[PATCH] objtool: support '-mtune=atom' stack frame setup instruction

[Josh Poimboeuf]

Arnd reported that enabling CONFIG_MATOM results in a bunch of objtool
false positive frame pointer warnings:

  arch/x86/events/intel/ds.o: warning: objtool: intel_pmu_pebs_del()+0x43: call without frame pointer save/setup
  security/keys/keyring.o: warning: objtool: keyring_read()+0x59: call without frame pointer save/setup
  kernel/signal.o: warning: objtool: __dequeue_signal()+0xd8: call without frame pointer save/setup
  ...

objtool gets confused by the fact that the '-mtune=atom' gcc option
sometimes uses 'lea (%rsp),%rbp' instead of 'mov %rsp,%rbp'.  The
instructions are effectively the same, but objtool doesn't know about
the 'lea' variant.

Fix the false warnings by adding support for 'lea (%rsp),%rbp' in the
objtool decoder.

Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 tools/objtool/arch/x86/decode.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index c0c0b26..b63a31b 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -98,6 +98,15 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
 			*type = INSN_FP_SETUP;
 		break;
 
+	case 0x8d:
+		if (insn.rex_prefix.bytes &&
+		    insn.rex_prefix.bytes[0] == 0x48 &&
+		    insn.modrm.nbytes && insn.modrm.bytes[0] == 0x2c &&
+		    insn.sib.nbytes && insn.sib.bytes[0] == 0x24)
+			/* lea %(rsp), %rbp */
+			*type = INSN_FP_SETUP;
+		break;
+
 	case 0x90:
 		*type = INSN_NOP;
 		break;
-- 
2.7.4