[Python-Dev] PyArg_ParseTupe(): parse unsigned integer and check for overflow

Guido van Rossum guido at python.org
Thu Jun 27 00:26:05 CEST 2013

On Wed, Jun 26, 2013 at 3:07 PM, Victor Stinner
<victor.stinner at gmail.com> wrote:
> I don't understand why the B, H, I, k formats of PyArg_ParseTupe() do
> not check for overflow, whereas formats for signed numbers do check
> for overflow. What is the useful of ignoring overflow?
> http://docs.python.org/3/c-api/arg.html

I think this is really old DNA. It comes from times when 0xffffffff
and -1 were equivalent -- we borrowed this cavalier attitude from old
C code.

I worry that "fixing" this would break some old libraries relying on
the feature, so I hope we can avoid changes in this area (the older
the DNA, the more hidden dependencies it has ;-).

> I would to parse an integer in [0; UINT_MAX] to fix the zlib module on
> 64-bit system:
> http://bugs.python.org/issue18294
> How should I implement that? Use "O" format and then use
> PyLong_Check(), PyLong_AsLong(), and check value <= UINT_MAX?

Why can't you use the K format? It won't reject out-of-range values,
but it will convert them to in-range so there aren't any attacks
possible based on bypassing the range check. I'm probably
misunderstanding something -- I don't completely understand that bug
report. :-(

--Guido van Rossum (python.org/~guido)

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