[PATCH v4 1/4] rust: sync: change `<Arc<T> as ForeignOwnable>::PointedTo` to `T`

[Andreas Hindborg <a.hindborg@kernel.org>]

Using `ArcInner` as `PoinedTo` in the `ForeignOwnable` implementation for
`Arc` is a bit unfortunate. Using `T` as `PointedTo` does not remove any
functionality, but allows `ArcInner` to be private. Further, it allows
downstream users to write code that is generic over `Box` and `Arc`, when
downstream users need access to `T` after calling `into_foreign`.

Reviewed-by: Fiona Behrens <me@kloenk.dev>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---

This patch is a dependency for Rust `configfs` abstractions. It allows both
`Box` and `Arc` to be used as pointer types in the `configfs` hierarchy.
---
 rust/kernel/sync/arc.rs | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index dfe4abf82c25..3d77a31e116f 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -143,7 +143,7 @@ pub struct Arc<T: ?Sized> {
 #[doc(hidden)]
 #[pin_data]
 #[repr(C)]
-pub struct ArcInner<T: ?Sized> {
+struct ArcInner<T: ?Sized> {
     refcount: Opaque<bindings::refcount_t>,
     data: T,
 }
@@ -345,18 +345,25 @@ pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
 
 // SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
 unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
-    type PointedTo = ArcInner<T>;
+    type PointedTo = T;
     type Borrowed<'a> = ArcBorrow<'a, T>;
     type BorrowedMut<'a> = Self::Borrowed<'a>;
 
     fn into_foreign(self) -> *mut Self::PointedTo {
-        ManuallyDrop::new(self).ptr.as_ptr()
+        let this = ManuallyDrop::new(self).ptr.as_ptr();
+        // SAFETY: `x` is a valid pointer to `Self` so the projection below is
+        // in bounds of the allocation.
+        unsafe { core::ptr::addr_of_mut!((*this).data) }
     }
 
     unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self {
+        // SAFETY: We did the reverse offset calculation in `into_foreign`, so
+        // the offset calculation below is in bounds of the allocation.
+        let inner_ptr = unsafe { kernel::container_of!(ptr, ArcInner<T>, data).cast_mut() };
+
         // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
         // call to `Self::into_foreign`.
-        let inner = unsafe { NonNull::new_unchecked(ptr) };
+        let inner = unsafe { NonNull::new_unchecked(inner_ptr) };
 
         // SAFETY: By the safety requirement of this function, we know that `ptr` came from
         // a previous call to `Arc::into_foreign`, which guarantees that `ptr` is valid and
@@ -365,9 +372,13 @@ unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self {
     }
 
     unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> ArcBorrow<'a, T> {
+        // SAFETY: We did the reverse offset calculation in `into_foreign`, so
+        // the offset calculation below is in bounds of the allocation.
+        let inner_ptr = unsafe { kernel::container_of!(ptr, ArcInner<T>, data).cast_mut() };
+
         // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
         // call to `Self::into_foreign`.
-        let inner = unsafe { NonNull::new_unchecked(ptr) };
+        let inner = unsafe { NonNull::new_unchecked(inner_ptr) };
 
         // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
         // for the lifetime of the returned value.

-- 
2.47.0