1 files changed, 52 insertions, 5 deletions

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 172f563976a9..77cdbcf7bd2e 100644
--- a/rust/kernel/sync/arc.rs
+++ b/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;
 
@@ -73,6 +73,7 @@ mod std_vendor;
 /// assert_eq!(cloned.b, 20);
 ///
 /// // The refcount drops to zero when `cloned` goes out of scope, and the memory is freed.
+/// # Ok::<(), Error>(())
 /// ```
 ///
 /// Using `Arc<T>` as the type of `self`:
@@ -98,6 +99,7 @@ mod std_vendor;
 /// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
 /// obj.use_reference();
 /// obj.take_over();
+/// # Ok::<(), Error>(())
 /// ```
 ///
 /// Coercion from `Arc<Example>` to `Arc<dyn MyTrait>`:
@@ -121,6 +123,7 @@ mod std_vendor;
 ///
 /// // `coerced` has type `Arc<dyn MyTrait>`.
 /// let coerced: Arc<dyn MyTrait> = obj;
+/// # Ok::<(), Error>(())
 /// ```
 pub struct Arc<T: ?Sized> {
     ptr: NonNull<ArcInner<T>>,
@@ -212,6 +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
@@ -299,7 +344,7 @@ impl<T: ?Sized> Drop for Arc<T> {
             // The count reached zero, we must free the memory.
             //
             // SAFETY: The pointer was initialised from the result of `Box::leak`.
-            unsafe { Box::from_raw(self.ptr.as_ptr()) };
+            unsafe { drop(Box::from_raw(self.ptr.as_ptr())) };
         }
     }
 }
@@ -336,7 +381,7 @@ impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
 /// # Example
 ///
 /// ```
-/// use crate::sync::{Arc, ArcBorrow};
+/// use kernel::sync::{Arc, ArcBorrow};
 ///
 /// struct Example;
 ///
@@ -349,12 +394,13 @@ impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
 ///
 /// // Assert that both `obj` and `cloned` point to the same underlying object.
 /// assert!(core::ptr::eq(&*obj, &*cloned));
+/// # Ok::<(), Error>(())
 /// ```
 ///
 /// Using `ArcBorrow<T>` as the type of `self`:
 ///
 /// ```
-/// use crate::sync::{Arc, ArcBorrow};
+/// use kernel::sync::{Arc, ArcBorrow};
 ///
 /// struct Example {
 ///     a: u32,
@@ -369,6 +415,7 @@ impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
 ///
 /// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
 /// obj.as_arc_borrow().use_reference();
+/// # Ok::<(), Error>(())
 /// ```
 pub struct ArcBorrow<'a, T: ?Sized + 'a> {
     inner: NonNull<ArcInner<T>>,