Re: [PATCH 1/2] Compiler Attributes: Add __counted_by_ptr macro

[Bill Wendling]

On Fri, Nov 21, 2025 at 11:40 AM Bill Wendling <morbo@xxxxxxxxxx> wrote:
>
> Clang and GCC are expanding the '__counted_by' attribute to support
> pointers in structs. Clang has support for it since version 21. This
> requires defining a separate macro, '__counted_by_ptr', because, while
> the attribute has the same name for both a pointer and a flexible array
> member, minimal compiler versions need to catch up.
>
> The effect of this feature is the same as for __counted_by on flexible
> array members. It provides hardening the ability to perform run-time
> bounds checking on otherwise unknown-size pointers.
>
> Cc: Kees Cook <kees@xxxxxxxxxx>
> Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
> Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> Cc: Nick Desaulniers <nick.desaulniers+lkml@xxxxxxxxx>
> Cc: Justin Stitt <justinstitt@xxxxxxxxxx>
> Cc: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Heiko Carstens <hca@xxxxxxxxxxxxx>
> Cc: Marc Herbert <Marc.Herbert@xxxxxxxxxxxxxxx>
> Cc: Uros Bizjak <ubizjak@xxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Jeff Xu <jeffxu@xxxxxxxxxxxx>
> Cc: "Michal Koutný" <mkoutny@xxxxxxxx>
> Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Cc: "Thomas Weißschuh" <thomas.weissschuh@xxxxxxxxxxxxx>
> Cc: John Stultz <jstultz@xxxxxxxxxx>
> Cc: Christian Brauner <brauner@xxxxxxxxxx>
> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Cc: Brian Gerst <brgerst@xxxxxxxxx>
> Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-hardening@xxxxxxxxxxxxxxx
> Cc: llvm@xxxxxxxxxxxxxxx
> Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx>
> ---
>  include/linux/compiler_types.h | 11 +++++++++++
>  init/Kconfig                   |  5 +++++
>  2 files changed, 16 insertions(+)
>
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 0a1b9598940d..2b0251bb951c 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -351,6 +351,17 @@ struct ftrace_likely_data {
>  # define __assume(expr)
>  #endif
>
> +/*
> + * Optional: only supported since clang >= 21
> + *
> + * clang: https://github.com/llvm/llvm-project/pull/137250
> + */
> +#ifdef CONFIG_CC_HAS_COUNTED_BY_FOR_POINTER
> +#define __counted_by_ptr(member)       __attribute__((__counted_by__(member)))
> +#else
> +#define __counted_by_ptr(member)
> +#endif
> +
>  /*
>   * Optional: only supported since gcc >= 15
>   * Optional: only supported since clang >= 18
> diff --git a/init/Kconfig b/init/Kconfig
> index cab3ad28ca49..298c94c4c1b1 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -139,6 +139,11 @@ config CC_HAS_COUNTED_BY
>         # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
>         default y if CC_IS_GCC && GCC_VERSION >= 150100
>
> +config CC_HAS_COUNTED_BY_ON_POINTERS
> +       bool
> +       # Needs clang 21.1.0 or higher.
> +       default y if CC_IS_CLANG && CLANG_VERSION >= 210100
> +
I mistakenly left out GCC from here. I'll roll that in with v2.

-bw

>  config CC_HAS_MULTIDIMENSIONAL_NONSTRING
>         def_bool $(success,echo 'char tag[][4] __attribute__((__nonstring__)) = { };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)
>
> --
> 2.52.0.rc2.455.g230fcf2819-goog
>