Input: fix regression when re-registering input handlers

Commit d469647bafd9 ("Input: simplify event handling logic") introduced
code that would set handler->events() method to either
input_handler_events_filter() or input_handler_events_default() or
input_handler_events_null(), depending on the kind of input handler
(a filter or a regular one) we are dealing with. Unfortunately this
breaks cases when we try to re-register the same filter (as is the case
with sysrq handler): after initial registration the handler will have 2
event handling methods defined, and will run afoul of the check in
input_handler_check_methods():

input: input_handler_check_methods: only one event processing method can be defined (sysrq)
sysrq: Failed to register input handler, error -22

Fix this by adding handle_events() method to input_handle structure and
setting it up when registering a new input handle according to event
handling methods defined in associated input_handler structure, thus
avoiding modifying the input_handler structure.

Reported-by: "Ned T. Crigler" <[e-mail]>
Reported-by: Christian Heusel <[e-mail]>
Tested-by: "Ned T. Crigler" <[e-mail]>
Tested-by: Peter Seiderer <[e-mail]>
Fixes: d469647bafd9 ("Input: simplify event handling logic")
Link: https://lore.kernel.org/r/Zx2iQp6csn42PJA7@xavtug
Cc: [e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/input.c

···
 
 	handle = rcu_dereference(dev->grab);
 	if (handle) {
-		count = handle->handler->events(handle, vals, count);
+		count = handle->handle_events(handle, vals, count);
 	} else {
 		list_for_each_entry_rcu(handle, &dev->h_list, d_node)
 			if (handle->open) {
-				count = handle->handler->events(handle, vals,
-								count);
+				count = handle->handle_events(handle, vals,
+							      count);
 				if (!count)
 					break;
 			}
···
 	return 0;
 }
 
-/*
- * An implementation of input_handler's events() method that simply
- * invokes handler->event() method for each event one by one.
- */
-static unsigned int input_handler_events_default(struct input_handle *handle,
-						 struct input_value *vals,
-						 unsigned int count)
-{
-	struct input_handler *handler = handle->handler;
-	struct input_value *v;
-
-	for (v = vals; v != vals + count; v++)
-		handler->event(handle, v->type, v->code, v->value);
-
-	return count;
-}
-
-/*
- * An implementation of input_handler's events() method that invokes
- * handler->filter() method for each event one by one and removes events
- * that were filtered out from the "vals" array.
- */
-static unsigned int input_handler_events_filter(struct input_handle *handle,
-						struct input_value *vals,
-						unsigned int count)
-{
-	struct input_handler *handler = handle->handler;
-	struct input_value *end = vals;
-	struct input_value *v;
-
-	for (v = vals; v != vals + count; v++) {
-		if (handler->filter(handle, v->type, v->code, v->value))
-			continue;
-		if (end != v)
-			*end = *v;
-		end++;
-	}
-
-	return end - vals;
-}
-
-/*
- * An implementation of input_handler's events() method that does nothing.
- */
-static unsigned int input_handler_events_null(struct input_handle *handle,
-					      struct input_value *vals,
-					      unsigned int count)
-{
-	return count;
-}
-
 /**
  * input_register_handler - register a new input handler
  * @handler: handler to be registered
···
 		return error;
 
 	INIT_LIST_HEAD(&handler->h_list);
-
-	if (handler->filter)
-		handler->events = input_handler_events_filter;
-	else if (handler->event)
-		handler->events = input_handler_events_default;
-	else if (!handler->events)
-		handler->events = input_handler_events_null;
 
 	error = mutex_lock_interruptible(&input_mutex);
 	if (error)
···
 }
 EXPORT_SYMBOL(input_handler_for_each_handle);
 
+/*
+ * An implementation of input_handle's handle_events() method that simply
+ * invokes handler->event() method for each event one by one.
+ */
+static unsigned int input_handle_events_default(struct input_handle *handle,
+						struct input_value *vals,
+						unsigned int count)
+{
+	struct input_handler *handler = handle->handler;
+	struct input_value *v;
+
+	for (v = vals; v != vals + count; v++)
+		handler->event(handle, v->type, v->code, v->value);
+
+	return count;
+}
+
+/*
+ * An implementation of input_handle's handle_events() method that invokes
+ * handler->filter() method for each event one by one and removes events
+ * that were filtered out from the "vals" array.
+ */
+static unsigned int input_handle_events_filter(struct input_handle *handle,
+					       struct input_value *vals,
+					       unsigned int count)
+{
+	struct input_handler *handler = handle->handler;
+	struct input_value *end = vals;
+	struct input_value *v;
+
+	for (v = vals; v != vals + count; v++) {
+		if (handler->filter(handle, v->type, v->code, v->value))
+			continue;
+		if (end != v)
+			*end = *v;
+		end++;
+	}
+
+	return end - vals;
+}
+
+/*
+ * An implementation of input_handle's handle_events() method that does nothing.
+ */
+static unsigned int input_handle_events_null(struct input_handle *handle,
+					     struct input_value *vals,
+					     unsigned int count)
+{
+	return count;
+}
+
+/*
+ * Sets up appropriate handle->event_handler based on the input_handler
+ * associated with the handle.
+ */
+static void input_handle_setup_event_handler(struct input_handle *handle)
+{
+	struct input_handler *handler = handle->handler;
+
+	if (handler->filter)
+		handle->handle_events = input_handle_events_filter;
+	else if (handler->event)
+		handle->handle_events = input_handle_events_default;
+	else if (handler->events)
+		handle->handle_events = handler->events;
+	else
+		handle->handle_events = input_handle_events_null;
+}
+
 /**
  * input_register_handle - register a new input handle
  * @handle: handle to register
···
 	struct input_dev *dev = handle->dev;
 	int error;
 
+	input_handle_setup_event_handler(handle);
 	/*
 	 * We take dev->mutex here to prevent race with
 	 * input_release_device().

include/linux/input.h

···
  * @name: name given to the handle by handler that created it
  * @dev: input device the handle is attached to
  * @handler: handler that works with the device through this handle
+ * @handle_events: event sequence handler. It is set up by the input core
+ *	according to event handling method specified in the @handler. See
+ *	input_handle_setup_event_handler().
+ *	This method is being called by the input core with interrupts disabled
+ *	and dev->event_lock spinlock held and so it may not sleep.
  * @d_node: used to put the handle on device's list of attached handles
  * @h_node: used to put the handle on handler's list of handles from which
  *	it gets events
  */
 struct input_handle {
-
 	void *private;
 
 	int open;
···
 
 	struct input_dev *dev;
 	struct input_handler *handler;
+
+	unsigned int (*handle_events)(struct input_handle *handle,
+				      struct input_value *vals,
+				      unsigned int count);
 
 	struct list_head	d_node;
 	struct list_head	h_node;