Re: [PATCH] container_of: add type safety

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Mar 08, 2023 at 12:35:03PM -0500, Michael S. Tsirkin wrote:
> Using a wrong member in container_of will result in an error.
> No so for container_of_const - it is just a cast so will
> happily give you a wrong pointer.
> 
> Use logic from container_of to add safety.
> 
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: Jason Gunthorpe <jgg@nvidia.com>
> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
> Cc: Rafael J. Wysocki <rafael@kernel.org>
> Link: https://lore.kernel.org/r/20221205121206.166576-1-gregkh@linuxfoundation.org

That's the wrong link, that's not this patch, that was an old patch.

> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

I did not sign off on this.  NEVER do that, you just made a legal
statement in my name, why?

Why did you not sign off on it?

totally confused...

> ---
>  include/linux/container_of.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> index 1d898f9158b4..5d87faf72e0a 100644
> --- a/include/linux/container_of.h
> +++ b/include/linux/container_of.h
> @@ -29,10 +29,13 @@
>   * @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, member)				\
> +#define container_of_const(ptr, type, member) ({			\
> +	static_assert(__same_type(*(ptr), ((type *)0)->member) ||	\
> +		      __same_type(*(ptr), void),			\
> +		      "pointer type mismatch in container_of()");	\

Why is this needed because:


>  	_Generic(ptr,							\
>  		const typeof(*(ptr)) *: ((const type *)container_of(ptr, type, member)),\
>  		default: ((type *)container_of(ptr, type, member))	\

container_of() is used here, so shouldn't the assert trigger there if
you get things wrong?

So why is this change needed at all?

thanks,

greg k-h