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Message-ID: <cca0cb20-f9e2-4094-840b-fb0f8810cd34@default>
Date:   Fri, 2 Nov 2018 01:45:41 -0700 (PDT)
From:   Zhenzhong Duan <zhenzhong.duan@oracle.com>
To:     <linux-kernel@vger.kernel.org>
Cc:     <mingo@redhat.com>, <luto@kernel.org>, <konrad.wilk@oracle.com>,
        <dwmw@amazon.co.uk>, <tglx@linutronix.de>,
        <srinivas.eeda@oracle.com>, <bp@suse.de>, <daniel@iogearbox.net>,
        <yamada.masahiro@socionext.com>, <michal.lkml@markovi.net>,
        <peterz@infradead.org>, <hpa@zytor.com>
Subject: [PATCH v3 1/2] retpolines: Enable retpoline support only when
 compiler support it
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Sender: linux-kernel-owner@vger.kernel.org
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Since retpoline capable compilers are widely available, make
CONFIG_RETPOLINE hard depend on it. This is achieved by breaking
the build early and showing an error like below:

"arch/x86/Makefile:226: *** You are building kernel with non-retpoline
compiler, please update your compiler..  Stop."

[peterz@infradead.org: initial suggestion]
Link: https://lore.kernel.org/lkml/caeb3213f13e57191e362ca04f2892cfa2ccda65=
.camel@infradead.org
[dwmw@amazon.co.uk: fail the build with non-retpoline compiler]
Link: https://lore.kernel.org/lkml/alpine.DEB.2.21.1811011317400.1642@nanos=
.tec.linutronix.de
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Michal Marek <michal.lkml@markovi.net>
---
 arch/x86/Kconfig                     |  4 ----
 arch/x86/Makefile                    |  5 +++--
 arch/x86/include/asm/nospec-branch.h | 10 ++++++----
 arch/x86/kernel/cpu/bugs.c           |  2 +-
 scripts/Makefile.build               |  2 --
 5 files changed, 10 insertions(+), 13 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c51c989..fcb7f37 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -443,10 +443,6 @@ config RETPOLINE
 =09  branches. Requires a compiler with -mindirect-branch=3Dthunk-extern
 =09  support for full protection. The kernel may run slower.
=20
-=09  Without compiler support, at least indirect branches in assembler
-=09  code are eliminated. Since this includes the syscall entry path,
-=09  it is not entirely pointless.
-
 config INTEL_RDT
 =09bool "Intel Resource Director Technology support"
 =09depends on X86 && CPU_SUP_INTEL
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5b562e4..6d94fe8 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -222,9 +222,10 @@ KBUILD_CFLAGS +=3D -fno-asynchronous-unwind-tables
=20
 # Avoid indirect branches in kernel to deal with Spectre
 ifdef CONFIG_RETPOLINE
-ifneq ($(RETPOLINE_CFLAGS),)
-  KBUILD_CFLAGS +=3D $(RETPOLINE_CFLAGS) -DRETPOLINE
+ifeq ($(RETPOLINE_CFLAGS),)
+  $(error You are building kernel with non-retpoline compiler, please upda=
te your compiler.)
 endif
+  KBUILD_CFLAGS +=3D $(RETPOLINE_CFLAGS)
 endif
=20
 archscripts: scripts_basic
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/no=
spec-branch.h
index 80dc144..8b09cbb 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -162,11 +162,12 @@
 =09_ASM_PTR " 999b\n\t"=09=09=09=09=09\
 =09".popsection\n\t"
=20
-#if defined(CONFIG_X86_64) && defined(RETPOLINE)
+#ifdef CONFIG_RETPOLINE
+#ifdef CONFIG_X86_64
=20
 /*
- * Since the inline asm uses the %V modifier which is only in newer GCC,
- * the 64-bit one is dependent on RETPOLINE not CONFIG_RETPOLINE.
+ * Inline asm uses the %V modifier which is only in newer GCC
+ * which is ensured when CONFIG_RETPOLINE is defined.
  */
 # define CALL_NOSPEC=09=09=09=09=09=09\
 =09ANNOTATE_NOSPEC_ALTERNATIVE=09=09=09=09\
@@ -181,7 +182,7 @@
 =09X86_FEATURE_RETPOLINE_AMD)
 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
=20
-#elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
+#else /* CONFIG_X86_32 */
 /*
  * For i386 we use the original ret-equivalent retpoline, because
  * otherwise we'll run out of registers. We don't care about CET
@@ -211,6 +212,7 @@
 =09X86_FEATURE_RETPOLINE_AMD)
=20
 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
+#endif
 #else /* No retpoline for C / inline asm */
 # define CALL_NOSPEC "call *%[thunk_target]\n"
 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index c37e66e..d0108fb 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -252,7 +252,7 @@ static void __init spec2_print_if_secure(const char *re=
ason)
=20
 static inline bool retp_compiler(void)
 {
-=09return __is_defined(RETPOLINE);
+=09return __is_defined(CONFIG_RETPOLINE);
 }
=20
 static inline bool match_option(const char *arg, int arglen, const char *o=
pt)
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index a8e7ba9..6a6be9f 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -236,10 +236,8 @@ ifdef CONFIG_GCOV_KERNEL
 objtool_args +=3D --no-unreachable
 endif
 ifdef CONFIG_RETPOLINE
-ifneq ($(RETPOLINE_CFLAGS),)
   objtool_args +=3D --retpoline
 endif
-endif
=20
=20
 ifdef CONFIG_MODVERSIONS
--=20
1.8.3.1