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

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Message-ID: <20250731154919.4132-3-dakr@kernel.org>
Date: Thu, 31 Jul 2025 17:48:07 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: 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,
	aliceryhl@...gle.com,
	tmgross@...ch.edu,
	maarten.lankhorst@...ux.intel.com,
	mripard@...nel.org,
	tzimmermann@...e.de,
	airlied@...il.com,
	simona@...ll.ch
Cc: rust-for-linux@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org,
	Danilo Krummrich <dakr@...nel.org>
Subject: [PATCH 2/4] rust: drm: ensure kmalloc() compatible Layout

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>
---
 rust/kernel/drm/device.rs | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index 3bb7c83966cf..d19410deaf6c 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -5,6 +5,7 @@
 //! C header: [`include/linux/drm/drm_device.h`](srctree/include/linux/drm/drm_device.h)
 
 use crate::{
+    alloc::allocator::Kmalloc,
     bindings, device, drm,
     drm::driver::AllocImpl,
     error::from_err_ptr,
@@ -12,7 +13,7 @@
     prelude::*,
     types::{ARef, AlwaysRefCounted, Opaque},
 };
-use core::{mem, ops::Deref, ptr, ptr::NonNull};
+use core::{alloc::Layout, mem, ops::Deref, ptr, ptr::NonNull};
 
 #[cfg(CONFIG_DRM_LEGACY)]
 macro_rules! drm_legacy_fields {
@@ -96,6 +97,10 @@ impl<T: drm::Driver> Device<T> {
 
     /// Create a new `drm::Device` for a `drm::Driver`.
     pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<ARef<Self>> {
+        // `__drm_dev_alloc` uses `kmalloc()` to allocate memory, hence ensure a `kmalloc()`
+        // compatible `Layout`.
+        let layout = Kmalloc::aligned_layout(Layout::new::<Self>());
+
         // SAFETY:
         // - `VTABLE`, as a `const` is pinned to the read-only section of the compilation,
         // - `dev` is valid by its type invarants,
@@ -103,7 +108,7 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<A
             bindings::__drm_dev_alloc(
                 dev.as_raw(),
                 &Self::VTABLE,
-                mem::size_of::<Self>(),
+                layout.size(),
                 mem::offset_of!(Self, dev),
             )
         }
-- 
2.50.0