ctypes resize and byref/addressof is not thread-safe under free-threaded build

[sergey-miryanov]

Bug report

Bug description:

I'm reviewing the https://github.com/python/cpython/blob/main/Modules/_ctypes/callproc.c. I believe I found a possible UB if resize and byref/addressof are used from different threads without any locking (AFAIU it is valid for free-threaded build and not for GIL-enabled).

resize does realloc -

cpython/Modules/_ctypes/callproc.c

Lines 1934 to 1938 in d07e9eb

	void * ptr = PyMem_Realloc(obj->b_ptr, size);
	if (ptr == NULL)
	return PyErr_NoMemory();
	obj->b_ptr = ptr;
	obj->b_size = size;

After realloc the old value of obj->b_ptr is no longer valid, and any access to it is UB. If another thread calls addressof

cpython/Modules/_ctypes/callproc.c

Line 1847 in d07e9eb

return PyLong_FromVoidPtr(((CDataObject *)obj)->b_ptr);

or byref

cpython/Modules/_ctypes/callproc.c

Line 1827 in d07e9eb

parg->value.p = (char *)((CDataObject *)obj)->b_ptr + offset;

, it may potentially get UB under heavy contention (I believe it is zero or less real cases so far).

Should we protect them with LOCK_PTR?

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

[sergey-miryanov]

Related to gh-127945