linux-kernel - Re: [tip:x86/pti] objtool, retpolines: Integrate objtool with retpoline support more closely

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Message-ID: <20180221131207.GS25235@hirez.programming.kicks-ass.net>
Date:   Wed, 21 Feb 2018 14:12:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     hpa@...or.com, luto@...nel.org, arjan@...ux.intel.com,
        tglx@...utronix.de, dan.j.williams@...el.com,
        torvalds@...ux-foundation.org, gregkh@...uxfoundation.org,
        mingo@...nel.org, bp@...en8.de, linux-kernel@...r.kernel.org,
        dave.hansen@...ux.intel.com, jpoimboe@...hat.com,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/pti] objtool, retpolines: Integrate objtool with
 retpoline support more closely

On Wed, Feb 21, 2018 at 01:53:57PM +0100, Peter Zijlstra wrote:
> That's because this crossed with the llvm cruft, right? I'll have a
> look.

Best I could come up with that seems to work is something like the
below. Ingo, can you backmerge or stuff on top as appropriate?

---
 Makefile               |  8 ++++----
 arch/x86/Makefile      | 10 +++-------
 scripts/Makefile.build |  2 +-
 3 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/Makefile b/Makefile
index e1a155a50fdc..3aa0a115eafb 100644
--- a/Makefile
+++ b/Makefile
@@ -489,10 +489,10 @@ KBUILD_CFLAGS += $(CLANG_TARGET) $(CLANG_GCC_TC)
 KBUILD_AFLAGS += $(CLANG_TARGET) $(CLANG_GCC_TC)
 endif
 
-ifneq ($(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register),)
-  CC_HAS_RETPOLINE := 1
-endif
-export CC_HAS_RETPOLINE
+RETPOLINE_CFLAGS_GCC := -mindirect-branch=thunk-extern -mindirect-branch-register
+RETPOLINE_CFLAGS_CLANG := -mretpoline-external-thunk
+RETPOLINE_CFLAGS := $(call cc-option,$(RETPOLINE_CFLAGS_GCC),$(call cc-option,$(RETPOLINE_CFLAGS_CLANG)))
+export RETPOLINE_CFLAGS
 
 ifeq ($(config-targets),1)
 # ===========================================================================
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index dbc7d0ed2eaa..498c1b812300 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -232,13 +232,9 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
 
 # Avoid indirect branches in kernel to deal with Spectre
 ifdef CONFIG_RETPOLINE
-    RETPOLINE_CFLAGS_GCC := -mindirect-branch=thunk-extern -mindirect-branch-register
-    RETPOLINE_CFLAGS_CLANG := -mretpoline-external-thunk
-
-    RETPOLINE_CFLAGS += $(call cc-option,$(RETPOLINE_CFLAGS_GCC),$(call cc-option,$(RETPOLINE_CFLAGS_CLANG)))
-    ifneq ($(RETPOLINE_CFLAGS),)
-        KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE
-    endif
+ifneq ($(RETPOLINE_CFLAGS),)
+  KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE
+endif
 endif
 
 archscripts: scripts_basic
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 07e5802e324b..4f2b25d43ec9 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -267,7 +267,7 @@ else
 objtool_args += $(call cc-ifversion, -lt, 0405, --no-unreachable)
 endif
 ifdef CONFIG_RETPOLINE
-ifdef CC_HAS_RETPOLINE
+ifneq ($(RETPOLINE_CFLAGS),)
   objtool_args += --retpoline
 endif
 endif