objtool: Fix for unreachable instruction warning

On Sat, Jan 28, 2023 at 11:06:34AM -0800, Paul E. McKenney wrote:
> Sorry to be a nag, but I am still seeing this.  Not a huge problem because
> I now filter it out so that it does not get in the way of other bugs,
> but I figured that I should follow up.

Does this fix it? (works for me)

If so, I'll need to extend it for all arches.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

arch/x86/kernel/process.c

@@ -712,6 +712,7 @@ void arch_cpu_idle_enter(void)
 void arch_cpu_idle_dead(void)
 {
 	play_dead();
+	BUG();
 }
 
 /*

include/linux/cpu.h

@@ -185,7 +185,7 @@ void arch_cpu_idle(void);
 void arch_cpu_idle_prepare(void);
 void arch_cpu_idle_enter(void);
 void arch_cpu_idle_exit(void);
-void arch_cpu_idle_dead(void);
+void __noreturn arch_cpu_idle_dead(void);
 
 int cpu_report_state(int cpu);
 int cpu_check_up_prepare(int cpu);

kernel/sched/idle.c

@@ -71,7 +71,7 @@ static noinline int __cpuidle cpu_idle_poll(void)
 void __weak arch_cpu_idle_prepare(void) { }
 void __weak arch_cpu_idle_enter(void) { }
 void __weak arch_cpu_idle_exit(void) { }
-void __weak arch_cpu_idle_dead(void) { }
+void __weak arch_cpu_idle_dead(void) { while (1); }
 void __weak arch_cpu_idle(void)
 {
 	cpu_idle_force_poll = 1;

tools/objtool/check.c

@@ -167,6 +167,7 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
 		"__reiserfs_panic",
 		"__stack_chk_fail",
 		"__ubsan_handle_builtin_unreachable",
+		"arch_cpu_idle_dead",
 		"cpu_bringup_and_idle",
 		"cpu_startup_entry",
 		"do_exit",