linux-kernel - Re: [PATCH v2 3/5] rust: types: implement `ForeignOwnable` for `Box<T>`

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Date:   Wed, 01 Feb 2023 10:56:24 +0100
From:   Andreas Hindborg <nmi@...aspace.dk>
To:     Wedson Almeida Filho <wedsonaf@...il.com>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org,
        Vincenzo Palazzo <vincenzopalazzodev@...il.com>,
        Alice Ferrazzi <alice.ferrazzi@...aclelinux.com>,
        Andreas Hindborg <a.hindborg@...sung.com>
Subject: Re: [PATCH v2 3/5] rust: types: implement `ForeignOwnable` for
 `Box<T>`


Wedson Almeida Filho <wedsonaf@...il.com> writes:

> 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@...il.com>
> Reviewed-by: Gary Guo <gary@...yguo.net>
> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@...il.com>
> Reviewed-by: Alice Ferrazzi <alice.ferrazzi@...aclelinux.com>
> ---

Reviewed-by: Andreas Hindborg <a.hindborg@...sung.com>

> v1 -> v2: Add `use alloc::boxed::Box`, which wasn't needed before
>
>  rust/kernel/types.rs | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
>
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 72710b7442a3..411655eca3e9 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -2,6 +2,7 @@
>  
>  //! Kernel types.
>  
> +use alloc::boxed::Box;
>  use core::{
>      cell::UnsafeCell,
>      mem::MaybeUninit,
> @@ -62,6 +63,28 @@ pub trait ForeignOwnable: Sized {
>      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.