linux-kernel - Re: [PATCH v3 2/2] rust: types: require `ForeignOwnable::into

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Message-Id: <DAKN1HO7WUXY.QS098VXTDICU@kernel.org>
Date: Thu, 12 Jun 2025 16:52:15 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Andreas Hindborg" <a.hindborg@...nel.org>, "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>
Cc: <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] rust: types: require
 `ForeignOwnable::into_foreign` return non-null

On Thu Jun 12, 2025 at 3:09 PM CEST, Andreas Hindborg wrote:
> The intended implementations of `ForeignOwnable` will not return null
> pointers from `into_foreign`, as this would render the implementation of
> `try_from_foreign` useless. Current users of `ForeignOwnable` rely on
> `into_foreign` returning non-null pointers. So require `into_foreign` to
> return non-null pointers.
>
> Suggested-by: Benno Lossin <lossin@...nel.org>
> Suggested-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
>  rust/kernel/types.rs | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index c156808a78d3..63a2559a545f 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -43,6 +43,7 @@ pub unsafe trait ForeignOwnable: Sized {
>      /// # Guarantees
>      ///
>      /// - Minimum alignment of returned pointer is [`Self::FOREIGN_ALIGN`].
> +    /// - The returned pointer is not null.

This also needs to be mentioned in the `Safety` section of this trait.
Alternatively you can put "Implementers must ensure the guarantees on
[`into_foreign`] are upheld." or similar.

---
Cheers,
Benno

>      ///
>      /// [`from_foreign`]: Self::from_foreign
>      /// [`try_from_foreign`]: Self::try_from_foreign