Compiler Attributes: disable __counted_by for clang < 19.1.3

This patch disables __counted_by for clang versions < 19.1.3 because
of the two issues listed below. It does this by introducing
CONFIG_CC_HAS_COUNTED_BY.

1. clang < 19.1.2 has a bug that can lead to __bdos returning 0:
llvm/llvm-project#110497

2. clang < 19.1.3 has a bug that can lead to __bdos being off by 4:
llvm/llvm-project#112636

Fixes: c8248fa ("Compiler Attributes: counted_by: Adjust name and identifier expansion")
Cc: [email protected] # 6.6.x: 16c31dd: Compiler Attributes: counted_by: bump min gcc version
Cc: [email protected] # 6.6.x: 2993eb7: Compiler Attributes: counted_by: fixup clang URL
Cc: [email protected] # 6.6.x: 231dc3f: lkdtm/bugs: Improve warning message for compilers without counted_by support
Cc: [email protected] # 6.6.x
Reported-by: Nathan Chancellor <[email protected]>
Closes: https://lore.kernel.org/all/20240913164630.GA4091534@thelio-3990X/
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-lkp/[email protected]
Link: https://lore.kernel.org/all/Zw8iawAF5W2uzGuh@archlinux/T/#m204c09f63c076586a02d194b87dffc7e81b8de7b
Suggested-by: Nathan Chancellor <[email protected]>
Signed-off-by: Jan Hendrik Farr <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Reviewed-by: Miguel Ojeda <[email protected]>
Reviewed-by: Thorsten Blum <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

Author: Cydox

drivers/misc/lkdtm/bugs.c

@@ -445,7 +445,7 @@ static void lkdtm_FAM_BOUNDS(void)
 
 	pr_err("FAIL: survived access of invalid flexible array member index!\n");
 
-	if (!__has_attribute(__counted_by__))
+	if (!IS_ENABLED(CONFIG_CC_HAS_COUNTED_BY))
 		pr_warn("This is expected since this %s was built with a compiler that does not support __counted_by\n",
 			lkdtm_kernel_info);
 	else if (IS_ENABLED(CONFIG_UBSAN_BOUNDS))

include/linux/compiler_attributes.h

@@ -94,19 +94,6 @@
 # define __copy(symbol)
 #endif
 
-/*
- * 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
- */
-#if __has_attribute(__counted_by__)
-# define __counted_by(member)		__attribute__((__counted_by__(member)))
-#else
-# define __counted_by(member)
-#endif
-
 /*
  * Optional: not supported by gcc
  * Optional: only supported since clang >= 14.0

include/linux/compiler_types.h

@@ -323,6 +323,25 @@ struct ftrace_likely_data {
 #define __no_sanitize_or_inline __always_inline
 #endif
 
+/*
+ * 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
+ *
+ * __bdos on clang < 19.1.2 can erroneously return 0:
+ * https://github.com/llvm/llvm-project/pull/110497
+ *
+ * __bdos on clang < 19.1.3 can be off by 4:
+ * https://github.com/llvm/llvm-project/pull/112636
+ */
+#ifdef CONFIG_CC_HAS_COUNTED_BY
+# define __counted_by(member)		__attribute__((__counted_by__(member)))
+#else
+# define __counted_by(member)
+#endif
+
 /*
  * Apply __counted_by() when the Endianness matches to increase test coverage.
  */

init/Kconfig

@@ -116,6 +116,15 @@ config CC_HAS_ASM_INLINE
 config CC_HAS_NO_PROFILE_FN_ATTR
 	def_bool $(success,echo '__attribute__((no_profile_instrument_function)) int x();' | $(CC) -x c - -c -o /dev/null -Werror)
 
+config CC_HAS_COUNTED_BY
+	# TODO: when gcc 15 is released remove the build test and add
+	# a gcc version check
+	def_bool $(success,echo 'struct flex { int count; int array[] __attribute__((__counted_by__(count))); };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)
+	# clang needs to be at least 19.1.3 to avoid __bdos miscalculations
+	# https://github.com/llvm/llvm-project/pull/110497
+	# https://github.com/llvm/llvm-project/pull/112636
+	depends on !(CC_IS_CLANG && CLANG_VERSION < 190103)
+
 config PAHOLE_VERSION
 	int
 	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))

lib/overflow_kunit.c

@@ -1187,7 +1187,7 @@ static void DEFINE_FLEX_test(struct kunit *test)
 {
 	/* Using _RAW_ on a __counted_by struct will initialize "counter" to zero */
 	DEFINE_RAW_FLEX(struct foo, two_but_zero, array, 2);
-#if __has_attribute(__counted_by__)
+#ifdef CONFIG_CC_HAS_COUNTED_BY
 	int expected_raw_size = sizeof(struct foo);
 #else
 	int expected_raw_size = sizeof(struct foo) + 2 * sizeof(s16);