rust: types: add FOREIGN_ALIGN to ForeignOwnable

The current implementation of `ForeignOwnable` is leaking the type of the
opaque pointer to consumers of the API. This allows consumers of the opaque
pointer to rely on the information that can be extracted from the pointer
type.

To prevent this, change the API to the version suggested by Maira
Canal (link below): Remove `ForeignOwnable::PointedTo` in favor of a
constant, which specifies the alignment of the pointers returned by
`into_foreign`.

With this change, `ArcInner` no longer needs `pub` visibility, so change it
to private.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Suggested-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20240309235927.168915-3-mcanal@igalia.com
Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250612-pointed-to-v3-1-b009006d86a1@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 13 insertions, 11 deletions

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 499175f637a7..63a66761d0c7 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -19,6 +19,7 @@
 use crate::{
     alloc::{AllocError, Flags, KBox},
     bindings,
+    ffi::c_void,
     init::InPlaceInit,
     try_init,
     types::{ForeignOwnable, Opaque},
@@ -141,10 +142,9 @@ pub struct Arc<T: ?Sized> {
     _p: PhantomData<ArcInner<T>>,
 }
 
-#[doc(hidden)]
 #[pin_data]
 #[repr(C)]
-pub struct ArcInner<T: ?Sized> {
+struct ArcInner<T: ?Sized> {
     refcount: Opaque<bindings::refcount_t>,
     data: T,
 }
@@ -373,20 +373,22 @@ impl<T: ?Sized> Arc<T> {
     }
 }
 
-// SAFETY: The `into_foreign` function returns a pointer that is well-aligned.
+// SAFETY: The pointer returned by `into_foreign` comes from a well aligned
+// pointer to `ArcInner<T>`.
 unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
-    type PointedTo = ArcInner<T>;
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<ArcInner<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()
+    fn into_foreign(self) -> *mut c_void {
+        ManuallyDrop::new(self).ptr.as_ptr().cast()
     }
 
-    unsafe fn from_foreign(ptr: *mut Self::PointedTo) -> Self {
+    unsafe fn from_foreign(ptr: *mut c_void) -> Self {
         // 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(ptr.cast::<ArcInner<T>>()) };
 
         // 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
@@ -394,17 +396,17 @@ unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
         unsafe { Self::from_inner(inner) }
     }
 
-    unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> ArcBorrow<'a, T> {
+    unsafe fn borrow<'a>(ptr: *mut c_void) -> ArcBorrow<'a, T> {
         // 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(ptr.cast::<ArcInner<T>>()) };
 
         // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
         // for the lifetime of the returned value.
         unsafe { ArcBorrow::new(inner) }
     }
 
-    unsafe fn borrow_mut<'a>(ptr: *mut Self::PointedTo) -> ArcBorrow<'a, T> {
+    unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> ArcBorrow<'a, T> {
         // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety
         // requirements for `borrow`.
         unsafe { <Self as ForeignOwnable>::borrow(ptr) }