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rust: drm: ensure kmalloc() compatible Layout

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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: 1e4b8896c0 ("rust: drm: add device abstraction")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250731154919.4132-3-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
This commit is contained in:
Danilo Krummrich 2025-07-31 17:48:07 +02:00
parent fde578c862
commit 22ab0641b9
1 changed files with 7 additions and 2 deletions
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9
rust/kernel/drm/device.rs
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@ -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 @@ use crate::{
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 @@ impl<T: drm::Driver> Device<T> {
bindings::__drm_dev_alloc(
dev.as_raw(),
&Self::VTABLE,
mem::size_of::<Self>(),
layout.size(),
mem::offset_of!(Self, dev),
)
}