linux-kernel - [PATCH 4.9 31/34] kbuild: add -fcf-protection=none when using retpoline flags

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Message-Id: <20191108174653.027224327@linuxfoundation.org>
Date:   Fri,  8 Nov 2019 19:50:38 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Seth Forshee <seth.forshee@...onical.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.9 31/34] kbuild: add -fcf-protection=none when using retpoline flags

From: Seth Forshee <seth.forshee@...onical.com>

[ Upstream commit 29be86d7f9cb18df4123f309ac7857570513e8bc ]

The gcc -fcf-protection=branch option is not compatible with
-mindirect-branch=thunk-extern. The latter is used when
CONFIG_RETPOLINE is selected, and this will fail to build with
a gcc which has -fcf-protection=branch enabled by default. Adding
-fcf-protection=none when building with retpoline enabled
prevents such build failures.

Signed-off-by: Seth Forshee <seth.forshee@...onical.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 Makefile | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Makefile b/Makefile
index 62ce3766032c9..1d50905aaf425 100644
--- a/Makefile
+++ b/Makefile
@@ -840,6 +840,12 @@ KBUILD_CFLAGS   += $(call cc-option,-Werror=designated-init)
 # change __FILE__ to the relative path from the srctree
 KBUILD_CFLAGS	+= $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
 
+# ensure -fcf-protection is disabled when using retpoline as it is
+# incompatible with -mindirect-branch=thunk-extern
+ifdef CONFIG_RETPOLINE
+KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
+endif
+
 # use the deterministic mode of AR if available
 KBUILD_ARFLAGS := $(call ar-option,D)
 
-- 
2.20.1