[PATCH 4/4] x86/retpoline: Support retpoline build with Clang

David Woodhouse
2018-02-14T13:45:27Z

Signed-off-by: David Woodhouse <dwmw@amazon.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