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

["Björn Roy Baron" <bjorn3_gh@protonmail.com>]

On Wednesday, March 22nd, 2023 at 20:45, Björn Roy Baron <bjorn3_gh@protonmail.com> wrote:

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

Actually Opaque<dma_resv> may be better as no fields of dma_resv are used on the rust side.

> 
> > +
> >      /// 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

Cheers,
Bjorn