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

[Alice Ryhl]

On Fri, Aug 1, 2025 at 11:29 AM Danilo Krummrich <dakr at kernel.org> wrote:
>
> 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>>().

Yes, you're right. I misunderstood how __drm_dev_alloc works.

Alice