X509Extension __str__() method raises exception for unknown extension types

[ich199]

Issue

When calling the crypto.X509Extension method __str__(), if the extension type is not supported by OpenSSL, an exception_type error is raised:

python -c 'from OpenSSL.crypto import X509Extension; print(str(X509Extension(b"1.2.3.4.5.6.7", False, b"DER:05:00")))'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/iain/pyopenssl-test/lib64/python3.11/site-packages/OpenSSL/crypto.py", line 882, in __str__
    _openssl_assert(print_result != 0)
  File "/home/iain/pyopenssl-test/lib64/python3.11/site-packages/OpenSSL/_util.py", line 71, in openssl_assert
    exception_from_error_queue(error)
  File "/home/iain/pyopenssl-test/lib64/python3.11/site-packages/OpenSSL/_util.py", line 57, in exception_from_error_queue
    raise exception_type(errors)
OpenSSL.crypto.Error: []

Environment:

$ python --version
Python 3.11.4

$ pip list
Package      Version
------------ -------
cffi         1.15.1
cryptography 41.0.3
pip          23.2.1
pycparser    2.21
pyOpenSSL    23.2.0
setuptools   62.6.0

$ openssl version
OpenSSL 3.0.9 30 May 2023 (Library: OpenSSL 3.0.9 30 May 2023)

Possible Cause

The current call to X509V3_EXT_print() in crypto.X509Extension sets the flags parameter (3rd parameter in the call) to 0, which causes OpenSSL to return an error for unknown extension types:

print_result = _lib.X509V3_EXT_print(bio, self._extension, 0, 0)

Possible Fix

Update X509V3_EXT_print() to set the flags parameter to one of the other valid values so that it returns success for unknown extension types.

eg. amending the call to use the value 1 << 16 or 65536 (X509V3_EXT_ERROR_UNKNOWN in OpenSSL) results in the call succeeding and printing <Not Supported> for the unknown extension type:

print_result = _lib.X509V3_EXT_print(bio, self._extension, 1 << 16, 0)

python -c 'from OpenSSL.crypto import X509Extension; print(str(X509Extension(b"1.2.3.4.5.6.7", False, b"DER:05:00")))'
<Not Supported>

[ich199]

Possibly related to #270