Unsound usages of unsafe implementation from PyObject to T

[llooFlashooll]

Hi, I am scanning the cpython in the latest version with my own static analyzer tool.

Unsafe conversion found at: src/py_class/mod.rs:96:15: 96:62.

#[inline]
#[doc(hidden)]
pub unsafe fn data_init<'a, T>(_py: Python<'a>, obj: &'a PyObject, offset: usize, value: T)
where
    T: Send + 'static,
{
    let ptr = (obj.as_ptr() as *mut u8).add(offset) as *mut T;
    ptr::write(ptr, value)
}

This unsound implementation would create a misalignment issues if the type size of PyObject is smaller than the type size of T.

This would potentially cause undefined behaviors in Rust. If we further manipulate the problematic converted types, it would potentially lead to different consequences such as access out-of-bound. I am reporting this issue for your attention.

[dgrunwald]

It's not unsound, it's just an unsafe function with an undocumented precondition:
The caller is supposed to pass an allocation that contains an ffi::PyObject followed by a T (with offset being sizeof(PyObject) plus maybe a bit more if required for alignment).