[Python-Dev] Deprecate invalid ctypes call protection on Windows

[Steve Dower]

One of the main reasons we are stuck with an old libffi fork in CPython 
is because the newer versions do not support protection from calling 
functions with too few/many arguments:

https://docs.python.org/3/library/ctypes.html?highlight=ctypes#calling-functions

There are a number of caveats here, including "this only works on 
Windows", but since it is documented we cannot just remove the behaviour 
without a deprecation period.

I'd like to propose a highly-accelerated deprecation period for this 
specific feature, starting in CPython 3.6.2 and being "completed" in 
3.7.0, when we will hopefully move onto a newer libffi.

In general, the "feature" is a misfeature anyway, since calling a native 
function with incorrect arguments is unsupported and a very easy way to 
cause information leakage or code execution vulnerabilities. There may 
be an argument for removing the functionality immediately, but honestly 
I think changing libffi in a point release is higher risk.

Once the special protection is removed, most of these cases will become 
OSError due to the general protection against segmentation faults. Some 
will undoubtedly fall through the cracks and crash the entire 
interpreter, but these are unavoidable (and really ought to crash to 
avoid potential exploits).

Does anyone have any reasons to oppose this? It already has votes from 
another Windows expert and the 3.6/3.7 Release Manager, but we wanted to 
see if anyone has a concern we haven't thought of.

Cheers,
Steve