Re: [PATCH v2] Compiler Attributes: Add __counted_by macro

[Nathan Chancellor <nathan@kernel.org>]

On Wed, May 17, 2023 at 12:08:44PM -0700, Kees Cook wrote:
> In an effort to annotate all flexible array members with their run-time
> size information, the "element_count" attribute is being introduced by
> Clang[1] and GCC[2] in future releases. This annotation will provide
> the CONFIG_UBSAN_BOUNDS and CONFIG_FORTIFY_SOURCE features the ability
> to perform run-time bounds checking on otherwise unknown-size flexible
> arrays.
> 
> Even though the attribute is under development, we can start the
> annotation process in the kernel. This requires defining a macro for
> it, even if we have to change the name of the actual attribute later.
> Since it is likely that this attribute may change its name to "counted_by"
> in the future (to better align with a future total bytes "sized_by"
> attribute), name the wrapper macro "__counted_by", which also reads more
> clearly (and concisely) in structure definitions.
> 
> [1] https://reviews.llvm.org/D148381
> [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
> 
> Cc: Miguel Ojeda <ojeda@kernel.org>
> Cc: Bill Wendling <morbo@google.com>
> Cc: Qing Zhao <qing.zhao@oracle.com>
> Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Tom Rix <trix@redhat.com>
> Cc: llvm@lists.linux.dev
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

> ---
> v2: - update "Optional" comments
> v1: https://lore.kernel.org/all/20230504181636.never.222-kees@kernel.org/
> ---
>  include/linux/compiler_attributes.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index e659cb6fded3..a92d8887e8f0 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -123,6 +123,19 @@
>  # define __designated_init
>  #endif
>  
> +/*
> + * Optional: only supported since gcc >= 14
> + * Optional: only supported since clang >= 17
> + *
> + *   gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
> + * clang: https://reviews.llvm.org/D148381
> + */
> +#if __has_attribute(__element_count__)
> +# define __counted_by(member)		__attribute__((__element_count__(member)))
> +#else
> +# define __counted_by(member)
> +#endif
> +
>  /*
>   * Optional: only supported since clang >= 14.0
>   *
> -- 
> 2.34.1
>