linux-kernel - [PATCH] x86/alternatives: add stack frame dependency to alternative_call

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Date:   Fri, 23 Sep 2016 16:49:39 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH] 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-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 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
-- 
2.7.4