objtool: Silence warnings for functions which use IRET

Previously, objtool ignored functions which have the IRET instruction
in them.  That's because it assumed that such functions know what
they're doing with respect to frame pointers.

With the new "objtool 2.0" changes, it stopped ignoring such functions,
and started complaining about them:

  arch/x86/kernel/alternative.o: warning: objtool: do_sync_core()+0x1b: unsupported instruction in callable function
  arch/x86/kernel/alternative.o: warning: objtool: text_poke()+0x1a8: unsupported instruction in callable function
  arch/x86/kernel/ftrace.o: warning: objtool: do_sync_core()+0x16: unsupported instruction in callable function
  arch/x86/kernel/cpu/mcheck/mce.o: warning: objtool: machine_check_poll()+0x166: unsupported instruction in callable function
  arch/x86/kernel/cpu/mcheck/mce.o: warning: objtool: do_machine_check()+0x147: unsupported instruction in callable function

Silence those warnings for now.  They can be re-enabled later, once we
have unwind hints which will allow the code to annotate the IRET usages.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: live-patching@vger.kernel.org
Fixes: baa4146 ("objtool: Implement stack validation 2.0")
Link: http://lkml.kernel.org/r/20170630140934.mmwtpockvpupahro@treble
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/check.c

@@ -100,6 +100,7 @@ static bool gcov_enabled(struct objtool_file *file)
 static bool ignore_func(struct objtool_file *file, struct symbol *func)
 {
 	struct rela *rela;
+	struct instruction *insn;
 
 	/* check for STACK_FRAME_NON_STANDARD */
 	if (file->whitelist && file->whitelist->rela)
@@ -112,6 +113,11 @@ static bool ignore_func(struct objtool_file *file, struct symbol *func)
 				return true;
 		}
 
+	/* check if it has a context switching instruction */
+	func_for_each_insn(file, func, insn)
+		if (insn->type == INSN_CONTEXT_SWITCH)
+			return true;
+
 	return false;
 }
 
@@ -1446,14 +1452,6 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 
 			return 0;
 
-		case INSN_CONTEXT_SWITCH:
-			if (func) {
-				WARN_FUNC("unsupported instruction in callable function",
-					  sec, insn->offset);
-				return 1;
-			}
-			return 0;
-
 		case INSN_STACK:
 			if (update_insn_state(insn, &state))
 				return -1;