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

Reproduce el ejemplo de campos de bits de ctypes.Structure en x86 de 32 bits e inspecciona el manejo de las unidades de almacenamiento de campos de bits descrito en el informe. Se considera terminado cuando se conserva el tamaño de estructura compatible con GCC sin leer ni escribir memoria más allá de la estructura contenedora.