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

Andreas Hindborg (Samsung)
2023-06-02T12:50:19Z

Alice Ryhl <aliceryhl@google.com> writes:

> 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>
> ---
> 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>);

Thanks for updating the comment with the link. I looked into this and I
find that what we are doing here, even though it works, does not feel
right at all. We should be able to do this:

let metadata = core::ptr::metadata(ptr);
let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);

but the way `Pointee::Metadata` is defined will not allow this, even
though we know it is valid. I would suggest the following instead:

let metadata = core::ptr::metadata(ptr);
// Convert <T as Pointee>::Metadata to <ArcInner<T> as
// Pointee>::Metadata. We know they have identical representation and thus this is OK.
let metadata: <ArcInner<T> as Pointee>::Metadata = *unsafe {
&*((&metadata as *const <T as Pointee>::Metadata as *const ())
as *const <ArcInner<T> as Pointee>::Metadata)
};
let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);

Even though it is a bit more complex, it captures what we are trying to
do better.

Best regards,
Andreas

> + 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