Re: [PATCH 1/4] container_of: add container_of_const() that preserves const-ness of the pointer

[Sakari Ailus]

Hi Greg,

On Thu, Dec 01, 2022 at 08:30:54PM +0100, Greg Kroah-Hartman wrote:
> container_of does not preserve the const-ness of a pointer that is
> passed into it, which can cause C code that passes in a const pointer to
> get a pointer back that is not const and then scribble all over the data
> in it.  To prevent this, container_of_const() will preserve the const
> status of the pointer passed into it using the newly available _Generic()
> method.
> 
> Co-developed-by: Jason Gunthorpe <jgg@xxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> 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>
> ---
>  include/linux/container_of.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> index 2008e9f4058c..3c290e865151 100644
> --- a/include/linux/container_of.h
> +++ b/include/linux/container_of.h
> @@ -22,4 +22,18 @@
>  		      "pointer type mismatch in container_of()");	\
>  	((type *)(__mptr - offsetof(type, member))); })
>  
> +/**
> + * container_of_const - cast a member of a structure out to the containing
> + *			structure and preserve the const-ness of the pointer
> + * @ptr_type:		the type of the pointer @ptr
> + * @ptr:		the pointer to the member
> + * @member_type:	the type of the container struct this is embedded in.
> + * @member:		the name of the member within the struct.
> + */
> +#define container_of_const(ptr_type, ptr, member_type, member)		\

I missed earlier you had four arguments for the macro instead of three.

With default: this can be done with just three:

#define container_of_const(ptr, member_type, member)		\
	_Generic(ptr,							\
		 const typeof(*(ptr)) *: ((const member_type *)container_of(ptr, member_type, member)),\
		 default: ((member_type *)container_of(ptr, member_type, member))	\
	)

The const typeof(*(ptr)) * will match if ptr is const, otherwise default
matches. But you can't have typeof(*(ptr)) * instead of default as the two
types are still the same, hence default.

I've tested this on GCC 10.2.1 and clang 11.0.1.

This should also make it a bit easier for existing users to switch to the
new macro and hopefully eventual rename back to container_of() once all
users have been converted.

> +	_Generic(ptr,							\
> +		const ptr_type *: ((const member_type *)container_of(ptr, member_type, member)),\
> +		ptr_type *: ((member_type *)container_of(ptr, member_type, member))	\
> +	)
> +
>  #endif	/* _LINUX_CONTAINER_OF_H */

-- 
Kind regards,

Sakari Ailus