[PATCH 1/3] kbuild: Change CFI_CLANG to depend on __builtin_function_start

[Sami Tolvanen <samitolvanen@google.com>]

Clang 14 added support for the __builtin_function_start()
built-in function, which allows us to implement function_nocfi()
without architecture-specific inline assembly. This patch changes
CONFIG_CFI_CLANG to depend on the built-in and effectively upgrades
the minimum supported compiler version for CFI to Clang 14.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
 arch/Kconfig | 5 +----
 init/Kconfig | 3 +++
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 29b0167c088b..eecfc2809781 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -723,10 +723,7 @@ config ARCH_SUPPORTS_CFI_CLANG
 config CFI_CLANG
 	bool "Use Clang's Control Flow Integrity (CFI)"
 	depends on LTO_CLANG && ARCH_SUPPORTS_CFI_CLANG
-	# Clang >= 12:
-	# - https://bugs.llvm.org/show_bug.cgi?id=46258
-	# - https://bugs.llvm.org/show_bug.cgi?id=47479
-	depends on CLANG_VERSION >= 120000
+	depends on CC_HAS_BUILTIN_FUNCTION_START
 	select KALLSYMS
 	help
 	  This option enables Clang’s forward-edge Control Flow Integrity
diff --git a/init/Kconfig b/init/Kconfig
index ddcbefe535e9..f024fd353373 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -86,6 +86,9 @@ 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_BUILTIN_FUNCTION_START
+	def_bool $(success,echo 'void f(void) {}; void *p = __builtin_function_start(f);' | $(CC) -x c - -c -o /dev/null)
+
 config PAHOLE_VERSION
 	int
 	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))
-- 
2.35.0