Re: [PATCH] rust: types: add FOREIGN_ALIGN to ForeignOwnable

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

"Alice Ryhl" <aliceryhl@google.com> writes:

> On Wed, Jun 11, 2025 at 12:46 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>
>> "Alice Ryhl" <aliceryhl@google.com> writes:
>>
>> > On Thu, Jun 5, 2025 at 10:00 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>> >>
>> >> 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`.
>> >>
>> >> Suggested-by: Alice Ryhl <aliceryhl@google.com>
>> >> Suggested-by: Maíra Canal <mcanal@igalia.com>
>> >> Link: https://lore.kernel.org/r/20240309235927.168915-3-mcanal@igaliacom
>> >> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>> >
>> > One nit below. With that and things other folks mentioned fixed, you may add:
>> >
>> > Reviewed-by: Alice Ryhl <aliceryhl@google.com>
>> >
>> >> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
>> >> index 22985b6f6982..025c619a2195 100644
>> >> --- a/rust/kernel/types.rs
>> >> +++ b/rust/kernel/types.rs
>> >> @@ -21,15 +21,11 @@
>> >>  ///
>> >>  /// # Safety
>> >>  ///
>> >> -/// Implementers must ensure that [`into_foreign`] returns a pointer which meets the alignment
>> >> -/// requirements of [`PointedTo`].
>> >> -///
>> >> -/// [`into_foreign`]: Self::into_foreign
>> >> -/// [`PointedTo`]: Self::PointedTo
>> >> +/// Implementers must ensure that [`Self::into_foreign`] return pointers with alignment that is an
>> >> +/// integer multiple of [`Self::FOREIGN_ALIGN`].
>> >
>> > We should require non-null:
>>
>> What is your rationale for this?
>
> The rationale is that the implementation of XArray assumes that the
> pointers are non-null. If we allow null pointers, we will need to fix
> the XArray.

OK, thanks. Also, `try_from_foreign` as Benno highlighted.


Best regards,
Andreas Hindborg