[PATCH 4/6] drm: Change drm_class from pointer to const struct class

[Dmitry Baryshkov]

On Sun, Sep 08, 2024 at 02:11:25PM GMT, Heiner Kallweit wrote:
> Define class drm statically and constify it. This ensure that no user
> of the exported struct class can tamper with it.
> 
> Signed-off-by: Heiner Kallweit <hkallweit1 at gmail.com>
> ---
>  drivers/gpu/drm/drm_internal.h       |  2 +-
>  drivers/gpu/drm/drm_privacy_screen.c |  2 +-
>  drivers/gpu/drm/drm_sysfs.c          | 32 ++++++++++++++--------------
>  3 files changed, 18 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
> index 1705bfc90..6e0df44b6 100644
> --- a/drivers/gpu/drm/drm_internal.h
> +++ b/drivers/gpu/drm/drm_internal.h
> @@ -139,7 +139,7 @@ bool drm_master_internal_acquire(struct drm_device *dev);
>  void drm_master_internal_release(struct drm_device *dev);
>  
>  /* drm_sysfs.c */
> -extern struct class *drm_class;
> +extern const struct class drm_class;
>  
>  int drm_sysfs_init(void);
>  void drm_sysfs_destroy(void);
> diff --git a/drivers/gpu/drm/drm_privacy_screen.c b/drivers/gpu/drm/drm_privacy_screen.c
> index 6cc39e307..2fbd24ba5 100644
> --- a/drivers/gpu/drm/drm_privacy_screen.c
> +++ b/drivers/gpu/drm/drm_privacy_screen.c
> @@ -401,7 +401,7 @@ struct drm_privacy_screen *drm_privacy_screen_register(
>  	mutex_init(&priv->lock);
>  	BLOCKING_INIT_NOTIFIER_HEAD(&priv->notifier_head);
>  
> -	priv->dev.class = drm_class;
> +	priv->dev.class = &drm_class;
>  	priv->dev.type = &drm_privacy_screen_type;
>  	priv->dev.parent = parent;
>  	priv->dev.release = drm_privacy_screen_device_release;
> diff --git a/drivers/gpu/drm/drm_sysfs.c b/drivers/gpu/drm/drm_sysfs.c
> index f8577043e..f443f9a76 100644
> --- a/drivers/gpu/drm/drm_sysfs.c
> +++ b/drivers/gpu/drm/drm_sysfs.c
> @@ -58,7 +58,15 @@ static struct device_type drm_sysfs_device_connector = {
>  	.name = "drm_connector",
>  };
>  
> -struct class *drm_class;
> +static char *drm_devnode(const struct device *dev, umode_t *mode)
> +{
> +	return kasprintf(GFP_KERNEL, "dri/%s", dev_name(dev));
> +}
> +
> +const struct class drm_class = {
> +	.name = "drm",
> +	.devnode = drm_devnode,
> +};
>  
>  #ifdef CONFIG_ACPI
>  static bool drm_connector_acpi_bus_match(struct device *dev)
> @@ -93,11 +101,6 @@ static void drm_sysfs_acpi_register(void) { }
>  static void drm_sysfs_acpi_unregister(void) { }
>  #endif
>  
> -static char *drm_devnode(const struct device *dev, umode_t *mode)
> -{
> -	return kasprintf(GFP_KERNEL, "dri/%s", dev_name(dev));
> -}
> -

Please keep this function in place and move drm_class declarattion next
to it. It simplifies reviewing the code.

>  static int typec_connector_bind(struct device *dev,
>  				struct device *typec_connector, void *data)
>  {
> @@ -138,14 +141,12 @@ static const struct component_ops typec_connector_ops = {
>   */
>  int drm_sysfs_init(void)
>  {
> -	drm_class = class_create("drm");
> -	if (IS_ERR(drm_class))
> -		return PTR_ERR(drm_class);
> +	int ret = class_register(&drm_class);
>  
> -	drm_class->devnode = drm_devnode;
> +	if (!ret)
> +		drm_sysfs_acpi_register();
>  
> -	drm_sysfs_acpi_register();
> -	return 0;
> +	return ret;
>  }
>  
>  /**
> @@ -156,8 +157,7 @@ int drm_sysfs_init(void)
>  void drm_sysfs_destroy(void)
>  {
>  	drm_sysfs_acpi_unregister();
> -	class_destroy(drm_class);
> -	drm_class = NULL;
> +	class_unregister(&drm_class);

This code makes me wonder: can we define static classes in unloadable
modules? What happens if userspace holds the reference on the class in
sysfs, while we remove the module ?

>  }
>  
>  static void drm_sysfs_release(struct device *dev)
> @@ -337,7 +337,7 @@ int drm_sysfs_connector_add(struct drm_connector *connector)
>  		return -ENOMEM;
>  
>  	device_initialize(kdev);
> -	kdev->class = drm_class;
> +	kdev->class = &drm_class;
>  	kdev->type = &drm_sysfs_device_connector;
>  	kdev->parent = dev->primary->kdev;
>  	kdev->groups = connector_dev_groups;
> @@ -516,7 +516,7 @@ struct device *drm_sysfs_minor_alloc(struct drm_minor *minor)
>  			minor_str = "card%d";
>  
>  		kdev->devt = MKDEV(DRM_MAJOR, minor->index);
> -		kdev->class = drm_class;
> +		kdev->class = &drm_class;
>  		kdev->type = &drm_sysfs_device_minor;
>  	}
>  
> -- 
> 2.46.0
> 
> 

-- 
With best wishes
Dmitry