linux-kernel - [PATCH v2 09/12] x86/retpoline: Finally enable retpoline for C code

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Date:   Wed,  3 Jan 2018 18:00:16 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     tglx@...utronix.de
Cc:     torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        linux-kernel@...r.kernel.org, tim.c.chen@...ux.intel.com,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: [PATCH v2 09/12] x86/retpoline: Finally enable retpoline for C code

From: Dave Hansen <dave.hansen@...ux.intel.com>

From: David Woodhouse <dwmw@...zon.co.uk>

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]

v2: Use CONFIG option to enable
Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
 arch/x86/Makefile | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 3e73bc255e4e..b02e35350244 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -230,6 +230,17 @@ KBUILD_CFLAGS += -Wno-sign-compare
 #
 KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
 
+#
+ifdef CONFIG_RETPOLINE
+	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. System may be insecure.)
+	endif
+endif
+
 archscripts: scripts_basic
 	$(Q)$(MAKE) $(build)=arch/x86/tools relocs
 
-- 
2.14.3