[PATCH 09/11] x86/retpoline: Finally enable retpoline for C code

[Andi Kleen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

Add retpoline compile option in Makefile

Update Makefile with retpoline compile options.  This requires a gcc with the
retpoline compiler patches enabled.

Print a warning when the compiler doesn't support retpoline

[Originally from David and Tim, but hacked by AK]

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 arch/x86/Makefile | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 3e73bc255e4e..dad4b24abdc9 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -230,6 +230,15 @@ KBUILD_CFLAGS += -Wno-sign-compare
 #
 KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
 
+#
+RETPOLINE_CFLAGS += $(call cc-option,-mindirect-branch=thunk-extern)
+ifneq ($(RETPOLINE_CFLAGS),)
+	KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE
+	KBUILD_AFLAGS += -DRETPOLINE
+else
+        $(warning Retpoline not supported in compiler. Kernel may be insecure.)
+endif
+
 archscripts: scripts_basic
 	$(Q)$(MAKE) $(build)=arch/x86/tools relocs
 
-- 
2.14.3