UPSTREAM: 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: Gary Guo <[email protected]>
Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
Reviewed-by: Benno Lossin <[email protected]>
Reviewed-by: Andreas Hindborg <[email protected]>
Reviewed-by: Boqun Feng <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>
[ Upstream commit a832177 ]
Change-Id: I3e02e5596bb18ac8255b8659e8ec9a76c5b2ac92
Signed-off-by: Alice Ryhl <[email protected]>

rust/kernel/lib.rs

@@ -16,6 +16,7 @@
 #![feature(coerce_unsized)]
 #![feature(dispatch_from_dyn)]
 #![feature(new_uninit)]
+#![feature(ptr_metadata)]
 #![feature(receiver_trait)]
 #![feature(unsize)]
 

rust/kernel/sync/arc.rs

@@ -24,13 +24,13 @@ use crate::{
 };
 use alloc::boxed::Box;
 use core::{
- alloc::AllocError,
+ alloc::{AllocError, Layout},
  fmt,
  marker::{PhantomData, Unsize},
  mem::{ManuallyDrop, MaybeUninit},
  ops::{Deref, DerefMut},
  pin::Pin,
- ptr::NonNull,
+ ptr::{NonNull, Pointee},
 };
 use macros::pin_data;
 
@@ -215,6 +215,48 @@ 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`].
+ ///
+ /// # Safety
+ ///
+ /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
+ /// must not be called more than 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 = Layout::for_value(unsafe { &*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 };
+
+ 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>.
+ let metadata: <ArcInner<T> as Pointee>::Metadata =
+ unsafe { core::mem::transmute_copy(&metadata) };
+ // 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);
+
+ // 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