linux-kernel - [PATCH 4/4] rust: drm: don't pass the address of drm::Device to drm_dev

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Message-ID: <20250731154919.4132-5-dakr@kernel.org>
Date: Thu, 31 Jul 2025 17:48:09 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: lorenzo.stoakes@...cle.com,
	vbabka@...e.cz,
	Liam.Howlett@...cle.com,
	urezki@...il.com,
	ojeda@...nel.org,
	alex.gaynor@...il.com,
	boqun.feng@...il.com,
	gary@...yguo.net,
	bjorn3_gh@...tonmail.com,
	lossin@...nel.org,
	a.hindborg@...nel.org,
	aliceryhl@...gle.com,
	tmgross@...ch.edu,
	maarten.lankhorst@...ux.intel.com,
	mripard@...nel.org,
	tzimmermann@...e.de,
	airlied@...il.com,
	simona@...ll.ch
Cc: rust-for-linux@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org,
	Danilo Krummrich <dakr@...nel.org>
Subject: [PATCH 4/4] rust: drm: don't pass the address of drm::Device to drm_dev_put()

In drm_dev_put() call in AlwaysRefCounted::dec_ref() we rely on struct
drm_device to be the first field in drm::Device, whereas everywhere
else we correctly obtain the address of the actual struct drm_device.

Analogous to the from_drm_device() helper, provide the
into_drm_device() helper in order to address this.

Fixes: 1e4b8896c0f3 ("rust: drm: add device abstraction")
Signed-off-by: Danilo Krummrich <dakr@...nel.org>
---
 rust/kernel/drm/device.rs | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index d0a9528121f1..d29c477e89a8 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -120,9 +120,13 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<A
         // - `raw_data` is a valid pointer to uninitialized memory.
         // - `raw_data` will not move until it is dropped.
         unsafe { data.__pinned_init(raw_data) }.inspect_err(|_| {
-            // SAFETY: `__drm_dev_alloc()` was successful, hence `raw_drm` must be valid and the
+            // SAFETY: `raw_drm` is a valid pointer to `Self`, given that `__drm_dev_alloc` was
+            // successful.
+            let drm_dev = unsafe { Self::into_drm_device(raw_drm) };
+
+            // SAFETY: `__drm_dev_alloc()` was successful, hence `drm_dev` must be valid and the
             // refcount must be non-zero.
-            unsafe { bindings::drm_dev_put(ptr::addr_of_mut!((*raw_drm.as_ptr()).dev).cast()) };
+            unsafe { bindings::drm_dev_put(drm_dev) };
         })?;
 
         // SAFETY: The reference count is one, and now we take ownership of that reference as a
@@ -143,6 +147,14 @@ unsafe fn from_drm_device(ptr: *const bindings::drm_device) -> *mut Self {
         unsafe { crate::container_of!(Opaque::cast_from(ptr), Self, dev) }.cast_mut()
     }
 
+    /// # Safety
+    ///
+    /// `ptr` must be a valid pointer to `Self`.
+    unsafe fn into_drm_device(ptr: NonNull<Self>) -> *mut bindings::drm_device {
+        // SAFETY: By the safety requirements of this function, `ptr` is a valid pointer to `Self`.
+        unsafe { &raw mut (*ptr.as_ptr()).dev }.cast()
+    }
+
     /// Not intended to be called externally, except via declare_drm_ioctls!()
     ///
     /// # Safety
@@ -192,8 +204,11 @@ fn inc_ref(&self) {
     }
 
     unsafe fn dec_ref(obj: NonNull<Self>) {
+        // SAFETY: `obj` is a valid pointer to `Self`.
+        let drm_dev = unsafe { Self::into_drm_device(obj) };
+
         // SAFETY: The safety requirements guarantee that the refcount is non-zero.
-        unsafe { bindings::drm_dev_put(obj.cast().as_ptr()) };
+        unsafe { bindings::drm_dev_put(drm_dev) };
     }
 }
 
-- 
2.50.0