[PATCH v3 3/23] rust: drm: kms: constrain connector encoder attachment

drm_connector_attach_encoder() requires both objects to belong to the same
DRM device. The safe wrapper previously accepted any AsRawEncoder,
including an encoder from another driver or device.

Accept only an UnregisteredEncoder from the same KMS driver and reject a
different device instance before entering C.

Fixes: 322a9b8d699b ("rust: drm/kms: Add UnregisteredConnector::attach_encoder()")

Assisted-by: Claude:claude-opus-5
Signed-off-by: Mike Lothian <[email protected]>
---
 rust/kernel/drm/kms/connector.rs | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/drm/kms/connector.rs b/rust/kernel/drm/kms/connector.rs
index 78b08b94587b..b36d138ae950 100644
--- a/rust/kernel/drm/kms/connector.rs
+++ b/rust/kernel/drm/kms/connector.rs
@@ -404,11 +404,22 @@ pub fn new<'a>(
 
     /// Attach an encoder to this [`Connector`].
     #[must_use]
-    pub fn attach_encoder(&self, encoder: &impl AsRawEncoder) -> Result {
+    pub fn attach_encoder<E>(&self, encoder: &UnregisteredEncoder<E>) -> Result
+    where
+        E: DriverEncoder<Driver = T::Driver>,
+    {
+        // SAFETY: Both unregistered objects have been initialized, so their parent device
+        // pointers are valid and invariant for their lifetimes.
+        let same_device = unsafe { (*self.as_raw()).dev == (*encoder.as_raw()).dev };
+        if !same_device {
+            return Err(EINVAL);
+        }
+
         // SAFETY:
-        // - Both as_raw() calls are guaranteed to return a valid pointer
-        // - We're guaranteed this connector is not registered via our type invariants, thus this
-        //   function is safe to call
+        // - Both `as_raw()` calls return valid pointers.
+        // - The generic bound and check above prove that both objects belong to the same driver
+        //   and device.
+        // - `self` is unregistered, as required by the C API.
         to_result(unsafe {
             bindings::drm_connector_attach_encoder(self.as_raw(), encoder.as_raw())
         })