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

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Jul 31, 2025 at 05:48:07PM +0200, Danilo Krummrich 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,
>   }

I'm not convinced this patch is right.

Imagine this scenario: T::Data has size and alignment both equal to 16,
and lets say that drm_device has a size that is a multiple of 8 but not
16 such as 72. In that case, you will allocate 72+16=88 bytes for
Device, but actually the size of Device is 96 because there is 8 bytes
of padding between dev and data.

Alice

> 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@kernel.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
>