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Date: Thu, 8 Aug 2024 19:44:09 +0200
From: Danilo Krummrich <dakr@kernel.org>
To: Benno Lossin <benno.lossin@proton.me>
Cc: ojeda@kernel.org, alex.gaynor@gmail.com, wedsonaf@gmail.com,
	boqun.feng@gmail.com, gary@garyguo.net, bjorn3_gh@protonmail.com,
	a.hindborg@samsung.com, aliceryhl@google.com,
	akpm@linux-foundation.org, daniel.almeida@collabora.com,
	faith.ekstrand@collabora.com, boris.brezillon@collabora.com,
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	airlied@redhat.com, ajanulgu@redhat.com, lyude@redhat.com,
	linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH v4 09/28] rust: alloc: implement kernel `Box`
Message-ID: <ZrUD6ZyfK9Kg1mrn@pollux>
References: <20240805152004.5039-1-dakr@kernel.org>
 <20240805152004.5039-10-dakr@kernel.org>
 <a1c1e273-2d40-4114-b3b9-f27c73e3d122@proton.me>
 <ZrKrMrg5E85y7jkj@pollux>
 <012f5a12-2408-4658-8318-55fa8d4285e1@proton.me>
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In-Reply-To: <012f5a12-2408-4658-8318-55fa8d4285e1@proton.me>

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
>