cpu/hotplug: Prevent overwriting of callbacks

Developers manage to overwrite states blindly without thought. That's fatal
and hard to debug. Add sanity checks to make it fail.

This requries to restructure the code so that the dynamic state allocation
happens in the same lock protected section as the actual store. Otherwise
the previous assignment of 'Reserved' to the name field would trigger the
overwrite check.

Signed-off-by: Thomas Gleixner <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Sebastian Siewior <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Thomas Gleixner <[e-mail]>

kernel/cpu.c

···
 	return 0;
 }
 
-static void cpuhp_store_callbacks(enum cpuhp_state state,
-				  const char *name,
-				  int (*startup)(unsigned int cpu),
-				  int (*teardown)(unsigned int cpu),
-				  bool multi_instance)
+/*
+ * Returns a free for dynamic slot assignment of the Online state. The states
+ * are protected by the cpuhp_slot_states mutex and an empty slot is identified
+ * by having no name assigned.
+ */
+static int cpuhp_reserve_state(enum cpuhp_state state)
+{
+	enum cpuhp_state i;
+
+	for (i = CPUHP_AP_ONLINE_DYN; i <= CPUHP_AP_ONLINE_DYN_END; i++) {
+		if (!cpuhp_ap_states[i].name)
+			return i;
+	}
+	WARN(1, "No more dynamic states available for CPU hotplug\n");
+	return -ENOSPC;
+}
+
+static int cpuhp_store_callbacks(enum cpuhp_state state, const char *name,
+				 int (*startup)(unsigned int cpu),
+				 int (*teardown)(unsigned int cpu),
+				 bool multi_instance)
 {
 	/* (Un)Install the callbacks for further cpu hotplug operations */
 	struct cpuhp_step *sp;
+	int ret = 0;
 
 	mutex_lock(&cpuhp_state_mutex);
+
+	if (state == CPUHP_AP_ONLINE_DYN) {
+		ret = cpuhp_reserve_state(state);
+		if (ret < 0)
+			goto out;
+		state = ret;
+	}
 	sp = cpuhp_get_step(state);
+	if (name && sp->name) {
+		ret = -EBUSY;
+		goto out;
+	}
 	sp->startup.single = startup;
 	sp->teardown.single = teardown;
 	sp->name = name;
 	sp->multi_instance = multi_instance;
 	INIT_HLIST_HEAD(&sp->list);
+out:
 	mutex_unlock(&cpuhp_state_mutex);
+	return ret;
 }
 
 static void *cpuhp_get_teardown_cb(enum cpuhp_state state)
···
 	}
 }
 
-/*
- * Returns a free for dynamic slot assignment of the Online state. The states
- * are protected by the cpuhp_slot_states mutex and an empty slot is identified
- * by having no name assigned.
- */
-static int cpuhp_reserve_state(enum cpuhp_state state)
-{
-	enum cpuhp_state i;
-
-	mutex_lock(&cpuhp_state_mutex);
-	for (i = CPUHP_AP_ONLINE_DYN; i <= CPUHP_AP_ONLINE_DYN_END; i++) {
-		if (cpuhp_ap_states[i].name)
-			continue;
-
-		cpuhp_ap_states[i].name = "Reserved";
-		mutex_unlock(&cpuhp_state_mutex);
-		return i;
-	}
-	mutex_unlock(&cpuhp_state_mutex);
-	WARN(1, "No more dynamic states available for CPU hotplug\n");
-	return -ENOSPC;
-}
-
 int __cpuhp_state_add_instance(enum cpuhp_state state, struct hlist_node *node,
 			       bool invoke)
 {
···
 
 /**
  * __cpuhp_setup_state - Setup the callbacks for an hotplug machine state
- * @state:	The state to setup
- * @invoke:	If true, the startup function is invoked for cpus where
- *		cpu state >= @state
- * @startup:	startup callback function
- * @teardown:	teardown callback function
+ * @state:		The state to setup
+ * @invoke:		If true, the startup function is invoked for cpus where
+ *			cpu state >= @state
+ * @startup:		startup callback function
+ * @teardown:		teardown callback function
+ * @multi_instance:	State is set up for multiple instances which get
+ *			added afterwards.
  *
  * Returns:
  *   On success:
···
 			bool multi_instance)
 {
 	int cpu, ret = 0;
-	int dyn_state = 0;
 
 	if (cpuhp_cb_check(state) || !name)
 		return -EINVAL;
 
 	get_online_cpus();
 
-	/* currently assignments for the ONLINE state are possible */
-	if (state == CPUHP_AP_ONLINE_DYN) {
-		dyn_state = 1;
-		ret = cpuhp_reserve_state(state);
-		if (ret < 0)
-			goto out;
-		state = ret;
-	}
+	ret = cpuhp_store_callbacks(state, name, startup, teardown,
+				    multi_instance);
 
-	cpuhp_store_callbacks(state, name, startup, teardown, multi_instance);
-
-	if (!invoke || !startup)
+	if (ret || !invoke || !startup)
 		goto out;
 
 	/*
···
 	}
 out:
 	put_online_cpus();
-	if (!ret && dyn_state)
+	/*
+	 * If the requested state is CPUHP_AP_ONLINE_DYN, return the
+	 * dynamically allocated state in case of success.
+	 */
+	if (!ret && state == CPUHP_AP_ONLINE_DYN)
 		return state;
 	return ret;
 }