Date:   Fri, 27 Jan 2023 13:56:25 +0000
From:   Gary Guo <gary@garyguo.net>
To:     Wedson Almeida Filho <wedsonaf@gmail.com>
Cc:     rust-for-linux@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/5] rust: types: implement `ForeignOwnable` for
 `Box<T>`
Message-ID: <20230127135625.6630332a.gary@garyguo.net>
In-Reply-To: <20230119174036.64046-3-wedsonaf@gmail.com>
References: <20230119174036.64046-1-wedsonaf@gmail.com>
        <20230119174036.64046-3-wedsonaf@gmail.com>
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On Thu, 19 Jan 2023 14:40:34 -0300
Wedson Almeida Filho <wedsonaf@gmail.com> wrote:

> This allows us to hand ownership of Rust dynamically allocated
> objects to the C side of the kernel.
> 
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/types.rs | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 5475f6163002..e037c262f23e 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -63,6 +63,28 @@ pub trait ForeignOwnable {
>      unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self;
>  }
>  
> +impl<T: 'static> ForeignOwnable for Box<T> {
> +    type Borrowed<'a> = &'a T;
> +
> +    fn into_foreign(self) -> *const core::ffi::c_void {
> +        Box::into_raw(self) as _
> +    }
> +
> +    unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> &'a T {
> +        // SAFETY: The safety requirements for this function ensure that the object is still alive,
> +        // so it is safe to dereference the raw pointer.
> +        // The safety requirements of `from_foreign` also ensure that the object remains alive for
> +        // the lifetime of the returned value.
> +        unsafe { &*ptr.cast() }
> +    }
> +
> +    unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
> +        // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
> +        // call to `Self::into_foreign`.
> +        unsafe { Box::from_raw(ptr as _) }
> +    }
> +}
> +
>  /// Runs a cleanup function/closure when dropped.
>  ///
>  /// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.