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Date: Thu, 12 Jun 2025 16:52:15 +0200
Message-Id: <DAKN1HO7WUXY.QS098VXTDICU@kernel.org>
From: "Benno Lossin" <lossin@kernel.org>
To: "Andreas Hindborg" <a.hindborg@kernel.org>, "Danilo Krummrich"
 <dakr@kernel.org>, "Miguel Ojeda" <ojeda@kernel.org>, "Alex Gaynor"
 <alex.gaynor@gmail.com>, "Boqun Feng" <boqun.feng@gmail.com>, "Gary Guo"
 <gary@garyguo.net>, =?utf-8?q?Bj=C3=B6rn_Roy_Baron?=
 <bjorn3_gh@protonmail.com>, "Alice Ryhl" <aliceryhl@google.com>, "Trevor
 Gross" <tmgross@umich.edu>, "Bjorn Helgaas" <bhelgaas@google.com>,
 =?utf-8?q?Krzysztof_Wilczy=C5=84ski?= <kwilczynski@kernel.org>, "Greg
 Kroah-Hartman" <gregkh@linuxfoundation.org>, "Rafael J. Wysocki"
 <rafael@kernel.org>, "Tamir Duberstein" <tamird@gmail.com>, "Viresh Kumar"
 <viresh.kumar@linaro.org>
Cc: <rust-for-linux@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
 <linux-pci@vger.kernel.org>
Subject: Re: [PATCH v3 2/2] rust: types: require
 `ForeignOwnable::into_foreign` return non-null
X-Mailer: aerc 0.20.1
References: <20250612-pointed-to-v3-0-b009006d86a1@kernel.org>
 <20250612-pointed-to-v3-2-b009006d86a1@kernel.org>
In-Reply-To: <20250612-pointed-to-v3-2-b009006d86a1@kernel.org>

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@kernel.org>
> Suggested-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.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