ctypes.Structure: bitfield of underaligned type can cause read of unrelated memory

Bug report

Bug description:

(This is an exotic edge case found by tests that are perhaps too stringent, but, we prefer issues over XXX comments in the code, so, here goes.)

A bitfield of an “underaligned” type (one whose alignment is smaller than its size) can cause the “storage unit” that ctypes uses for handling the bitfield to extend past the containing Structure.

For example, on 32-bit x86 architecture, where int64_t is 8 bytes long but only aligned to 4 bytes, we have:

>>> import ctypes
>>> class S(ctypes.Structure):
...     _fields_ = [('f', ctypes.c_int64, 1)]
>>> ctypes.sizeof(S)
4
>>> ctypes.sizeof(ctypes.c_int64)
8

This matches GCC struct layout:

#include <stdio.h>
#include <stdint.h>

struct S {
    int64_t f: 1;
};

int main() {
    printf("%zd\n", sizeof(struct S));  // -> 4
}

ctypes handles bitfield reads/writes by reading the entire storage unit, masking/shifting, and (for writes) writing the entire unit back. So, in this case it can read/write memory that doesn't belong to the struct.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

32 ビット x86 で ctypes.Structure のビットフィールド例を再現し、レポートで説明されているビットフィールドの格納単位の扱いを調査します。包含する構造体の外部のメモリを読み書きせずに、GCC 互換の構造体サイズを維持できれば完了です。