Re: [PATCH 2/4] rust: drm: gem: Refactor IntoGEMObject::from_gem_obj() to as_ref()

[Lyude Paul <lyude@redhat.com>]

On Fri, 2025-05-09 at 18:37 -0300, Daniel Almeida wrote:
> Hi Lyude
> 
> > On 1 May 2025, at 15:33, Lyude Paul <lyude@redhat.com> wrote:
> > 
> > There's a few issues with this function, mainly:
> > 
> > * This function -probably- should have been unsafe from the start. Pointers
> >   are not always necessarily valid, but you want a function that does
> >   field-projection for a pointer that can travel outside of the original
> >   struct to be unsafe, at least if I understand properly.
> > * *mut Self is not terribly useful in this context, the majority of uses of
> >   from_gem_obj() grab a *mut Self and then immediately convert it into a
> >   &'a Self. It also goes against the ffi conventions we've set in the rest
> >   of the kernel thus far.
> > * from_gem_obj() also doesn't follow the naming conventions in the rest of
> >   the DRM bindings at the moment, as_ref() would be a better name.
> > 
> > So, let's:
> > 
> > * Make from_gem_obj() unsafe
> > * Convert it to return &'a Self
> > * Rename it to as_ref()
> > * Update all call locations
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > ---
> > rust/kernel/drm/gem/mod.rs | 67 ++++++++++++++++++++++++--------------
> > 1 file changed, 42 insertions(+), 25 deletions(-)
> > 
> > diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
> > index df8f9fdae5c22..f70531889c21f 100644
> > --- a/rust/kernel/drm/gem/mod.rs
> > +++ b/rust/kernel/drm/gem/mod.rs
> > @@ -45,8 +45,12 @@ pub trait IntoGEMObject: Sized + super::private::Sealed {
> >      #[allow(clippy::wrong_self_convention)]
> >      fn into_gem_obj(&self) -> &Opaque<bindings::drm_gem_object>;
> > 
> > -    /// Converts a pointer to a `struct drm_gem_object` into a pointer to `Self`.
> > -    fn from_gem_obj(obj: *mut bindings::drm_gem_object) -> *mut Self;
> > +    /// Converts a pointer to a `struct drm_gem_object` into a reference to `Self`.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// `self_ptr` must be a valid pointer to `Self`.
> 
> Must also obey the reference rules. This is a bit obvious but it should
> probably be mentioned regardless.

By "reference rules" I assume that you mean lifetime rules?

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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