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

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Message-Id: <DAIRJ71UP655.2NF13FJSA0G68@kernel.org>
Date: Tue, 10 Jun 2025 11:58:07 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Andreas Hindborg" <a.hindborg@...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

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..."?

Otherwise looks good!

---
Cheers,
Benno