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

[Danilo Krummrich <dakr@kernel.org>]

On Thu, May 01, 2025 at 02:33:17PM -0400, Lyude Paul wrote:
> 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`.

Is this really a requirement? I think this should just be "`ptr` must point to
a `struct drm_gem_object` represented through `Self`". How exactly the
implementer does the conversion depends on Self, no?

> @@ -144,11 +147,25 @@ fn lookup_handle(
>      ) -> Result<ARef<Self>> {
>          // SAFETY: The arguments are all valid per the type invariants.
>          let ptr = unsafe { bindings::drm_gem_object_lookup(file.as_raw().cast(), handle) };
> -        let ptr = <Self as IntoGEMObject>::from_gem_obj(ptr);
> -        let ptr = NonNull::new(ptr).ok_or(ENOENT)?;
>  
> -        // SAFETY: We take ownership of the reference of `drm_gem_object_lookup()`.
> -        Ok(unsafe { ARef::from_raw(ptr) })
> +        // SAFETY:
> +        // - A `drm::Driver` can only have a single `File` implementation.
> +        // - `file` uses the same `drm::Driver` as `Self`.
> +        // - Therefore, we're guaranteed that `ptr` must be a gem object embedded within `Self`.
> +        // - And we check if the pointer is null befoe calling as_ref(), ensuring that `ptr` is a
> +        //   valid pointer to an initialized `Self`.
> +        // XXX: The expect lint here is to workaround
> +        // https://github.com/rust-lang/rust-clippy/issues/13024
> +        #[expect(clippy::undocumented_unsafe_blocks)]
> +        let obj = (!ptr.is_null())
> +            .then(|| unsafe { Self::as_ref(ptr) })
> +            .ok_or(ENOENT)?;

Maybe simply go for

	if ptr.is_null() {
	   return Err(ENOENT);
	}

which should be much easier to parse for kernel developers just starting to look
at Rust code anyways.