linux-kernel - [PATCH v2 4/4] x86/retpoline: Support retpoline build with Clang

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Date:   Wed, 14 Feb 2018 23:29:18 +0000
From:   David Woodhouse <dwmw@...zon.co.uk>
To:     tglx@...utronix.de, karahmed@...zon.de, x86@...nel.org,
        kvm@...r.kernel.org, torvalds@...ux-foundation.org,
        pbonzini@...hat.com, linux-kernel@...r.kernel.org, bp@...en8.de,
        peterz@...radead.org, jmattson@...gle.com, rkrcmar@...hat.com,
        arjan.van.de.ven@...el.com, dave.hansen@...el.com, mingo@...nel.org
Subject: [PATCH v2 4/4] x86/retpoline: Support retpoline build with Clang

Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
---
 arch/x86/Makefile              | 5 ++++-
 include/linux/compiler-clang.h | 5 +++++
 include/linux/compiler-gcc.h   | 4 ++++
 include/linux/init.h           | 8 ++++----
 4 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index fad5516..dbc7d0e 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -232,7 +232,10 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
 
 # Avoid indirect branches in kernel to deal with Spectre
 ifdef CONFIG_RETPOLINE
-    RETPOLINE_CFLAGS += $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
+    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
diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index 3b609ed..be3aef6 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -19,3 +19,8 @@
 
 #define randomized_struct_fields_start	struct {
 #define randomized_struct_fields_end	};
+
+/* Clang doesn't have a way to turn it off per-function, yet. */
+#ifdef __noretpoline
+#undef __noretpoline
+#endif
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 631354a..2e8a287 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -93,6 +93,10 @@
 #define __weak		__attribute__((weak))
 #define __alias(symbol)	__attribute__((alias(#symbol)))
 
+#ifdef RETPOLINE
+#define __noretpoline __attribute__((indirect_branch("keep")))
+#endif
+
 /*
  * it doesn't make sense on ARM (currently the only user of __naked)
  * to trace naked functions because then mcount is called without
diff --git a/include/linux/init.h b/include/linux/init.h
index 506a981..bc27cf0 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -6,10 +6,10 @@
 #include <linux/types.h>
 
 /* Built-in __init functions needn't be compiled with retpoline */
-#if defined(RETPOLINE) && !defined(MODULE)
-#define __noretpoline __attribute__((indirect_branch("keep")))
+#if defined(__noretpoline) && !defined(MODULE)
+#define __noinitretpoline __noretpoline
 #else
-#define __noretpoline
+#define __noinitretpoline
 #endif
 
 /* These macros are used to mark some functions or 
@@ -47,7 +47,7 @@
 
 /* These are for everybody (although not all archs will actually
    discard it in modules) */
-#define __init		__section(.init.text) __cold  __latent_entropy __noretpoline
+#define __init		__section(.init.text) __cold  __latent_entropy __noinitretpoline
 #define __initdata	__section(.init.data)
 #define __initconst	__section(.init.rodata)
 #define __exitdata	__section(.exit.data)
-- 
2.7.4