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Message-ID: <CAGG=3QV7-W5vEPNtABghg=ktn6kKs45_e-PWBT9oCUzURod1bg@mail.gmail.com>
Date: Mon, 20 Oct 2025 15:53:55 -0700
From: Bill Wendling <morbo@...gle.com>
To: Kees Cook <kees@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Justin Stitt <justinstitt@...gle.com>,
Peter Zijlstra <peterz@...radead.org>, Marco Elver <elver@...gle.com>,
Przemek Kitszel <przemyslaw.kitszel@...el.com>, Andrew Morton <akpm@...ux-foundation.org>,
Masahiro Yamada <masahiroy@...nel.org>, Christophe Leroy <christophe.leroy@...roup.eu>,
Johannes Weiner <hannes@...xchg.org>, llvm@...ts.linux.dev,
Al Viro <viro@...iv.linux.org.uk>, Arnd Bergmann <arnd@...db.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
Nicolas Schier <nicolas.schier@...ux.dev>, Shuah Khan <shuah@...nel.org>,
"Gustavo A. R. Silva" <gustavoars@...nel.org>,
Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
Tamir Duberstein <tamird@...il.com>, Michael Kelley <mhklinux@...look.com>,
kernel test robot <lkp@...el.com>, Heiko Carstens <hca@...ux.ibm.com>, Uros Bizjak <ubizjak@...il.com>,
Jan Hendrik Farr <kernel@...rr.cc>, Yafang Shao <laoar.shao@...il.com>,
Marc Herbert <Marc.Herbert@...ux.intel.com>, Christopher Ferris <cferris@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Alexander Lobakin <aleksander.lobakin@...el.com>,
Paolo Abeni <pabeni@...hat.com>, Tejun Heo <tj@...nel.org>, Jeff Xu <jeffxu@...omium.org>,
Michal Koutný <mkoutny@...e.com>,
Shakeel Butt <shakeel.butt@...ux.dev>, Randy Dunlap <rdunlap@...radead.org>,
Brian Gerst <brgerst@...il.com>, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-kbuild@...r.kernel.org,
linux-kselftest@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/3] compiler_types: Introduce __counted_by_ptr()
On Mon, Oct 20, 2025 at 3:01 PM Kees Cook <kees@...nel.org> wrote:
>
> Introduce __counted_by_ptr(), which works like __counted_by(), but for
> pointer struct members:
>
> struct foo {
> int a, b, c;
> char *buffer __counted_by_ptr(bytes);
> short nr_bars;
> struct bar *bars __counted_by_ptr(nr_bars);
> size_t bytes;
> };
>
> Since "counted_by" can only be applied to pointer members in very recent
> compiler versions, its application ends up needing to be distinct from
> flexible array "counted_by" annotations, hence a separate macro.
>
> Unfortunately, this annotation cannot be used for "void *" members
> (since such a member is considered a pointer to an incomplete type,
> and neither Clang nor GCC developers could be convinced otherwise[1],
> even in the face of the GNU extension that "void *" has size "1 byte"
> for pointer arithmetic). For "void *" members, we must use the coming
> "sized_by" attribute.
>
I'm pretty sure that "sized_by" is available in Clang right now.
-bw
> Link: https://gcc.gnu.org/pipermail/gcc-patches/2025-May/683136.html [1]
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
> Cc: Miguel Ojeda <ojeda@...nel.org>
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Nick Desaulniers <nick.desaulniers+lkml@...il.com>
> Cc: Bill Wendling <morbo@...gle.com>
> Cc: Justin Stitt <justinstitt@...gle.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Marco Elver <elver@...gle.com>
> Cc: Przemek Kitszel <przemyslaw.kitszel@...el.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Masahiro Yamada <masahiroy@...nel.org>
> Cc: Christophe Leroy <christophe.leroy@...roup.eu>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: <llvm@...ts.linux.dev>
> ---
> init/Kconfig | 11 +++++++++++
> Makefile | 4 ++++
> include/linux/compiler_types.h | 21 ++++++++++++++++++++-
> include/uapi/linux/stddef.h | 4 ++++
> 4 files changed, 39 insertions(+), 1 deletion(-)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index cab3ad28ca49..54691b086bc6 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -139,6 +139,17 @@ 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_PTR_BARE
> + def_bool $(success,echo 'struct foo { int *ptr __attribute__((__counted_by__(count))); int count; };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)
> +
> +config CC_HAS_COUNTED_BY_PTR_EXP
> + def_bool $(success,echo 'struct foo { int *ptr __attribute__((__counted_by__(count))); int count; };' | $(CC) $(CLANG_FLAGS) -fexperimental-late-parse-attributes -x c - -c -o /dev/null -Werror)
> + depends on !CC_HAS_COUNTED_BY_PTR_BARE
> +
> +config CC_HAS_COUNTED_BY_PTR
> + def_bool y
> + depends on CC_HAS_COUNTED_BY_PTR_BARE || CC_HAS_COUNTED_BY_PTR_EXP
> +
> config CC_HAS_MULTIDIMENSIONAL_NONSTRING
> def_bool $(success,echo 'char tag[][4] __attribute__((__nonstring__)) = { };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)
>
> diff --git a/Makefile b/Makefile
> index d14824792227..1b297dcbb0df 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -933,6 +933,10 @@ KBUILD_CFLAGS += $(CC_AUTO_VAR_INIT_ZERO_ENABLER)
> endif
> endif
>
> +ifdef CONFIG_CC_HAS_COUNTED_BY_PTR_EXP
> +KBUILD_CFLAGS += -fexperimental-late-parse-attributes
> +endif
> +
> # Explicitly clear padding bits during variable initialization
> KBUILD_CFLAGS += $(call cc-option,-fzero-init-padding-bits=all)
>
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 59288a2c1ad2..f197ea03b593 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -353,11 +353,14 @@ struct ftrace_likely_data {
> #endif
>
> /*
> + * Runtime track number of flexible array member elements for use by
> + * CONFIG_FORTIFY_SOURCE and CONFIG_UBSAN_BOUNDS.
> + *
> * Optional: only supported since gcc >= 15
> * Optional: only supported since clang >= 18
> *
> * gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
> - * clang: https://github.com/llvm/llvm-project/pull/76348
> + * clang: https://clang.llvm.org/docs/AttributeReference.html#counted-by-counted-by-or-null-sized-by-sized-by-or-null
> *
> * __bdos on clang < 19.1.2 can erroneously return 0:
> * https://github.com/llvm/llvm-project/pull/110497
> @@ -371,6 +374,22 @@ struct ftrace_likely_data {
> # define __counted_by(member)
> #endif
>
> +/*
> + * Runtime track number of objects pointed to by a pointer member for
> + * use by CONFIG_FORTIFY_SOURCE and CONFIG_UBSAN_BOUNDS.
> + *
> + * Optional: only supported since gcc >= 16
> + * Optional: only supported since clang >= 20
> + *
> + * gcc: https://gcc.gnu.org/pipermail/gcc-patches/2025-April/681727.html
> + * clang: ...
> + */
> +#ifdef CONFIG_CC_HAS_COUNTED_BY_PTR
> +# define __counted_by_ptr(member) __attribute__((__counted_by__(member)))
> +#else
> +# define __counted_by_ptr(member)
> +#endif
> +
> /*
> * Optional: only supported since gcc >= 15
> * Optional: not supported by Clang
> diff --git a/include/uapi/linux/stddef.h b/include/uapi/linux/stddef.h
> index 9a28f7d9a334..111b097ec00b 100644
> --- a/include/uapi/linux/stddef.h
> +++ b/include/uapi/linux/stddef.h
> @@ -72,6 +72,10 @@
> #define __counted_by_be(m)
> #endif
>
> +#ifndef __counted_by_ptr
> +#define __counted_by_ptr(m)
> +#endif
> +
> #ifdef __KERNEL__
> #define __kernel_nonstring __nonstring
> #else
> --
> 2.34.1
>
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