[PATCH v2 1/2] Compiler Attributes: Add __counted_by_ptr macro

[Bill Wendling]

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: Qing Zhao <qing.zhao@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>
---
v2 - Add support for GCC.
---
 include/linux/compiler_types.h | 11 +++++++++++
 init/Kconfig                   |  7 +++++++
 2 files changed, 18 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..f947f242bca8 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -139,6 +139,13 @@ 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_ON_POINTERS
+	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.rc2.455.g230fcf2819-goog