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

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Message-ID: <42e37b4d-000d-4077-ad28-bc8660890b72@proton.me>
Date: Thu, 08 Aug 2024 19:44:12 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com, boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...sung.com, aliceryhl@...gle.com, akpm@...ux-foundation.org, daniel.almeida@...labora.com, faith.ekstrand@...labora.com, boris.brezillon@...labora.com, lina@...hilina.net, mcanal@...lia.com, zhiw@...dia.com, acurrid@...dia.com, cjia@...dia.com, jhubbard@...dia.com, airlied@...hat.com, ajanulgu@...hat.com, lyude@...hat.com, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4 09/28] rust: alloc: implement kernel `Box`

On 08.08.24 19:44, Danilo Krummrich wrote:
> 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?

Oh yeah that is true, good catch! Here is a playground example [1].

[1]: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c439494a0582bb287232c7a05e21bd23

---
Cheers,
Benno

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