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Message-ID: <87edn6do1b.fsf@metaspace.dk>
Date: Wed, 24 May 2023 12:20:36 +0200
From: Andreas Hindborg <nmi@...aspace.dk>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
Wedson Almeida Filho <wedsonaf@...il.com>,
Tejun Heo <tj@...nel.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Alex Gaynor <alex.gaynor@...il.com>,
Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
Björn Roy Baron <bjorn3_gh@...tonmail.com>,
Benno Lossin <benno.lossin@...ton.me>,
linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}`
Alice Ryhl <aliceryhl@...gle.com> writes:
> From: Wedson Almeida Filho <walmeida@...rosoft.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@...rosoft.com>
> Co-developed-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
> rust/kernel/lib.rs | 1 +
> rust/kernel/sync/arc.rs | 44 +++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 45 insertions(+)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index cdf9fe999328..82854c86e65d 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -22,6 +22,7 @@
> #![feature(generic_associated_types)]
> #![feature(new_uninit)]
> #![feature(pin_macro)]
> +#![feature(ptr_metadata)]
> #![feature(receiver_trait)]
> #![feature(unsize)]
>
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index e6d206242465..7c55a9178dfb 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -210,6 +210,50 @@ impl<T: ?Sized> Arc<T> {
> }
> }
>
> + /// 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 {
> + // SAFETY: The safety requirement ensures that the pointer is valid.
> + let val_align = core::mem::align_of_val(unsafe { &*ptr });
> + let refcount_size = core::mem::size_of::<Opaque<bindings::refcount_t>>();
> +
> + // Use the `repr(C)` algorithm to compute the offset of `data` in `ArcInner`.
> + //
> + // Pseudo-code for the `#[repr(C)]` algorithm can be found here:
> + // <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>
> + let mut val_offset = refcount_size;
> + let val_misalign = val_offset % val_align;
> + if val_misalign > 0 {
> + val_offset += val_align - val_misalign;
> + }
> +
> + // This preserves the metadata in the pointer, if any.
> + let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
I can't follow this. How does this work? `ptr` was for field
`inner.data: T`, but we are casting to `ArcInner<T>`.
> + let ptr = (ptr as *mut u8).wrapping_sub(val_offset) as *mut ();
> + let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
Metadata was obtained from a pointer pointing to `inner.data`, we then
move it back to beginning of `ArcInner<T>` and then reconstruct the
potentially fat pointer with metadata from the pointer to `T`? How can
this be right?
BR Andreas
> +
> + // 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
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