When converting a Box<T> into a void pointer, the allocator might
guarantee a higher alignment than the type itself does, and in that case
it is guaranteed that the void pointer has that higher alignment.

This is quite useful when combined with the XArray, which you can only
create using a ForeignOwnable whose FOREIGN_ALIGN is at least 4. This
means that you can now always use a Box<T> with the XArray no matter the
alignment of T.

Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/alloc/kbox.rs | 13 +++++++++----
 rust/kernel/sync/arc.rs   |  6 +++---
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index 856d05aa60f13485d8afc98f0b7fe7593867b5a1..eedab0be1eff78a8150524346348ec0759f852e2 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -401,12 +401,17 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
 }
 
 // SAFETY: The pointer returned by `into_foreign` comes from a well aligned
-// pointer to `T`.
+// pointer to `T` allocated by `A`.
 unsafe impl<T: 'static, A> ForeignOwnable for Box<T, A>
 where
     A: Allocator,
 {
-    const FOREIGN_ALIGN: usize = core::mem::align_of::<T>();
+    const FOREIGN_ALIGN: usize = if core::mem::align_of::<T>() < A::MIN_ALIGN {
+        A::MIN_ALIGN
+    } else {
+        core::mem::align_of::<T>()
+    };
+
     type Borrowed<'a> = &'a T;
     type BorrowedMut<'a> = &'a mut T;
 
@@ -435,12 +440,12 @@ unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> &'a mut T {
 }
 
 // SAFETY: The pointer returned by `into_foreign` comes from a well aligned
-// pointer to `T`.
+// pointer to `T` allocated by `A`.
 unsafe impl<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
 where
     A: Allocator,
 {
-    const FOREIGN_ALIGN: usize = core::mem::align_of::<T>();
+    const FOREIGN_ALIGN: usize = <Box<T, A> as ForeignOwnable>::FOREIGN_ALIGN;
     type Borrowed<'a> = Pin<&'a T>;
     type BorrowedMut<'a> = Pin<&'a mut T>;
 
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 63a66761d0c7d752e09ce7372bc230661b2f7c6d..74121cf935f364c16799b5c31cc88714dfd6b702 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -373,10 +373,10 @@ pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
     }
 }
 
-// SAFETY: The pointer returned by `into_foreign` comes from a well aligned
-// pointer to `ArcInner<T>`.
+// SAFETY: The pointer returned by `into_foreign` was originally allocated as an
+// `KBox<ArcInner<T>>`, so that type is what determines the alignment.
 unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
-    const FOREIGN_ALIGN: usize = core::mem::align_of::<ArcInner<T>>();
+    const FOREIGN_ALIGN: usize = <KBox<ArcInner<T>> as ForeignOwnable>::FOREIGN_ALIGN;
 
     type Borrowed<'a> = ArcBorrow<'a, T>;
     type BorrowedMut<'a> = Self::Borrowed<'a>;

-- 
2.50.1.703.g449372360f-goog