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

["Danilo Krummrich" <dakr@kernel.org>]

On Fri Aug 1, 2025 at 11:18 AM CEST, Alice Ryhl wrote:
> 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.

Are you saying that there is an issue with

  (1) the existing implementation with uses mem::size_of::<Self>() or

  (2) the proper one that uses Kmalloc::aligned_layout(Layout::new::<Self>())?

I think neither has, because we're not allocating
size_of::<Opaque<bindings::drm_device>>() + size_of::<T::Data>() as you seem to
assume above, but size_of::<Device<T>>().

>> 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
>>