[PATCH v2] x86/retpoline: Add clang support

[Guenter Roeck <linux@roeck-us.net>]

clang has its own set of compiler options for retpoline support.

Link: https://github.com/llvm-mirror/clang/commit/0d816739a82da29748caf88570affb9715e18b69
Link: https://github.com/llvm-mirror/llvm/commit/fd5a8723ce9f2a6b250e85972ef859e4253ea95d
Link: https://github.com/llvm-mirror/llvm/commit/59b64490fda69d29bb42cfdf7eec37bcc31ff833
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: gnomes@lxorguk.ukuu.org.uk
Cc: Rik van Riel <riel@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: thomas.lendacky@amd.com
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Kees Cook <keescook@google.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org>
Cc: Paul Turner <pjt@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
v2: llvm has been updated to use the same thunk names as gcc.
    Tested with:
    clang version 7.0.0
	(https://git.llvm.org/git/clang.git/
		848874aed95a913fb45f363120500cebfe54e2ef)
	(https://git.llvm.org/git/llvm.git/
		3afd566557f3616881505db0d69f5d19bf55ae14)
    cross-checked with gcc 7.3.0 (x86_64-linux-gcc.br_real (Buildroot
    2018.02-rc1) 7.3.0).

Tested with 64-bit builds only; 32-bit images fail to build with clang
with various unrelated errors and are difficult to test.

I had to change '+=' to '=' below since make otherwise sets
RETPOLINE_CFLAGS to " ", and the subsequent ifneq would always match.
This is also the reason for the "ifeq ($(RETPOLINE_CFLAGS),)".
If there is another/different/better way to handle this, please let
me know.

There are curently lots of warnings when building an image with clang.

./include/linux/init.h:134:6: warning:
	unknown attribute 'indirect_branch' ignored

I was inclined to add "&& !defined(__clang__)" to the condition for the
__noretpoline define to fix the problem, but concluded that this should
be a separate patch unless it can be addressed in clang.

 arch/x86/Makefile | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index fad55160dcb9..c1a35cea88e1 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -232,7 +232,12 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
 
 # Avoid indirect branches in kernel to deal with Spectre
 ifdef CONFIG_RETPOLINE
-    RETPOLINE_CFLAGS += $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
+    # gcc
+    RETPOLINE_CFLAGS = $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
+    ifeq ($(RETPOLINE_CFLAGS),)
+	# clang
+	RETPOLINE_CFLAGS = $(call cc-option,-mretpoline-external-thunk)
+    endif
     ifneq ($(RETPOLINE_CFLAGS),)
         KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE
     endif
-- 
2.7.4