kernel/reboot: Change registration order of legacy power-off handler

We're unconditionally registering sys-off handler for the legacy
pm_power_off() callback, this causes problem for platforms that don't
use power-off handlers at all and should be halted. Now reboot syscall
assumes that there is a power-off handler installed and tries to power
off system instead of halting it.

To fix the trouble, move the handler's registration to the reboot syscall
and check the pm_power_off() presence.

Fixes: 0e2110d2 ("kernel/reboot: Add kernel_can_power_off()")
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/reboot.c

@@ -569,22 +569,6 @@ static int legacy_pm_power_off(struct sys_off_data *data)
 	return NOTIFY_DONE;
 }
 
-/*
- * Register sys-off handlers for legacy PM callbacks. This allows legacy
- * PM callbacks co-exist with the new sys-off API.
- *
- * TODO: Remove legacy handlers once all legacy PM users will be switched
- *       to the sys-off based APIs.
- */
-static int __init legacy_pm_init(void)
-{
-	register_sys_off_handler(SYS_OFF_MODE_POWER_OFF, SYS_OFF_PRIO_DEFAULT,
-				 legacy_pm_power_off, NULL);
-
-	return 0;
-}
-core_initcall(legacy_pm_init);
-
 static void do_kernel_power_off_prepare(void)
 {
 	blocking_notifier_call_chain(&power_off_prep_handler_list, 0, NULL);
@@ -646,6 +630,7 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
 		void __user *, arg)
 {
 	struct pid_namespace *pid_ns = task_active_pid_ns(current);
+	struct sys_off_handler *sys_off = NULL;
 	char buffer[256];
 	int ret = 0;
 
@@ -670,6 +655,21 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
 	if (ret)
 		return ret;
 
+	/*
+	 * Register sys-off handlers for legacy PM callback. This allows
+	 * legacy PM callbacks temporary co-exist with the new sys-off API.
+	 *
+	 * TODO: Remove legacy handlers once all legacy PM users will be
+	 *       switched to the sys-off based APIs.
+	 */
+	if (pm_power_off) {
+		sys_off = register_sys_off_handler(SYS_OFF_MODE_POWER_OFF,
+						   SYS_OFF_PRIO_DEFAULT,
+						   legacy_pm_power_off, NULL);
+		if (IS_ERR(sys_off))
+			return PTR_ERR(sys_off);
+	}
+
 	/* Instead of trying to make the power_off code look like
 	 * halt when pm_power_off is not set do it the easy way.
 	 */
@@ -727,6 +727,7 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
 		break;
 	}
 	mutex_unlock(&system_transition_mutex);
+	unregister_sys_off_handler(sys_off);
 	return ret;
 }