[tip:x86/pti] x86/retpoline: Make CONFIG_RETPOLINE depend on compiler support

[tip-bot for Zhenzhong Duan <tipbot@zytor.com>]

Commit-ID:  cf8f07269c7182f0e99abd3ac697870ea44d6f47
Gitweb:     https://git.kernel.org/tip/cf8f07269c7182f0e99abd3ac697870ea44d6f47
Author:     Zhenzhong Duan <zhenzhong.duan@oracle.com>
AuthorDate: Fri, 2 Nov 2018 01:45:41 -0700
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 6 Nov 2018 21:46:20 +0100

x86/retpoline: Make CONFIG_RETPOLINE depend on compiler support

Since retpoline capable compilers are widely available, make
CONFIG_RETPOLINE hard depend on the compiler capability.

Break the build when CONFIG_RETPOLINE is enabled and the compiler does not
support it. Emit an error message in that case:

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

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Borislav Petkov <bp@suse.de>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: H. Peter Anvin <hpa@zytor.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>
Cc: <srinivas.eeda@oracle.com>
Link: https://lkml.kernel.org/r/cca0cb20-f9e2-4094-840b-fb0f8810cd34@default

[dwmw@amazon.co.uk: fail the build with non-retpoline compiler]
---
 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 ba7e3464ee92..ea399744a18c 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -444,10 +444,6 @@ config RETPOLINE
 	  branches. Requires a compiler with -mindirect-branch=thunk-extern
 	  support for full protection. The kernel may run slower.
 
-	  Without compiler support, at least indirect branches in assembler
-	  code are eliminated. Since this includes the syscall entry path,
-	  it is not entirely pointless.
-
 config INTEL_RDT
 	bool "Intel Resource Director Technology support"
 	depends on X86 && CPU_SUP_INTEL
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5b562e464009..6d94fe897983 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -222,9 +222,10 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
 
 # Avoid indirect branches in kernel to deal with Spectre
 ifdef CONFIG_RETPOLINE
-ifneq ($(RETPOLINE_CFLAGS),)
-  KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE
+ifeq ($(RETPOLINE_CFLAGS),)
+  $(error You are building kernel with non-retpoline compiler, please update your compiler.)
 endif
+  KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
 endif
 
 archscripts: scripts_basic
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 80dc14422495..8b09cbb2d52c 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -162,11 +162,12 @@
 	_ASM_PTR " 999b\n\t"					\
 	".popsection\n\t"
 
-#if defined(CONFIG_X86_64) && defined(RETPOLINE)
+#ifdef CONFIG_RETPOLINE
+#ifdef CONFIG_X86_64
 
 /*
- * 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						\
 	ANNOTATE_NOSPEC_ALTERNATIVE				\
@@ -181,7 +182,7 @@
 	X86_FEATURE_RETPOLINE_AMD)
 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
 
-#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 @@
 	X86_FEATURE_RETPOLINE_AMD)
 
 # 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 c37e66e493bf..d0108fb6e4dd 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 *reason)
 
 static inline bool retp_compiler(void)
 {
-	return __is_defined(RETPOLINE);
+	return __is_defined(CONFIG_RETPOLINE);
 }
 
 static inline bool match_option(const char *arg, int arglen, const char *opt)
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index a8e7ba9f73e8..6a6be9f440cf 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -236,10 +236,8 @@ ifdef CONFIG_GCOV_KERNEL
 objtool_args += --no-unreachable
 endif
 ifdef CONFIG_RETPOLINE
-ifneq ($(RETPOLINE_CFLAGS),)
   objtool_args += --retpoline
 endif
-endif
 
 
 ifdef CONFIG_MODVERSIONS