes: 6 additions & 0 deletions PIL/PngImagePlugin.py
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Original file line number Diff line number Diff line change
Expand Up @@ -138,6 +138,12 @@ def call(self, cid, pos, length):
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):
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I'd prefer to ignore the error after it occurs. Of course, it is meaningless to do the check, if we are going to accept the result in any case. But in the future, we can add some warnings or other decoder info.

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Hm, why bother doing this now, if in both cases code has to be changed to throw a warning?
But anyway, I can keep crc check if you'd like. Would you like to ignore checksum mismatch or incomplete checksum as well?

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))
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28 changes: 28 additions & 0 deletions Tests/test_file_png.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@
from io import BytesIO

from PIL import Image
from PIL import ImageFile
from PIL import PngImagePlugin
import zlib

Expand Down Expand Up @@ -319,6 +320,33 @@ def test_verify_struct_error(self):
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
Expand Down