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

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Message-ID: <8734c6zd71.fsf@kernel.org>
Date: Wed, 11 Jun 2025 14:15:46 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
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>,  "Benno Lossin" <lossin@...nel.org>,  "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

"Alice Ryhl" <aliceryhl@...gle.com> writes:

> On Wed, Jun 11, 2025 at 12:46 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>>
>> "Alice Ryhl" <aliceryhl@...gle.com> writes:
>>
>> > On Thu, Jun 5, 2025 at 10:00 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>> >>
>> >> The current implementation of `ForeignOwnable` is leaking the type of the
>> >> opaque pointer to consumers of the API. This allows consumers of the opaque
>> >> pointer to rely on the information that can be extracted from the pointer
>> >> type.
>> >>
>> >> To prevent this, change the API to the version suggested by Maira
>> >> Canal (link below): Remove `ForeignOwnable::PointedTo` in favor of a
>> >> constant, which specifies the alignment of the pointers returned by
>> >> `into_foreign`.
>> >>
>> >> Suggested-by: Alice Ryhl <aliceryhl@...gle.com>
>> >> Suggested-by: Maíra Canal <mcanal@...lia.com>
>> >> Link: https://lore.kernel.org/r/20240309235927.168915-3-mcanal@igaliacom
>> >> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
>> >
>> > One nit below. With that and things other folks mentioned fixed, you may add:
>> >
>> > Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
>> >
>> >> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
>> >> index 22985b6f6982..025c619a2195 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`] return pointers with alignment that is an
>> >> +/// integer multiple of [`Self::FOREIGN_ALIGN`].
>> >
>> > We should require non-null:
>>
>> What is your rationale for this?
>
> The rationale is that the implementation of XArray assumes that the
> pointers are non-null. If we allow null pointers, we will need to fix
> the XArray.

OK, thanks. Also, `try_from_foreign` as Benno highlighted.

Best regards,
Andreas Hindborg