Message-Id: <036A1696-C113-4C71-93AD-D5EA92EA3552@collabora.com>
Date: Fri, 9 May 2025 18:37:20 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Lyude Paul <lyude@...hat.com>
Cc: dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>,
 Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>,
 Danilo Krummrich <dakr@...nel.org>,
 Asahi Lina <lina@...hilina.net>,
 Alyssa Rosenzweig <alyssa@...enzweig.io>
Subject: Re: [PATCH 2/4] rust: drm: gem: Refactor
 IntoGEMObject::from_gem_obj() to as_ref()

Hi Lyude

> On 1 May 2025, at 15:33, Lyude Paul <lyude@...hat.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@...hat.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.

> +    unsafe fn as_ref<'a>(self_ptr: *mut bindings::drm_gem_object) -> &'a Self;
> }
> 
> /// Trait which must be implemented by drivers using base GEM objects.
> @@ -63,14 +67,13 @@ extern "C" fn open_callback<T: BaseDriverObject<U>, U: BaseObject>(
>     let file = unsafe {
>         drm::File::<<<U as IntoGEMObject>::Driver as drm::Driver>::File>::as_ref(raw_file)
>     };
> -    let obj =
> -        <<<U as IntoGEMObject>::Driver as drm::Driver>::Object as IntoGEMObject>::from_gem_obj(
> -            raw_obj,
> -        );
> -
> -    // SAFETY: `from_gem_obj()` returns a valid pointer as long as the type is correct and the
> -    // `raw_obj` we got is valid.
> -    match T::open(unsafe { &*obj }, file) {
> +    // SAFETY: `open_callback` is specified in the AllocOps structure for `Object<T>`, ensuring that
> +    // `raw_obj` is indeed contained within a `Object<T>`.
> +    let obj = unsafe {
> +        <<<U as IntoGEMObject>::Driver as drm::Driver>::Object as IntoGEMObject>::as_ref(raw_obj)
> +    };

Ugh..IMHO we need to have aliases for all of these. This is, of course,
orthogonal to your patch. Just a nice-to-have for the future :)

> +
> +    match T::open(obj, file) {
>         Err(e) => e.to_errno(),
>         Ok(()) => 0,
>     }
> @@ -84,14 +87,13 @@ extern "C" fn close_callback<T: BaseDriverObject<U>, U: BaseObject>(
>     let file = unsafe {
>         drm::File::<<<U as IntoGEMObject>::Driver as drm::Driver>::File>::as_ref(raw_file)
>     };
> -    let obj =
> -        <<<U as IntoGEMObject>::Driver as drm::Driver>::Object as IntoGEMObject>::from_gem_obj(
> -            raw_obj,
> -        );
> -
> -    // SAFETY: `from_gem_obj()` returns a valid pointer as long as the type is correct and the
> -    // `raw_obj` we got is valid.
> -    T::close(unsafe { &*obj }, file);
> +    // SAFETY: `close_callback` is specified in the AllocOps structure for `Object<T>`, ensuring
> +    // that `raw_obj` is indeed contained within a `Object<T>`.
> +    let obj = unsafe {
> +        <<<U as IntoGEMObject>::Driver as drm::Driver>::Object as IntoGEMObject>::as_ref(raw_obj)
> +    };
> +
> +    T::close(obj, file);
> }
> 
> impl<T: DriverObject> IntoGEMObject for Object<T> {
> @@ -101,9 +103,10 @@ fn into_gem_obj(&self) -> &Opaque<bindings::drm_gem_object> {
>         &self.obj
>     }
> 
> -    fn from_gem_obj(obj: *mut bindings::drm_gem_object) -> *mut Self {
> -        // SAFETY: All of our objects are Object<T>.
> -        unsafe { crate::container_of!(obj, Object<T>, obj).cast_mut() }
> +    unsafe fn as_ref<'a>(self_ptr: *mut bindings::drm_gem_object) -> &'a Self {
> +        // SAFETY: `obj` is guaranteed to be in an `Object<T>` via the safety contract of this
> +        // function
> +        unsafe { &*crate::container_of!(self_ptr, Object<T>, obj) }
>     }
> }
> 
> @@ -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)?;
> +
> +        // SAFETY:
> +        // - We take ownership of the reference of `drm_gem_object_lookup()`.
> +        // - Our `NonNull` comes from an immutable reference, thus ensuring it is a valid pointer to
> +        //   `Self`.
> +        Ok(unsafe { ARef::from_raw(obj.into()) })
>     }
> 
>     /// Creates an mmap offset to map the object from userspace.
> -- 
> 2.48.1
> 
> 

With the extra safety requirement,

Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>

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