linux-kernel - Re: [PATCH] rust: types: add Opaque::from

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Message-ID: <aFFzi88miMbCZ0yQ@pollux>
Date: Tue, 17 Jun 2025 15:54:19 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>, dri-devel@...ts.freedesktop.org,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: types: add Opaque::from_raw

On Tue, Jun 17, 2025 at 01:36:47PM +0000, Alice Ryhl wrote:
> Since commit b20fbbc08a36 ("rust: check type of `$ptr` in
> `container_of!`") we have enforced that the field pointer passed to
> container_of! must match the declared field. This caused mismatches when
> using a pointer to bindings::x for fields of type Opaque<bindings::x>.
> 
> This situation encourages the user to simply pass field.cast() to the
> container_of! macro, but this is not great because you might
> accidentally pass a *mut bindings::y when the field type is
> Opaque<bindings::x>, which would be wrong.
> 
> To help catch this kind of mistake, add a new Opaque::from_raw that
> wraps a raw pointer in Opaque without changing the inner type.

The patch does more than that, it also adds a hint to container_of!() and fixes
up two occurences. I feel like we should split it up.

> +    /// The opposite operation of [`Opaque::raw_get`].
> +    pub const fn from_raw(this: *const T) -> *const Self {

Do we want to name this from_raw()? Usually from_raw() methods return either
Self or &'a Self.

Maybe something like cast_from() and rename raw_get() to cast_into()? I think
the latter may be confusing anyways, since it sounds like it would do somthing
with reference counting.

> +        this.cast()
> +    }