Re: [PATCH] rust: delete `ForeignOwnable::borrow_mut`

[Gary Guo <gary@garyguo.net>]

On Thu,  6 Jul 2023 09:46:15 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> We discovered that the current design of `borrow_mut` is problematic.
> This patch removes it until a better solution can be found.
> 
> Specifically, the current design gives you access to a `&mut T`, which
> lets you change where the `ForeignOwnable` points (e.g., with
> `core::mem::swap`). No upcoming user of this API intended to make that
> possible, making all of them unsound.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

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

> ---
>  rust/kernel/sync/arc.rs |  3 +--
>  rust/kernel/types.rs    | 22 ++--------------------
>  2 files changed, 3 insertions(+), 22 deletions(-)
> 
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index a89843cacaad..172f563976a9 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -243,8 +243,7 @@ unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, T> {
>          let inner = NonNull::new(ptr as *mut ArcInner<T>).unwrap();
>  
>          // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
> -        // for the lifetime of the returned value. Additionally, the safety requirements of
> -        // `ForeignOwnable::borrow_mut` ensure that no new mutable references are created.
> +        // for the lifetime of the returned value.
>          unsafe { ArcBorrow::new(inner) }
>      }
>  
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 1e5380b16ed5..d479f8da8f38 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -35,34 +35,16 @@ pub trait ForeignOwnable: Sized {
>      ///
>      /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
>      /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
> -    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow_mut`]
> -    /// for this object must have been dropped.
>      unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Self::Borrowed<'a>;
>  
> -    /// Mutably borrows a foreign-owned object.
> -    ///
> -    /// # Safety
> -    ///
> -    /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
> -    /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
> -    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
> -    /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
> -    unsafe fn borrow_mut(ptr: *const core::ffi::c_void) -> ScopeGuard<Self, fn(Self)> {
> -        // SAFETY: The safety requirements ensure that `ptr` came from a previous call to
> -        // `into_foreign`.
> -        ScopeGuard::new_with_data(unsafe { Self::from_foreign(ptr) }, |d| {
> -            d.into_foreign();
> -        })
> -    }
> -
>      /// Converts a foreign-owned object back to a Rust-owned one.
>      ///
>      /// # Safety
>      ///
>      /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
>      /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
> -    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
> -    /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
> +    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] for
> +    /// this object must have been dropped.
>      unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self;
>  }
>  
> 
> base-commit: d2e3115d717197cb2bc020dd1f06b06538474ac3