linux-kernel - Re: [PATCH 2/4] rust: drm: ensure kmalloc() compatible Layout

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Message-ID: <CAH5fLgh6x9FYdURedJJSB1nm9n7=HA7aK3Y741P=1g7cLKEQOg@mail.gmail.com>
Date: Mon, 4 Aug 2025 16:00:48 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: lorenzo.stoakes@...cle.com, vbabka@...e.cz, Liam.Howlett@...cle.com, 
	urezki@...il.com, ojeda@...nel.org, alex.gaynor@...il.com, 
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, 
	lossin@...nel.org, a.hindborg@...nel.org, tmgross@...ch.edu, 
	maarten.lankhorst@...ux.intel.com, mripard@...nel.org, tzimmermann@...e.de, 
	airlied@...il.com, simona@...ll.ch, rust-for-linux@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] rust: drm: ensure kmalloc() compatible Layout

On Thu, Jul 31, 2025 at 5:49 PM Danilo Krummrich <dakr@...nel.org> wrote:
>
> drm::Device is allocated through __drm_dev_alloc() (which uses
> kmalloc()) and the driver private data, <T as drm::Driver>::Data, is
> initialized in-place.
>
> Due to the order of fields in drm::Device
>
>   pub struct Device<T: drm::Driver> {
>      dev: Opaque<bindings::drm_device>,
>      data: T::Data,
>   }
>
> even with an arbitrary large alignment requirement of T::Data it can't
> happen that the size of Device is smaller than its alignment requirement.
>
> However, let's not rely on this subtle circumstance and create a proper
> kmalloc() compatible Layout.
>
> Fixes: 1e4b8896c0f3 ("rust: drm: add device abstraction")
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>

Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>