[PATCH 1/3] compiler_types: Introduce __counted_by_ptr()

[Kees Cook]

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.

Link: https://gcc.gnu.org/pipermail/gcc-patches/2025-May/683136.html [1]
Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
---
Cc: Miguel Ojeda <ojeda@xxxxxxxxxx>
Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
Cc: Nick Desaulniers <nick.desaulniers+lkml@xxxxxxxxx>
Cc: Bill Wendling <morbo@xxxxxxxxxx>
Cc: Justin Stitt <justinstitt@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Cc: Przemek Kitszel <przemyslaw.kitszel@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Cc: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: <llvm@xxxxxxxxxxxxxxx>
---
 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