linux-kernel - Re: [PATCH v3 1/2] rust: kernel: add `drop

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Message-ID: <Zq5Io6tKgczUYcf3@boqun-archlinux>
Date: Sat, 3 Aug 2024 08:11:31 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] rust: kernel: add `drop_contents` to `BoxExt`

On Sat, Aug 03, 2024 at 02:23:42PM +0000, Benno Lossin wrote:
> On 03.08.24 16:16, Benno Lossin wrote:
> > @@ -53,4 +69,12 @@ fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {
> >          // zero-sized types, we use `NonNull::dangling`.
> >          Ok(unsafe { Box::from_raw(ptr) })
> >      }
> > +
> > +    fn drop_contents(this: Self) -> Box<MaybeUninit<T>> {
> > +        let ptr = Box::into_raw(this);
> > +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
> > +        unsafe { ptr::drop_in_place(ptr) };
> > +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
> 
> I just noticed that I missed another comment from Boqun here. Got
> confused with the two mails. I would replace the comment above with
> 
>     // CAST: `T` and `MaybeUninit<T>` have the same layout.
>     let ptr = ptr.cast::<MaybeUninit<T>>();
>     // SAFETY: `ptr` is valid for writes, because it came from `Box::into_raw` and it is valid for
>     // reads, since the pointer came from `Box::into_raw` and the type is `MaybeUninit<T>`.
> 
> Let me know if you want another version.

Looks good to me, please do send an updated version.

Although, I would expect the "CAST" comment already explains that if
`ptr` is a valid, then the casting result is also valid, i.e. we put
"CAST" comments on the casting that matters to safety. But that seems
not matching what you use CAST for?

Regards,
Boqun

> 
> ---
> Cheers,
> Benno
> 
> > +        unsafe { Box::from_raw(ptr.cast()) }
> > +    }
> >  }
> > --
> > 2.45.2
> > 
> > 
> > 
>