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Escape NUL bytes in white diff admin text. #1726

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Merged
veluca93 merged 1 commit into from
Oct 5, 2026
+13 −2
Merged

Escape NUL bytes in white diff admin text. #1726

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8 changes: 6 additions & 2 deletions cms/grading/steps/whitediff.py
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Original file line number Diff line number Diff line change
Expand Up @@ -120,8 +120,12 @@ def _white_diff(output: typing.BinaryIO, res: typing.BinaryIO) -> tuple[bool, st
lout = lout[:LENGTH_LIMIT] + b"..."
if len(lres) > LENGTH_LIMIT:
lres = lres[:LENGTH_LIMIT] + b"..."
lout = lout.decode("utf-8", errors='backslashreplace')
lres = lres.decode("utf-8", errors='backslashreplace')
# NUL is valid UTF-8, so backslashreplace lets it through,
# but PostgreSQL cannot store it in a text column.
lout = lout.decode("utf-8", errors='backslashreplace') \
.replace("\x00", "\\x00")
lres = lres.decode("utf-8", errors='backslashreplace') \
.replace("\x00", "\\x00")
return False, f"Expected `{lres}`, found `{lout}` on line {line}"


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7 changes: 7 additions & 0 deletions cmstestsuite/unit_tests/grading/steps/whitediff_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -72,6 +72,13 @@ def test_diff_wrong_long_line(self):
line2 = line1 + "0"
self.assertFalse(self._diff(line1, line2))

def test_diff_nul_byte_escaped_in_admin_text(self):
correct, admin_text = _white_diff(
BytesIO(b"hello\x00world"), BytesIO(b"helloworld"))
self.assertFalse(correct)
self.assertNotIn("\x00", admin_text)
self.assertIn("hello\\x00world", admin_text)


if __name__ == "__main__":
unittest.main()
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