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, 23 insertions, 18 deletions

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index c386ff771d50..bffe72f44cb3 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -15,6 +15,7 @@ use core::pin::Pin;
 use core::ptr::NonNull;
 use core::result::Result;
 
+use crate::ffi::c_void;
 use crate::init::InPlaceInit;
 use crate::types::ForeignOwnable;
 use pin_init::{InPlaceWrite, Init, PinInit, ZeroableOption};
@@ -398,70 +399,74 @@ where
     }
 }
 
-// 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 `T`.
 unsafe impl<T: 'static, A> ForeignOwnable for Box<T, A>
 where
     A: Allocator,
 {
-    type PointedTo = T;
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<T>();
     type Borrowed<'a> = &'a T;
     type BorrowedMut<'a> = &'a mut T;
 
-    fn into_foreign(self) -> *mut Self::PointedTo {
-        Box::into_raw(self)
+    fn into_foreign(self) -> *mut c_void {
+        Box::into_raw(self).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`.
-        unsafe { Box::from_raw(ptr) }
+        unsafe { Box::from_raw(ptr.cast()) }
     }
 
-    unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> &'a T {
+    unsafe fn borrow<'a>(ptr: *mut c_void) -> &'a T {
         // SAFETY: The safety requirements of this method ensure that the object remains alive and
         // immutable for the duration of 'a.
-        unsafe { &*ptr }
+        unsafe { &*ptr.cast() }
     }
 
-    unsafe fn borrow_mut<'a>(ptr: *mut Self::PointedTo) -> &'a mut T {
+    unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> &'a mut T {
+        let ptr = ptr.cast();
         // SAFETY: The safety requirements of this method ensure that the pointer is valid and that
         // nothing else will access the value for the duration of 'a.
         unsafe { &mut *ptr }
     }
 }
 
-// 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 `T`.
 unsafe impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
 where
     A: Allocator,
 {
-    type PointedTo = T;
+    const FOREIGN_ALIGN: usize = core::mem::align_of::<T>();
     type Borrowed<'a> = Pin<&'a T>;
     type BorrowedMut<'a> = Pin<&'a mut T>;
 
-    fn into_foreign(self) -> *mut Self::PointedTo {
+    fn into_foreign(self) -> *mut c_void {
         // SAFETY: We are still treating the box as pinned.
-        Box::into_raw(unsafe { Pin::into_inner_unchecked(self) })
+        Box::into_raw(unsafe { Pin::into_inner_unchecked(self) }).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`.
-        unsafe { Pin::new_unchecked(Box::from_raw(ptr)) }
+        unsafe { Pin::new_unchecked(Box::from_raw(ptr.cast())) }
     }
 
-    unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> Pin<&'a T> {
+    unsafe fn borrow<'a>(ptr: *mut c_void) -> Pin<&'a T> {
         // SAFETY: The safety requirements for this function ensure that the object is still alive,
         // so it is safe to dereference the raw pointer.
         // The safety requirements of `from_foreign` also ensure that the object remains alive for
         // the lifetime of the returned value.
-        let r = unsafe { &*ptr };
+        let r = unsafe { &*ptr.cast() };
 
         // SAFETY: This pointer originates from a `Pin<Box<T>>`.
         unsafe { Pin::new_unchecked(r) }
     }
 
-    unsafe fn borrow_mut<'a>(ptr: *mut Self::PointedTo) -> Pin<&'a mut T> {
+    unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> Pin<&'a mut T> {
+        let ptr = ptr.cast();
         // SAFETY: The safety requirements for this function ensure that the object is still alive,
         // so it is safe to dereference the raw pointer.
         // The safety requirements of `from_foreign` also ensure that the object remains alive for