compiler_types.h: 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.

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@kernel.org>
Signed-off-by: Bill Wendling <morbo@google.com>
Link: https://patch.msgid.link/20260116005838.2419118-1-morbo@google.com
Signed-off-by: Kees Cook <kees@kernel.org>

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)
 

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 >= 22
+ *
+ *   gcc: https://gcc.gnu.org/pipermail/gcc-patches/2025-April/681727.html
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#counted-by-counted-by-or-null-sized-by-sized-by-or-null
+ */
+#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

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

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 22
+	default y if CC_IS_CLANG && CLANG_VERSION >= 220000
+	# 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)