rust: alloc: take the allocator into account for FOREIGN_ALIGN

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>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250811-align-min-allocator-v2-2-3386cc94f4fc@google.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

1 files changed, 9 insertions, 4 deletions

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index 856d05aa60f1..eedab0be1eff 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -401,12 +401,17 @@ where
 }
 
 // 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 @@ where
 }
 
 // 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>;