Re: [PATCH] container_of: add type safety

Greg Kroah-Hartman
2023-03-08T17:57:23Z

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