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Message-ID: <20260114193716.1740684-1-morbo@google.com>
Date: Wed, 14 Jan 2026 19:36:47 +0000
From: Bill Wendling <morbo@...gle.com>
Cc: Bill Wendling <morbo@...gle.com>, Kees Cook <kees@...nel.org>,
"Gustavo A. R. Silva" <gustavoars@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Justin Stitt <justinstitt@...gle.com>,
Miguel Ojeda <ojeda@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>, Heiko Carstens <hca@...ux.ibm.com>,
Marc Herbert <Marc.Herbert@...ux.intel.com>, Uros Bizjak <ubizjak@...il.com>,
Tejun Heo <tj@...nel.org>, Jeff Xu <jeffxu@...omium.org>,
"Michal Koutný" <mkoutny@...e.com>, Shakeel Butt <shakeel.butt@...ux.dev>,
"Thomas Weißschuh" <thomas.weissschuh@...utronix.de>, John Stultz <jstultz@...gle.com>,
Christian Brauner <brauner@...nel.org>, Randy Dunlap <rdunlap@...radead.org>,
Brian Gerst <brgerst@...il.com>, Masahiro Yamada <masahiroy@...nel.org>, linux-kernel@...r.kernel.org,
linux-hardening@...r.kernel.org, llvm@...ts.linux.dev,
Nicolas Schier <nsc@...nel.org>, Tamir Duberstein <tamird@...il.com>, Steven Rostedt <rostedt@...dmis.org>,
"Jason A. Donenfeld" <Jason@...c4.com>, "H. Peter Anvin" <hpa@...or.com>,
Naman Jain <namjain@...ux.microsoft.com>, Eric Dumazet <edumazet@...gle.com>,
Simon Horman <horms@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
Ingo Molnar <mingo@...nel.org>, Thomas Gleixner <tglx@...nel.org>,
Douglas Anderson <dianders@...omium.org>, linux-kbuild@...r.kernel.org
Subject: [PATCH 1/2] Compiler Attributes: Add __counted_by_ptr macro
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;
};
Because "counted_by" can only be applied to pointer members in very
recent compiler versions, its application ends up needing to be distinct
from flexibe array "counted_by" annotations, hence a separate macro.
Note that Clang's support for "void *" members will be in version 22.
So, when using Clang, you'll need to wait until its release before using
the feature with "void *". No such restriction applies to GCC's version
16.
This is a reworking of Kees' previous patch [1].
Link: https://lore.kernel.org/all/20251020220118.1226740-1-kees@kernel.org/ [1]
Co-developed-by: Kees Cook <kees@...nel.org>
Signed-off-by: Bill Wendling <morbo@...gle.com>
---
Cc: Kees Cook <kees@...nel.org>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>
Cc: Nick Desaulniers <nick.desaulniers+lkml@...il.com>
Cc: Justin Stitt <justinstitt@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Heiko Carstens <hca@...ux.ibm.com>
Cc: Marc Herbert <Marc.Herbert@...ux.intel.com>
Cc: Uros Bizjak <ubizjak@...il.com>
Cc: Tejun Heo <tj@...nel.org>
Cc: Jeff Xu <jeffxu@...omium.org>
Cc: "Michal Koutný" <mkoutny@...e.com>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: "Thomas Weißschuh" <thomas.weissschuh@...utronix.de>
Cc: John Stultz <jstultz@...gle.com>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Randy Dunlap <rdunlap@...radead.org>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>
Cc: linux-kernel@...r.kernel.org
Cc: linux-hardening@...r.kernel.org
Cc: llvm@...ts.linux.dev
---
v3 - Replace the previous code with a modified version of Kees' previous patch
[1].
- The question about the naming of the macro was considered, but we decided
to keep the original naming (__counted_by_ptr), because it mirrors the current
macros like "__counted_by_{le,be}".
v2 - Add support for GCC.
---
Makefile | 6 ++++++
include/linux/compiler_types.h | 18 +++++++++++++++++-
include/uapi/linux/stddef.h | 4 ++++
init/Kconfig | 7 +++++++
4 files changed, 34 insertions(+), 1 deletion(-)
diff --git a/Makefile b/Makefile
index 9d38125263fb..6b029f694bc2 100644
--- a/Makefile
+++ b/Makefile
@@ -952,6 +952,12 @@ KBUILD_CFLAGS += $(CC_AUTO_VAR_INIT_ZERO_ENABLER)
endif
endif
+ifdef CONFIG_CC_IS_CLANG
+ifdef CONFIG_CC_HAS_COUNTED_BY_PTR
+KBUILD_CFLAGS += -fexperimental-late-parse-attributes
+endif
+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 d3318a3c2577..e597c814d60b 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -369,7 +369,7 @@ struct ftrace_likely_data {
* 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
@@ -383,6 +383,22 @@ 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 >= 21.1
+ *
+ * gcc: https://gcc.gnu.org/pipermail/gcc-patches/2025-April/681727.html
+ * clang: https://github.com/llvm/llvm-project/pull/137250
+ */
+#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
diff --git a/init/Kconfig b/init/Kconfig
index fa79feb8fe57..dc27b998d111 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -143,6 +143,13 @@ 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
+ bool
+ # supported since clang 21.1.0
+ default y if CC_IS_CLANG && CLANG_VERSION >= 210100
+ # supported since gcc 16.0.0
+ default y if CC_IS_GCC && GCC_VERSION >= 160000
+
config CC_HAS_MULTIDIMENSIONAL_NONSTRING
def_bool $(success,echo 'char tag[][4] __attribute__((__nonstring__)) = { };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)
--
2.52.0.457.g6b5491de43-goog
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