linux-kernel - [tip:x86/asm] x86/alternatives: Add stack frame dependency to alternative_call

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Date:   Sat, 24 Sep 2016 09:28:32 -0700
From:   tip-bot for Josh Poimboeuf <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     jpoimboe@...hat.com, hpa@...or.com, acme@...hat.com,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
        dvlasenk@...hat.com, luto@...nel.org,
        torvalds@...ux-foundation.org, bp@...en8.de, peterz@...radead.org,
        brgerst@...il.com
Subject: [tip:x86/asm] x86/alternatives: Add stack frame dependency to
 alternative_call_2()

Commit-ID:  317c2ce77d8ab73c24f4fb9c75e5bb441fbe3e30
Gitweb:     http://git.kernel.org/tip/317c2ce77d8ab73c24f4fb9c75e5bb441fbe3e30
Author:     Josh Poimboeuf <jpoimboe@...hat.com>
AuthorDate: Fri, 23 Sep 2016 16:49:39 -0500
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Sat, 24 Sep 2016 09:30:03 +0200

x86/alternatives: Add stack frame dependency to alternative_call_2()

Linus reported the following objtool warning:

  kernel/signal.o: warning: objtool: .altinstr_replacement+0x54: call without frame pointer save/setup

The warning is valid.  It's caused by the fact that gcc placed the call
instruction in alternative_call_2()'s inline asm before the frame
pointer setup, which breaks frame pointer convention and can result in a
bad stack trace.

Force a stack frame to be created before the call instruction by listing
the stack pointer as an output operand in the inline asm statement.

Reported-and-tested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/20160923214939.j5o7c67nhepzmh3t@treble
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/include/asm/alternative.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index e77a644..1b02038 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -217,10 +217,14 @@ static inline int alternatives_text_reserved(void *start, void *end)
  */
 #define alternative_call_2(oldfunc, newfunc1, feature1, newfunc2, feature2,   \
 			   output, input...)				      \
+{									      \
+	register void *__sp asm(_ASM_SP);				      \
 	asm volatile (ALTERNATIVE_2("call %P[old]", "call %P[new1]", feature1,\
 		"call %P[new2]", feature2)				      \
-		: output : [old] "i" (oldfunc), [new1] "i" (newfunc1),	      \
-		[new2] "i" (newfunc2), ## input)
+		: output, "+r" (__sp)					      \
+		: [old] "i" (oldfunc), [new1] "i" (newfunc1),		      \
+		  [new2] "i" (newfunc2), ## input);			      \
+}
 
 /*
  * use this macro(s) if you need more than one output parameter