Re: [PATCH v4 09/28] rust: alloc: implement kernel `Box`

[Danilo Krummrich <dakr@kernel.org>]

On Wed, Aug 07, 2024 at 07:49:31AM +0000, Benno Lossin wrote:
> >>> +impl<T, A> Drop for Box<T, A>
> >>> +where
> >>> +    T: ?Sized,
> >>> +    A: Allocator,
> >>> +{
> >>> +    fn drop(&mut self) {
> >>> +        let ptr = self.0.as_ptr();
> >>> +
> >>> +        // SAFETY: `ptr` is always properly aligned, dereferenceable and points to an initialized
> >>> +        // instance of `T`.
> >>> +        let size = unsafe { core::mem::size_of_val(&*ptr) };
> >>
> >> 1. `size_of_val` is not `unsafe`.
> > 
> > Right, but dereferencing the `ptr` is unsafe.
> > 
> >> 2. why not use `&*self` instead of using the raw pointer? (then move the
> >>    let binding below this line)
> > 
> > If we ever support non-ZST `Allocator`s using `self` would not always evaluate
> > to the correct size. I think evaluating the size of `T` rather than `Box<T>` is
> > the correct thing to do.
> 
> I mean use `Box::deref` (that's what `&*self` should do), you don't need

Actually, this must either be `size_of_val(&**self)` or `size_of_val::<T>(self).

`size_of_val(&*self)` should indeed resolve to `&Box<T, A>`, right?

> to repeat the same SAFETY comment when it already is wrapped by a safe
> function.
> 
> ---
> Cheers,
> Benno
>