Re: [PATCH] iommufd/selftest: Make it clearer to gcc that the access is not out of bounds

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Dec 05, 2025 at 08:39:59PM -0400, Jason Gunthorpe wrote:
> GCC gets a bit confused and reports:
> 
>    In function '_test_cmd_get_hw_info',
>        inlined from 'iommufd_ioas_get_hw_info' at iommufd.c:779:3,
>        inlined from 'wrapper_iommufd_ioas_get_hw_info' at iommufd.c:752:1:
> >> iommufd_utils.h:804:37: warning: array subscript 'struct iommu_test_hw_info[0]' is partly outside array bounds of 'struct iommu_test_hw_info_buffer_smaller[1]' [-Warray-bounds=]
>      804 |                         assert(!info->flags);
>          |                                 ~~~~^~~~~~~
>    iommufd.c: In function 'wrapper_iommufd_ioas_get_hw_info':
>    iommufd.c:761:11: note: object 'buffer_smaller' of size 4
>      761 |         } buffer_smaller;
>          |           ^~~~~~~~~~~~~~
> 
> While it is true that "struct iommu_test_hw_info[0]" is partly out of
> bounds of the input pointer, it is not true that info->flags is out of
> bounds. Unclear why it warns on this.
> 
> Reuse an existing properly sized stack buffer and pass a truncated length
> instead to test the same thing.
> 
> Fixes: af4fde93c319 ("iommufd/selftest: Add coverage for IOMMU_GET_HW_INFO ioctl")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202512032344.kaAcKFIM-lkp@intel.com/
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
 
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>