linux-kernel - [PATCH v3 13/13] retpoline: Attempt to quiten objtool warning for unreachable code

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Date:   Thu,  4 Jan 2018 14:37:10 +0000
From:   David Woodhouse <dwmw@...zon.co.uk>
To:     ak@...ux.intel.com
Cc:     Paul Turner <pjt@...gle.com>, LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, tglx@...utronix.de,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk,
        jpoimboe@...hat.com
Subject: [PATCH v3 13/13] 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>
Not-Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
---
 arch/x86/lib/retpoline.S |  5 +++++
 include/linux/compiler.h | 10 +++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index bbdda5cc136e..8112feaea6ff 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -2,6 +2,7 @@
 
 #include <linux/stringify.h>
 #include <linux/linkage.h>
+#include <linux/compiler.h>
 #include <asm/dwarf2.h>
 #include <asm/cpufeatures.h>
 #include <asm/alternative-asm.h>
@@ -14,7 +15,9 @@ ENTRY(__x86.indirect_thunk.\reg)
 	ALTERNATIVE "call 2f", __stringify(jmp *%\reg), X86_BUG_NO_RETPOLINE
 1:
 	lfence
+	ASM_UNREACHABLE
 	jmp	1b
+	ASM_UNREACHABLE
 2:
 	mov	%\reg, (%\sp)
 	ret
@@ -40,7 +43,9 @@ ENTRY(__x86.indirect_thunk)
 	ALTERNATIVE "call 2f", "ret", X86_BUG_NO_RETPOLINE
 1:
 	lfence
+	ASM_UNREACHABLE
 	jmp	1b
+	ASM_UNREACHABLE
 2:
 	lea	4(%esp), %esp
 	ret
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