Re: [PATCH v2] rust: types: add FOREIGN_ALIGN to ForeignOwnable

["Benno Lossin" <lossin@kernel.org>]

On Wed Jun 11, 2025 at 12:43 PM CEST, Andreas Hindborg wrote:
> "Benno Lossin" <lossin@kernel.org> writes:
>
>> On Tue Jun 10, 2025 at 4:15 PM CEST, Alice Ryhl wrote:
>>> On Tue, Jun 10, 2025 at 4:10 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>>>
>>>> "Benno Lossin" <lossin@kernel.org> writes:
>>>>
>>>> > On Tue Jun 10, 2025 at 1:30 PM CEST, Andreas Hindborg wrote:
>>>> >> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
>>>> >> index 22985b6f6982..0ccef6b5a20a 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`] returns pointers aligned to
>>>> >> +/// [`Self::FOREIGN_ALIGN`].
>>>> >>  pub unsafe trait ForeignOwnable: Sized {
>>>> >> -    /// Type used when the value is foreign-owned. In practical terms only defines the alignment of
>>>> >> -    /// the pointer.
>>>> >> -    type PointedTo;
>>>> >> +    /// The alignment of pointers returned by `into_foreign`.
>>>> >> +    const FOREIGN_ALIGN: usize;
>>>> >>
>>>> >>      /// Type used to immutably borrow a value that is currently foreign-owned.
>>>> >>      type Borrowed<'a>;
>>>> >> @@ -39,18 +35,20 @@ pub unsafe trait ForeignOwnable: Sized {
>>>> >>
>>>> >>      /// Converts a Rust-owned object to a foreign-owned one.
>>>> >>      ///
>>>> >> +    /// The foreign representation is a pointer to void. Aside from the guarantees listed below,
>>>> >
>>>> > I feel like this reads better:
>>>> >
>>>> > s/guarantees/ones/
>>>> >
>>>> >> +    /// there are no other guarantees for this pointer. For example, it might be invalid, dangling
>>>> >
>>>> > We should also mention that it could be null. (or is that assumption
>>>> > wrong?)
>>>>
>>>> It is probably not going to be null, but it is allowed to. I can add it.
>>>>
>>>> The list does not claim to be exhaustive, and a null pointer is just a
>>>> special case of an invalid pointer.
>>>
>>> We probably should not allow null pointers. If we do, then
>>> try_from_foreign does not make sense.
>>
>> That's a good point. Then let's add that as a safety requirement for the
>> trait.
>
> I disagree. It does not matter for the safety of the trait.
>
> From the point of the user, the pointer is opaque and can be any value.
> In fact, one could do a safe implementation where the returned value is
> a key into some mapping structure. Probably not super fast, but the user
> should not care.

Then we'll have to remove `try_from_foreign`.

---
Cheers,
Benno