linux-kernel - Re: [PATCH v2] rust: types: add FOREIGN

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Message-ID: <87v7p3znz6.fsf@kernel.org>
Date: Tue, 10 Jun 2025 16:10:37 +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 v2] rust: types: add FOREIGN_ALIGN to ForeignOwnable

"Benno Lossin" <lossin@...nel.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.

Best regards,
Andreas Hindborg