Re: [PATCH v2][next] container_of: add container_first() macro

[Dan Carpenter <dan.carpenter@linaro.org>]

On Wed, Jan 29, 2025 at 10:29:24AM +0300, Dan Carpenter wrote:
> drivers/iommu/iommufd/iommufd_private.h
>    243  static inline struct iommufd_ioas *iommufd_get_ioas(struct iommufd_ctx *ictx,
>    244                                                      u32 id)
>    245  {
>    246          return container_of(iommufd_get_object(ictx, id,
>    247                                                 IOMMUFD_OBJ_IOAS),
>    248                              struct iommufd_ioas, obj);
>    249  }
> 
> It's just a cast like you say, but it looks like pointer math.  It would
> be more readable as container_of_first().

I left out the important bit...  iommufd_get_object() returns error pointers.
It doesn't make sense to pass error pointers to container_of() unless the
offset is zero.

Smatch could check that the offset is zero, but Coccinelle can't.  There are
a bunch of advantages to Coccinelle at times so this will improve the
readability and make it easier for static checkers as well.

regards,
dan carpenter