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

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

Hi Benno,

"Benno Lossin" <lossin@kernel.org> writes:

> The title should probably also mention that it removes `PointedTo`.
>
> On Thu Jun 5, 2025 at 9:55 PM CEST, Andreas Hindborg 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@igalia.com
>> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>
> A couple nits and documentation review below, with those fixed:
>
> Reviewed-by: Benno Lossin <lossin@kernel.org>
>
>> ---
>>  rust/kernel/alloc/kbox.rs | 40 ++++++++++++++++++++++------------------
>>  rust/kernel/miscdevice.rs | 10 +++++-----
>>  rust/kernel/pci.rs        |  2 +-
>>  rust/kernel/platform.rs   |  2 +-
>>  rust/kernel/sync/arc.rs   | 23 ++++++++++++-----------
>>  rust/kernel/types.rs      | 46 +++++++++++++++++++++-------------------------
>>  rust/kernel/xarray.rs     |  8 ++++----
>>  7 files changed, 66 insertions(+), 65 deletions(-)
>>
>> diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
>> index c386ff771d50..97f45bc4d74f 100644
>> --- a/rust/kernel/alloc/kbox.rs
>> +++ b/rust/kernel/alloc/kbox.rs
>> @@ -398,70 +398,74 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
>>      }
>>  }
>>
>> -// 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 crate::ffi::c_void {
>
> How about we import the prelude, then you can just write `*mut c_void`
> everywhere instead of having to write `crate::ffi` all the time.

OK.

>
>> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
>> index c7af0aa48a0a..6603079b05af 100644
>> --- a/rust/kernel/sync/arc.rs
>> +++ b/rust/kernel/sync/arc.rs
>> @@ -140,10 +140,9 @@ pub struct Arc<T: ?Sized> {
>>      _p: PhantomData<ArcInner<T>>,
>>  }
>>
>> -#[doc(hidden)]
>>  #[pin_data]
>>  #[repr(C)]
>> -pub struct ArcInner<T: ?Sized> {
>> +struct ArcInner<T: ?Sized> {
>
> I agree with this change, but let's mention it in the commit message.

Right.

>
>>      refcount: Opaque<bindings::refcount_t>,
>>      data: T,
>>  }
>
>> 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
>
> s/return/returns/

Thanks.

>
>> +/// integer multiple of [`Self::FOREIGN_ALIGN`].
>
> I would just write "returns pointers aligned to [`Self::FOREIGN_ALIGN`]".

OK.

>
>>  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 +35,17 @@ pub unsafe trait ForeignOwnable: Sized {
>>
>>      /// Converts a Rust-owned object to a foreign-owned one.
>>      ///
>> -    /// # Guarantees
>
> Why remove this section? I think we should streamline it, (make it use
> bullet points, shorten the sentences etc). We can keep the paragraph you
> wrote below as normal docs.

Not sure exactly what you are going for here. How is this:


  Converts a Rust-owned object to a foreign-owned one.

  The foreign representation is a pointer to void.

  # Guarantees

  - Minimum alignment of returned pointer is [`Self::FOREIGN_ALIGN`].

  There are no other guarantees for this pointer. For example, it might be invalid, dangling
  or pointing to uninitialized memory. Using it in any way except for [`from_foreign`],
  [`try_from_foreign`], [`borrow`], or [`borrow_mut`] can result in undefined behavior.



Best regards,
Andreas Hindborg