objtool: Fix sibling call detection logic

With some configs, objtool reports the following warning:

  arch/x86/kernel/ftrace.o: warning: objtool: ftrace_modify_code_direct()+0x2d: sibling call from callable instruction with modified stack frame

The instruction it's complaining about isn't actually a sibling call.
It's just a normal jump to an address inside the function.  Objtool
thought it was a sibling call because the instruction's jump_dest wasn't
initialized because the function was supposed to be ignored due to its
use of sync_core().

Objtool ended up validating the function instead of ignoring it because
it didn't properly recognize a sibling call to the function.  So fix the
sibling call logic.  Also add a warning to catch ignored functions being
validated so we'll get a more useful error message next time.
Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/96cc8ecbcdd8cb29ddd783817b4af918a6a171b0.1499437107.git.jpoimboe@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/check.c

@@ -1371,6 +1371,12 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 
 		func = insn->func;
 
+		if (func && insn->ignore) {
+			WARN_FUNC("BUG: why am I validating an ignored function?",
+				  sec, insn->offset);
+			return -1;
+		}
+
 		if (insn->visited) {
 			if (!!insn_state_match(insn, &state))
 				return 1;
@@ -1426,16 +1432,19 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 
 		case INSN_JUMP_CONDITIONAL:
 		case INSN_JUMP_UNCONDITIONAL:
-			if (insn->jump_dest) {
+			if (insn->jump_dest &&
+			    (!func || !insn->jump_dest->func ||
+			     func == insn->jump_dest->func)) {
 				ret = validate_branch(file, insn->jump_dest,
 						      state);
 				if (ret)
 					return 1;
+
 			} else if (func && has_modified_stack_frame(&state)) {
 				WARN_FUNC("sibling call from callable instruction with modified stack frame",
 					  sec, insn->offset);
 				return 1;
-			} /* else it's a sibling call */
+			}
 
 			if (insn->type == INSN_JUMP_UNCONDITIONAL)
 				return 0;