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

Alice Ryhl
2023-06-01T13:53:22Z

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>);
+ 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
--
2.41.0.rc0.172.g3f132b7071-goog