Re: [PATCH] rust: stop using ptr_metadata feature

[Trevor Gross]

On Mon, Feb 5, 2024 at 3:19 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> The `byte_sub` method was stabilized in Rust 1.75.0. By using that
> method, we no longer need the unstable `ptr_metadata` feature for
> implementing `Arc::from_raw`.
>
> This brings us one step closer towards not using unstable compiler
> features.
>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> ---
> This patch is based on rust-next because it depends on the patch [1]
> that upgrades to Rust 1.75.0.
>
> [1]: https://lore.kernel.org/all/20231224172128.271447-1-ojeda@xxxxxxxxxx/
>
>  rust/kernel/lib.rs      |  1 -
>  rust/kernel/sync/arc.rs | 10 ++++------
>  2 files changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> [...]
> @@ -239,18 +239,16 @@ pub unsafe fn from_raw(ptr: *const T) -> Self {
>          // binary, so its layout is not so large that it can trigger arithmetic overflow.
>          let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
>
> -        let metadata: <T as Pointee>::Metadata = core::ptr::metadata(ptr);
>          // SAFETY: The metadata of `T` and `ArcInner<T>` is the same because `ArcInner` is a struct
>          // with `T` as its last field.
>          //
>          // This is documented at:
>          // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.

The comment should be reworded, no more metadata and no unsafe block
so it doesn't have to be SAFETY.

> -        let metadata: <ArcInner<T> as Pointee>::Metadata =
> -            unsafe { core::mem::transmute_copy(&metadata) };
> +        let ptr = ptr as *mut ArcInner<T>;

Nit: this could be `.cast::<ArcInner<T>>().cast_mut()` to make the
intentional mutability change clear.

>          // SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the
>          // pointer, since it originates from a previous call to `Arc::into_raw` and is still valid.
> -        let ptr = unsafe { (ptr as *mut u8).sub(val_offset) as *mut () };
> -        let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
> +        let ptr = unsafe { ptr.byte_sub(val_offset) };
>
>          // SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
>          // reference count held then will be owned by the new `Arc` object.
>
> base-commit: f090f0d0eea9666a96702b29bc9a64cbabee85c5
> --
> 2.43.0.594.gd9cf4e227d-goog
>
>

Worth noting that the same change has been in upstream for a while,
since https://github.com/rust-lang/rust/pull/99113.

With the above changed or otherwise clarified:

Reviewed-by: Trevor Gross <tmgross@xxxxxxxxx>