[tip:x86/asm] x86/alternatives: Add stack frame dependency to alternative_call_2()

From: tip-bot for Josh Poimboeuf
Date: Sat Sep 24 2016 - 12:29:57 EST

Commit-ID: 317c2ce77d8ab73c24f4fb9c75e5bb441fbe3e30
Gitweb: http://git.kernel.org/tip/317c2ce77d8ab73c24f4fb9c75e5bb441fbe3e30
Author: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate: Fri, 23 Sep 2016 16:49:39 -0500
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
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@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20160923214939.j5o7c67nhepzmh3t@treble
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
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