On Fri, Nov 21, 2025 at 11:40 AM Bill Wendling <[email protected]> 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 <[email protected]>
> Cc: "Gustavo A. R. Silva" <[email protected]>
> Cc: Nathan Chancellor <[email protected]>
> Cc: Nick Desaulniers <[email protected]>
> Cc: Justin Stitt <[email protected]>
> Cc: Miguel Ojeda <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Heiko Carstens <[email protected]>
> Cc: Marc Herbert <[email protected]>
> Cc: Uros Bizjak <[email protected]>
> Cc: Tejun Heo <[email protected]>
> Cc: Jeff Xu <[email protected]>
> Cc: "Michal Koutný" <[email protected]>
> Cc: Shakeel Butt <[email protected]>
> Cc: "Thomas Weißschuh" <[email protected]>
> Cc: John Stultz <[email protected]>
> Cc: Christian Brauner <[email protected]>
> Cc: Randy Dunlap <[email protected]>
> Cc: Brian Gerst <[email protected]>
> Cc: Masahiro Yamada <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Signed-off-by: Bill Wendling <[email protected]>
> ---
> 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
>
