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

在 32 位元 x86 上重現 ctypes.Structure 位元欄位範例，並檢查報告中描述的位元欄位儲存單位處理方式。完成標準是在不讀取或寫入包含該結構之外的記憶體的情況下，維持與 GCC 相容的結構大小。