Re: [PATCH v2] driver core: allow kobj_to_dev() to take a const pointer

[Sakari Ailus]

Hi Greg,

On Sun, Oct 16, 2022 at 12:41:26PM +0200, Greg Kroah-Hartman wrote:
> If a const * to a kobject is passed to kobj_to_dev(), we want to return
> back a const * to a device as the driver core shouldn't be modifying a
> constant structure.  But when dealing with container_of() the pointer
> const attribute is cast away, so we need to manually handle this by
> determining the type of the pointer passed in to know the type of the
> pointer to pass out.

Alternatively container_of() could be fixed, but that will likely produce
lots of warnings currently.

> 
> Luckily _Generic can do this type of magic, and as the kernel now
> supports C11 it is availble to us to handle this type of build-time type
> detection.
> 
> Cc: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
> Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> ---
> v2 - use _Generic() to make this type safe as pointed out by Sakari
> 
>  include/linux/device.h | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 424b55df0272..023ea50b1916 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -680,11 +680,27 @@ struct device_link {
>  	bool supplier_preactivated; /* Owned by consumer probe. */
>  };
>  
> -static inline struct device *kobj_to_dev(struct kobject *kobj)
> +static inline struct device *__kobj_to_dev(struct kobject *kobj)
>  {
>  	return container_of(kobj, struct device, kobj);
>  }
>  
> +static inline const struct device *__kobj_to_dev_const(const struct kobject *kobj)
> +{
> +	return container_of(kobj, const struct device, kobj);
> +}
> +
> +/*
> + * container_of() will happily take a const * and spit back a non-const * as it
> + * is just doing pointer math.  But we want to be a bit more careful in the
> + * driver code, so manually force any const * of a kobject to also be a const *
> + * to a device.
> + */

container_of() documentation has (probably?) never warned about this.

Wouldn't such a comment be more appropriate there? Albeit it wouldn't be
needed if container_of() were fixed.

> +#define kobj_to_dev(kobj)					\
> +	_Generic((kobj),					\
> +		 const struct kobject *: __kobj_to_dev_const,	\
> +		 struct kobject *: __kobj_to_dev)(kobj)
> +
>  /**
>   * device_iommu_mapped - Returns true when the device DMA is translated
>   *			 by an IOMMU

-- 
Kind regards,

Sakari Ailus