[RFC PATCH 2/9] rust: drm: gem: add method to return DmaResv from GEMObject

[Björn Roy Baron]

On Friday, March 17th, 2023 at 13:12, Maíra Canal <mcanal at igalia.com> wrote:

> A GEMObject, related to struct drm_gem_object, holds a pointer
> to reservation object associated with the this GEM object. Therefore,
> expose this reservation object through the DmaResv safe abstraction.
> 
> Signed-off-by: Maíra Canal <mcanal at igalia.com>
> ---
>  rust/kernel/drm/gem/mod.rs | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
> index c4a42bb2f718..dae95e5748a7 100644
> --- a/rust/kernel/drm/gem/mod.rs
> +++ b/rust/kernel/drm/gem/mod.rs
> @@ -11,6 +11,7 @@ use alloc::boxed::Box;
> 
>  use crate::{
>      bindings,
> +    dma_resv::DmaResv,
>      drm::{device, drv, file},
>      error::{to_result, Result},
>      prelude::*,
> @@ -136,6 +137,12 @@ pub trait BaseObject: IntoGEMObject {
>          self.gem_obj().size
>      }
> 
> +    /// Returns the pointer to reservation object associated with this GEM object.
> +    fn resv(&self) -> DmaResv {
> +        // SAFETY: Every GEM object holds a reference to a reservation object
> +        unsafe { DmaResv::from_raw(self.gem_obj().resv) }
> +    }

There is nothing ensuring that DmaResv doesn't outlive self and thus may have been deallocated. You could change DmaResv to be a newtype around UnsafeCell<dma_resv> and then return an &DmaResv here. This way &DmaResv can't outlive &self.

> +
>      /// Sets the exportable flag, which controls whether the object can be exported via PRIME.
>      fn set_exportable(&mut self, exportable: bool) {
>          self.mut_gem_obj().exportable = exportable;
> --
> 2.39.2

Cheers,
Bjorn