linux-kernel - [PATCH v2 12/12] retpoline: Attempt to quiten objtool warning for unreachable code

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Date:   Wed,  3 Jan 2018 18:00:19 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     tglx@...utronix.de
Cc:     torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        linux-kernel@...r.kernel.org, tim.c.chen@...ux.intel.com,
        Andi Kleen <ak@...ux.intel.com>, jpoimboe@...hat.com
Subject: [PATCH v2 12/12] retpoline: Attempt to quiten objtool warning for unreachable code

From: Andi Kleen <ak@...ux.intel.com>

The speculative jump trampoline has to contain unreachable code.
objtool keeps complaining

arch/x86/lib/retpoline.o: warning: objtool: __x86.indirect_thunk()+0x8: unreachable instruction

I tried to fix it here by adding ASM_UNREACHABLE annotation (after
supporting them for pure assembler), but it still complains.
Seems like a objtool bug?

So it doesn't actually fix the warning oyet.
Of course it's just a warning so the kernel will still work fine.

Perhaps Josh can figure it out

Cc: jpoimboe@...hat.com
Not-Signed-off-by: Andi Kleen <ak@...ux.intel.com>
---
 arch/x86/lib/retpoline.S |  3 +++
 include/linux/compiler.h | 10 +++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index cb40781adbfe..f4ed556a90ee 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -11,6 +11,7 @@
  */
 
 #include <linux/linkage.h>
+#include <linux/compiler.h>
 #include <asm/dwarf2.h>
 #include <asm/export.h>
 
@@ -21,7 +22,9 @@ ENTRY(__x86.indirect_thunk)
 	call	retpoline_call_target
 2:
 	lfence		/* stop speculation */
+	ASM_UNREACHABLE
 	jmp	2b
+	ASM_UNREACHABLE
 retpoline_call_target:
 #ifdef CONFIG_64BIT
 	lea	8(%rsp), %rsp
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 52e611ab9a6c..cfba91acc79a 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -269,7 +269,15 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 
 #endif /* __KERNEL__ */
 
-#endif /* __ASSEMBLY__ */
+#else /* __ASSEMBLY__ */
+
+#define ASM_UNREACHABLE \
+	999:						\
+	.pushsection .discard.unreachable;		\
+	.long 999b - .;					\
+	.popsection
+
+#endif /* !__ASSEMBLY__ */
 
 /* Compile time object size, -1 for unknown */
 #ifndef __compiletime_object_size
-- 
2.14.3