linux-kernel - Re: [PATCH] rust: types: add FOREIGN

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Message-ID: <87bjqv3ncr.fsf@kernel.org>
Date: Tue, 10 Jun 2025 12:25:08 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Benno Lossin" <lossin@...nel.org>
Cc: "Danilo Krummrich" <dakr@...nel.org>,  "Miguel Ojeda"
 <ojeda@...nel.org>,  "Alex Gaynor" <alex.gaynor@...il.com>,  "Boqun Feng"
 <boqun.feng@...il.com>,  "Gary Guo" <gary@...yguo.net>,  Björn Roy Baron
 <bjorn3_gh@...tonmail.com>,  "Alice Ryhl" <aliceryhl@...gle.com>,  "Trevor
 Gross" <tmgross@...ch.edu>,  "Bjorn Helgaas" <bhelgaas@...gle.com>,
  Krzysztof Wilczyński <kwilczynski@...nel.org>,  "Greg
 Kroah-Hartman"
 <gregkh@...uxfoundation.org>,  "Rafael J. Wysocki" <rafael@...nel.org>,
  "Tamir Duberstein" <tamird@...il.com>,  "Viresh Kumar"
 <viresh.kumar@...aro.org>,  <rust-for-linux@...r.kernel.org>,
  <linux-kernel@...r.kernel.org>,  <linux-pci@...r.kernel.org>,
  Maíra
 Canal <mcanal@...lia.com>
Subject: Re: [PATCH] rust: types: add FOREIGN_ALIGN to ForeignOwnable

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

> On Tue Jun 10, 2025 at 11:27 AM CEST, Andreas Hindborg wrote:
>> Hi Benno,
>>
>> "Benno Lossin" <lossin@...nel.org> writes:
>>
>>> The title should probably also mention that it removes `PointedTo`.
>
> Just making sure that you saw this.
>
>>> On Thu Jun 5, 2025 at 9:55 PM CEST, Andreas Hindborg wrote:
>>>>  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.
>
> Maybe even move this paragraph above the `Guarantees` section and change
> the beginning of it to "Aside from the guarantees listed below, there
> are no other..."?

Alright.

Best regards,
Andreas Hindborg