rust: sync: add Arc::{from_raw, into_raw}

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: rust-lang/rust#81513 [1]
Link: rust-lang/rust#96283 [2]
Signed-off-by: Wedson Almeida Filho <[email protected]>
Co-developed-by: Alice Ryhl <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
Reviewed-by: Martin Rodriguez Reboredo <[email protected]>

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)]
 

rust/kernel/sync/arc.rs

@@ -24,7 +24,7 @@ use crate::{
 };
 use alloc::boxed::Box;
 use core::{
- alloc::AllocError,
+ alloc::{AllocError, Layout},
  fmt,
  marker::{PhantomData, Unsize},
  mem::{ManuallyDrop, MaybeUninit},
@@ -210,6 +210,46 @@ 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 {
+ 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