Re: [PATCH v2 1/4] container_of: add container_of_const() that preserves const-ness of the pointer

[Rafael J. Wysocki]

On Mon, Dec 5, 2022 at 1:12 PM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> 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.
>
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxx>
> Suggested-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Acked-by: Rafael J. Wysocki <rafael@xxxxxxxxxx>

> ---
> v2: - removed one parameter, now matches container_of(), thanks to
>       suggestion from Sakari
>     - changed Jason's tag to suggested-by and reviewed-by
>     - added Andy's tag
>
>  include/linux/container_of.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> index 2008e9f4058c..1d898f9158b4 100644
> --- a/include/linux/container_of.h
> +++ b/include/linux/container_of.h
> @@ -22,4 +22,17 @@
>                       "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:               the pointer to the 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, member)                          \
> +       _Generic(ptr,                                                   \
> +               const typeof(*(ptr)) *: ((const type *)container_of(ptr, type, member)),\
> +               default: ((type *)container_of(ptr, type, member))      \
> +       )
> +
>  #endif /* _LINUX_CONTAINER_OF_H */
> --
> 2.38.1
>