rust: types: add Opaque::cast_from

Since commit b20fbbc08a ("rust: check type of `$ptr` in
`container_of!`") we have enforced that the field pointer passed to
container_of! must match the declared field. This caused mismatches when
using a pointer to bindings::x for fields of type Opaque<bindings::x>.

This situation encourages the user to simply pass field.cast() to the
container_of! macro, but this is not great because you might
accidentally pass a *mut bindings::y when the field type is
Opaque<bindings::x>, which would be wrong.

To help catch this kind of mistake, add a new Opaque::cast_from that
wraps a raw pointer in Opaque without changing the inner type. Also
update the docs to reflect this as well as some existing users.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250624-opaque-from-raw-v2-1-e4da40bdc59c@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/drm/device.rs

@@ -135,11 +135,9 @@ impl<T: drm::Driver> Device<T> {
     ///
     /// `ptr` must be a valid pointer to a `struct device` embedded in `Self`.
     unsafe fn from_drm_device(ptr: *const bindings::drm_device) -> *mut Self {
-        let ptr: *const Opaque<bindings::drm_device> = ptr.cast();
-
         // SAFETY: By the safety requirements of this function `ptr` is a valid pointer to a
         // `struct drm_device` embedded in `Self`.
-        unsafe { crate::container_of!(ptr, Self, dev) }.cast_mut()
+        unsafe { crate::container_of!(Opaque::cast_from(ptr), Self, dev) }.cast_mut()
     }
 
     /// Not intended to be called externally, except via declare_drm_ioctls!()

rust/kernel/drm/gem/mod.rs

@@ -125,11 +125,9 @@ impl<T: DriverObject> IntoGEMObject for Object<T> {
     }
 
     unsafe fn as_ref<'a>(self_ptr: *mut bindings::drm_gem_object) -> &'a Self {
-        let self_ptr: *mut Opaque<bindings::drm_gem_object> = self_ptr.cast();
-
         // 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) }
+        unsafe { &*crate::container_of!(Opaque::cast_from(self_ptr), Object<T>, obj) }
     }
 }
 

rust/kernel/lib.rs

@@ -204,6 +204,13 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
 
 /// Produces a pointer to an object from a pointer to one of its fields.
 ///
+/// If you encounter a type mismatch due to the [`Opaque`] type, then use [`Opaque::raw_get`] or
+/// [`Opaque::cast_from`] to resolve the mismatch.
+///
+/// [`Opaque`]: crate::types::Opaque
+/// [`Opaque::raw_get`]: crate::types::Opaque::raw_get
+/// [`Opaque::cast_from`]: crate::types::Opaque::cast_from
+///
 /// # Safety
 ///
 /// The pointer passed to this macro, and the pointer returned by this macro, must both be in

rust/kernel/types.rs

@@ -413,6 +413,11 @@ impl<T> Opaque<T> {
     pub const fn raw_get(this: *const Self) -> *mut T {
         UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>()
     }
+
+    /// The opposite operation of [`Opaque::raw_get`].
+    pub const fn cast_from(this: *const T) -> *const Self {
+        this.cast()
+    }
 }
 
 /// Types that are _always_ reference counted.