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 the ctypes.Structure bitfield example on 32-bit x86 and inspect the bitfield storage-unit handling described in the report. Done means preserving the GCC-compatible structure size without reading or writing memory beyond the containing structure.