Python contains a lot of tests like this one:

    if (length > PY_SSIZE_T_MAX / 4)
         return PyErr_NoMemory();

where length type is Py_ssize_t.

This test uses signed integers. There is usually a "assert(length > 0);" before.

The issue #22110 enabled more compiler warnings and there are now warnings when the test uses an unsigned number. Example:

   if (size > PY_SSIZE_T_MAX - PyBytesObject_SIZE) ...

where PyBytesObject_SIZE is defined using offsetof() which returns a size_t.

I propose to always cast Py_ssize_t length to size_t to avoid undefined behaviour (I never know if the compiler chooses signed or unsigned at the end) to ensure that the test also fail for negative number. For example, the following test must fail for negative size:

   if ((size_t)size > (size_t)PY_SSIZE_T_MAX - PyBytesObject_SIZE) ...

Attached patch changes bytesobject.c, tupleobject.c and unicodeobject.c (and asdl.c). If the global approach is accepted, more files should be patched.

overflow2.patch: more changes for this issue (it doesn't replace my previous patch).

New changeset 97c70528b604 by Victor Stinner in branch 'default':
Issue #22207: Fix "comparison between signed and unsigned integers" warning in
http://hg.python.org/cpython/rev/97c70528b604

I commited the first part.

The remaining part is a long patch for unicodeobject.c.

I think there are too many changes. Here is simpler patch which get rid of all warnings in Objects/unicodeobject.c on my computer (gcc 4.6.3, 32-bit Linux).

> I think there are too many changes. Here is simpler patch which get rid of all warnings in Objects/unicodeobject.c on my computer (gcc 4.6.3, 32-bit Linux).

The purpose of my patch is not to make the compiler quiet, but to ensure that negative lengths are handled correctly: Python must raise an error if a length is negative.

For example, _PyUnicode_DecodeUnicodeInternal() doesn't check currently if the size is negative. I don't know exactly what happens if you pass a negative size.

Gregory just has committed an equivalent of unicode_2.patch in a404bf4db6a6.

modify it as you see fit, i hadn't noticed this issue; just the warnings.

Yes, these warning annoyed me too. But Victor's patch is purposed to check 
input data of public C API functions.

Here is original Victor's patch synchronized with the tip.

I have reviewed the patch and found only few checks which looks good to me. Also I found two possible overflows (not fixed by the patch). All other changes looks redundant to me and just churn a code.

Ok, let's close the issue in that case ;-)

I extracted the cases that really can happen in PR 2623.

New changeset 64e461be09e23705ecbab43a8b01722186641f71 by Serhiy Storchaka in branch 'master':
bpo-22207: Add checks for possible integer overflows in unicodeobject.c. (#2623)
https://github.com/python/cpython/commit/64e461be09e23705ecbab43a8b01722186641f71

New changeset 82a907560011c7b784badccc78082cef70ea7128 by Serhiy Storchaka in branch '3.6':
[3.6] bpo-22207: Add checks for possible integer overflows in unicodeobject.c. (GH-2623) (#2658)
https://github.com/python/cpython/commit/82a907560011c7b784badccc78082cef70ea7128

New changeset 44eb51e6fcf45f3c2bf21c16e18c4da48a23d2d3 by Serhiy Storchaka in branch '3.5':
[3.5] bpo-22207: Add checks for possible integer overflows in unicodeobject.c. (GH-2623) (#2659)
https://github.com/python/cpython/commit/44eb51e6fcf45f3c2bf21c16e18c4da48a23d2d3