Input: implement input filters

Sometimes it is desirable to suppress certain events from reaching
input handlers and thus user space. One such example is Mac mouse
button emulation code which catches certain key presses and converts
them into button clicks as if they were emitted by a virtual mouse.
The original key press events should be completely suppressed,
otherwise user space will be confused, and while keyboard driver
does it on its own evdev is blissfully unaware of this arrangement.

This patch adds notion of 'filter' to the standard input handlers,
which may flag event as filtered thus preventing it from reaching
other input handlers. Filters don't (nor will they ever) have a
notion of priority relative to each other, input core will run all
of them first and any one of them may mark event as filtered.

This patch is inspired by similar patch by Matthew Garret but the
implementation and intended usage are quite different.
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/input.c

@@ -86,12 +86,14 @@ static int input_defuzz_abs_event(int value, int old_val, int fuzz)
 }
 
 /*
- * Pass event through all open handles. This function is called with
+ * Pass event first through all filters and then, if event has not been
+ * filtered out, through all open handles. This function is called with
  * dev->event_lock held and interrupts disabled.
  */
 static void input_pass_event(struct input_dev *dev,
 			     unsigned int type, unsigned int code, int value)
 {
+	struct input_handler *handler;
 	struct input_handle *handle;
 
 	rcu_read_lock();
@@ -99,11 +101,25 @@ static void input_pass_event(struct input_dev *dev,
 	handle = rcu_dereference(dev->grab);
 	if (handle)
 		handle->handler->event(handle, type, code, value);
-	else
-		list_for_each_entry_rcu(handle, &dev->h_list, d_node)
-			if (handle->open)
-				handle->handler->event(handle,
-							type, code, value);
+	else {
+		bool filtered = false;
+
+		list_for_each_entry_rcu(handle, &dev->h_list, d_node) {
+			if (!handle->open)
+				continue;
+
+			handler = handle->handler;
+			if (!handler->filter) {
+				if (filtered)
+					break;
+
+				handler->event(handle, type, code, value);
+
+			} else if (handler->filter(handle, type, code, value))
+				filtered = true;
+		}
+	}
+
 	rcu_read_unlock();
 }
 
@@ -990,6 +1006,8 @@ static int input_handlers_seq_show(struct seq_file *seq, void *v)
 	union input_seq_state *state = (union input_seq_state *)&seq->private;
 
 	seq_printf(seq, "N: Number=%u Name=%s", state->pos, handler->name);
+	if (handler->filter)
+		seq_puts(seq, " (filter)");
 	if (handler->fops)
 		seq_printf(seq, " Minor=%d", handler->minor);
 	seq_putc(seq, '\n');
@@ -1803,7 +1821,16 @@ int input_register_handle(struct input_handle *handle)
 	error = mutex_lock_interruptible(&dev->mutex);
 	if (error)
 		return error;
-	list_add_tail_rcu(&handle->d_node, &dev->h_list);
+
+	/*
+	 * Filters go to the head of the list, normal handlers
+	 * to the tail.
+	 */
+	if (handler->filter)
+		list_add_rcu(&handle->d_node, &dev->h_list);
+	else
+		list_add_tail_rcu(&handle->d_node, &dev->h_list);
+
 	mutex_unlock(&dev->mutex);
 
 	/*

include/linux/input.h

@@ -1198,6 +1198,8 @@ struct input_handle;
  * @event: event handler. This method is being called by input core with
  *	interrupts disabled and dev->event_lock spinlock held and so
  *	it may not sleep
+ * @filter: similar to @event; separates normal event handlers from
+ *	"filters".
  * @connect: called when attaching a handler to an input device
  * @disconnect: disconnects a handler from input device
  * @start: starts handler for given handle. This function is called by
@@ -1219,6 +1221,11 @@ struct input_handle;
  * same time. All of them will get their copy of input event generated by
  * the device.
  *
+ * The very same structure is used to implement input filters. Input core
+ * allows filters to run first and will not pass event to regular handlers
+ * if any of the filters indicate that the event should be filtered (by
+ * returning %true from their filter() method).
+ *
  * Note that input core serializes calls to connect() and disconnect()
  * methods.
  */
@@ -1227,6 +1234,7 @@ struct input_handler {
 	void *private;
 
 	void (*event)(struct input_handle *handle, unsigned int type, unsigned int code, int value);
+	bool (*filter)(struct input_handle *handle, unsigned int type, unsigned int code, int value);
 	int (*connect)(struct input_handler *handler, struct input_dev *dev, const struct input_device_id *id);
 	void (*disconnect)(struct input_handle *handle);
 	void (*start)(struct input_handle *handle);