Ctypes segfaulting when false given as arg to strlen fixed

Closed Bitbucket Importer requested to merge bitbucket/declined-pr-544 into branch/default

Created originally on Bitbucket by Dodan (Dodan Mihai)

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I have changed the criteria allowing a fastpath. Functions that take char pointers as arguments are no longer allowed through the fast path, because they are more prone to segfaulting.(issues/466)

The issue was that the fast path does not verify if the parameters given to a ctype-function match the argument types specified.

This is not a problem if the function exits with an error, because it is handled afterwards (that is why you do not see this type of behaviour for specifying bool instead of c_int, for example). However, string functions (such as strlen) consider that the string end when they encounter the null char in the stack. Since passing a boolean / int or any other type does not null-terminate the string, there was a segfault.

The reason why I didn't eliminate the fast path altogether, is that it is 3x faster than the slowpath. Also, I am not aware of any libc function with other types of parameters that are as prone to segfaulting, so punishing the use of pointers to int/floats etc. seems redundant.

I have run a test to show the before and after effects. It calls strlen (just as in the example) and the abs(c_int) function of stdlib. There is a warmup of 10000 iterations for both tests, and a measured cycle of 5000000 runs. As you can see, the abs function sufferes no penalty, while strlen is 3 times slower, but it does not segfault anymore, but instead exits with ArgumentError, as expected.



10000 Abs 5000000 1.29707
10000 strlen 5000000 9.35953
Traceback (most recent call last):
  File "/home/mihaid/Documents/ctypes/standalone466.py", line 59, in <module>
    assert libc.strlen(False) == 5
  File "/home/mihaid/pypy2-v5.7.1-linux64/lib_pypy/_ctypes/function.py", line 354, in __call__
    self._convert_args(argtypes, args, kwargs))
  File "/home/mihaid/pypy2-v5.7.1-linux64/lib_pypy/_ctypes/function.py", line 583, in _convert_args
    raise ArgumentError(str(e))
ArgumentError: expected c_char_p instance instead of bool


10000 Abs 5000000 1.27580
10000 strlen 5000000 3.05891
Segmentation fault (core dumped)

Also, I have run the ctypes tests in pypy/module/test_lib_pypy/ctypes_tests and I have the same passing ratio as before.

Thank you for your consideration!

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