Re: [PATCH v2 3/8] rust: sync: add `Arc::{from_raw, into_raw}`

[Benno Lossin <benno.lossin@proton.me>]

On 01.06.23 15:49, Alice Ryhl wrote:
> From: Wedson Almeida Filho <walmeida@microsoft.com>
> 
> These methods can be used to turn an `Arc` into a raw pointer and back,
> in a way that preserves the metadata for fat pointers.
> 
> This is done using the unstable ptr_metadata feature [1]. However, it
> could also be done using the unstable pointer_byte_offsets feature [2],
> which is likely to have a shorter path to stabilization than
> ptr_metadata.
> 
> Link: https://github.com/rust-lang/rust/issues/81513 [1]
> Link: https://github.com/rust-lang/rust/issues/96283 [2]
> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
> Co-developed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

-- 
Cheers,
Benno

> ---
>   rust/kernel/lib.rs      |  1 +
>   rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++-
>   2 files changed, 42 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 7ea777b731e6..ad9142928fb1 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -17,6 +17,7 @@
>   #![feature(const_refs_to_cell)]
>   #![feature(dispatch_from_dyn)]
>   #![feature(new_uninit)]
> +#![feature(ptr_metadata)]
>   #![feature(receiver_trait)]
>   #![feature(unsize)]
> 
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index a89843cacaad..684be9f73aca 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -24,7 +24,7 @@
>   };
>   use alloc::boxed::Box;
>   use core::{
> -    alloc::AllocError,
> +    alloc::{AllocError, Layout},
>       fmt,
>       marker::{PhantomData, Unsize},
>       mem::{ManuallyDrop, MaybeUninit},
> @@ -212,6 +212,46 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
>           }
>       }
> 
> +    /// Convert the [`Arc`] into a raw pointer.
> +    ///
> +    /// The raw pointer has ownership of the refcount that this Arc object owned.
> +    pub fn into_raw(self) -> *const T {
> +        let ptr = self.ptr.as_ptr();
> +        core::mem::forget(self);
> +        // SAFETY: The pointer is valid.
> +        unsafe { core::ptr::addr_of!((*ptr).data) }
> +    }
> +
> +    /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
> +    ///
> +    /// This code relies on the `repr(C)` layout of structs as described in
> +    /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
> +    ///
> +    /// # Safety
> +    ///
> +    /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
> +    /// can only be called once for each previous call to [`Arc::into_raw`].
> +    pub unsafe fn from_raw(ptr: *const T) -> Self {
> +        let refcount_layout = Layout::new::<bindings::refcount_t>();
> +        // SAFETY: The caller guarantees that the pointer is valid.
> +        let val_layout = unsafe { Layout::for_value(&*ptr) };
> +        // SAFETY: We're computing the layout of a real struct that existed when compiling this
> +        // 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 };
> +
> +        // This preserves the metadata in the pointer, if any.
> +        //
> +        // Note that `*const T` and `*const ArcInner<T>` have the same metadata as documented at
> +        // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
> +        let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
> +        let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
> +        let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
> +
> +        // 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.
> +        unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
> +    }
> +
>       /// Returns an [`ArcBorrow`] from the given [`Arc`].
>       ///
>       /// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method
> --
> 2.41.0.rc0.172.g3f132b7071-goog
>