[v3,4/4] x86/retpoline: Support retpoline build with Clang

Commit Message

Woodhouse, David Feb. 19, 2018, 10:50 a.m. UTC

Signed-off-by: David Woodhouse <[e-mail]>
---
 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(-)

Comments

Thomas Gleixner Feb. 20, 2018, 8:36 a.m. UTC | #1

On Mon, 19 Feb 2018, David Woodhouse wrote:

Reviewed-by: Thomas Gleixner <[e-mail]>

> +/* Clang doesn't have a way to turn it off per-function, yet. */

Is that going to happen in the foreseable future? Hmm, why am I asking,
clang is going to break anyway once we require asm-gotos hard.

Thanks,

	tglx

David Woodhouse Feb. 20, 2018, 8:45 a.m. UTC | #2

On Tue, 2018-02-20 at 09:36 +0100, Thomas Gleixner wrote:
> On Mon, 19 Feb 2018, David Woodhouse wrote:
> 
> Reviewed-by: Thomas Gleixner <[e-mail]>
> 
> > 
> > +/* Clang doesn't have a way to turn it off per-function, yet. */
>
> Is that going to happen in the foreseable future? Hmm, why am I asking,
> clang is going to break anyway once we require asm-gotos hard.

It's only minor optimisation so I told them there was no massive
urgency to get it implemented. Unlike asm-goto.

Patch

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 d02a4df..d3f264a 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -27,3 +27,8 @@ 
 #if __has_feature(address_sanitizer)
 #define __SANITIZE_ADDRESS__
 #endif
+
+/* 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 73bc63e..673fbf9 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)