[PATCH 2/7] rust: drm: gem: gate open/close callbacks with RegistrationGuard

Wrap the GEM object open and close callbacks with a RegistrationGuard
(drm_dev_enter / drm_dev_exit) to ensure the driver callbacks only run
while the parent bus device is bound.

If the device has been unbound, open returns -ENODEV and close silently
returns. This prevents driver code from accessing device resources after
unbind and is a prerequisite for making drm::Driver::File
lifetime-parameterized, since GEM callbacks receive a &drm::File that
could otherwise be used to access invalidated file private data.

Signed-off-by: Danilo Krummrich <[email protected]>
---
 rust/kernel/drm/gem/mod.rs | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
index 80d8f524f9d5..560403ca8e38 100644
--- a/rust/kernel/drm/gem/mod.rs
+++ b/rust/kernel/drm/gem/mod.rs
@@ -128,6 +128,14 @@ extern "C" fn open_callback<T: DriverObject>(
     raw_obj: *mut bindings::drm_gem_object,
     raw_file: *mut bindings::drm_file,
 ) -> core::ffi::c_int {
+    // SAFETY: `raw_obj` is a valid pointer to a `struct drm_gem_object` with a valid `dev`.
+    let dev: &drm::Device<T::Driver, drm::Userspace> =
+        unsafe { drm::Device::from_raw((*raw_obj).dev) };
+
+    let Some(_guard) = dev.registration_guard() else {
+        return ENODEV.to_errno();
+    };
+
     // SAFETY: `open_callback` is only ever called with a valid pointer to a `struct drm_file`.
     let file = unsafe { DriverFile::<T>::from_raw(raw_file) };
 
@@ -148,6 +156,14 @@ extern "C" fn close_callback<T: DriverObject>(
     raw_obj: *mut bindings::drm_gem_object,
     raw_file: *mut bindings::drm_file,
 ) {
+    // SAFETY: `raw_obj` is a valid pointer to a `struct drm_gem_object` with a valid `dev`.
+    let dev: &drm::Device<T::Driver, drm::Userspace> =
+        unsafe { drm::Device::from_raw((*raw_obj).dev) };
+
+    let Some(_guard) = dev.registration_guard() else {
+        return;
+    };
+
     // SAFETY: `open_callback` is only ever called with a valid pointer to a `struct drm_file`.
     let file = unsafe { DriverFile::<T>::from_raw(raw_file) };
 
-- 
2.55.0