evdev: drain any pending evdev events on a device

open_restricted() doesn't always mean 'open the fd'. When the X server uses
systemd-logind, the fd is opened once before PreInit and then kept open across
devices being disabled and enabled through the protocol.

When the device is re-enabled and libinput_path_add_device is called for the
device, we may have events pending on the fd, leaking information that we
should just ignore.

There's also the potential of inconsistent state. The kernel updates the
device state whenever it processes an event, the evdev ioctls return that
state. If events are pending, the state we see is newer than the events we
process immediately after initialization. That can lead to confusion.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>

src/evdev.c

@@ -2202,6 +2202,17 @@ evdev_set_device_group(struct evdev_device *device,
 	return 0;
 }
 
+static inline void
+evdev_drain_fd(int fd)
+{
+	struct input_event ev[24];
+	size_t sz = sizeof ev;
+
+	while (read(fd, &ev, sz) == (int)sz) {
+		/* discard all pending events */
+	}
+}
+
 struct evdev_device *
 evdev_device_create(struct libinput_seat *seat,
 		    struct udev_device *udev_device)
@@ -2235,6 +2246,8 @@ evdev_device_create(struct libinput_seat *seat,
 	libinput_device_init(&device->base, seat);
 	libinput_seat_ref(seat);
 
+	evdev_drain_fd(fd);
+
 	rc = libevdev_new_from_fd(fd, &device->evdev);
 	if (rc != 0)
 		goto err;
@@ -2682,6 +2695,8 @@ evdev_device_resume(struct evdev_device *device)
 		return -ENODEV;
 	}
 
+	evdev_drain_fd(fd);
+
 	device->fd = fd;
 
 	if (evdev_need_mtdev(device)) {

test/misc.c

@@ -693,6 +693,87 @@ START_TEST(time_conversion)
 }
 END_TEST
 
+static int open_restricted_leak(const char *path, int flags, void *data)
+{
+	return *(int*)data;
+}
+
+static void close_restricted_leak(int fd, void *data)
+{
+	/* noop */
+}
+
+const struct libinput_interface leak_interface = {
+	.open_restricted = open_restricted_leak,
+	.close_restricted = close_restricted_leak,
+};
+
+static void
+simple_log_handler(struct libinput *libinput,
+		   enum libinput_log_priority priority,
+		   const char *format,
+		   va_list args)
+{
+	vfprintf(stderr, format, args);
+}
+
+START_TEST(fd_no_event_leak)
+{
+	struct libevdev_uinput *uinput;
+	struct libinput *li;
+	struct libinput_device *device;
+	int fd = -1;
+	const char *path;
+	struct libinput_event *event;
+
+	uinput = create_simple_test_device("litest test device",
+					   EV_REL, REL_X,
+					   EV_REL, REL_Y,
+					   EV_KEY, BTN_LEFT,
+					   EV_KEY, BTN_MIDDLE,
+					   EV_KEY, BTN_LEFT,
+					   -1, -1);
+	path = libevdev_uinput_get_devnode(uinput);
+
+	fd = open(path, O_RDWR | O_NONBLOCK | O_CLOEXEC);
+	ck_assert_int_gt(fd, -1);
+
+	li = libinput_path_create_context(&leak_interface, &fd);
+	libinput_log_set_priority(li, LIBINPUT_LOG_PRIORITY_DEBUG);
+	libinput_log_set_handler(li, simple_log_handler);
+
+	/* Add the device, trigger an event, then remove it again.
+	 * Without it, we get a SYN_DROPPED immediately and no events.
+	 */
+	device = libinput_path_add_device(li, path);
+	libevdev_uinput_write_event(uinput, EV_REL, REL_X, 1);
+	libevdev_uinput_write_event(uinput, EV_SYN, SYN_REPORT, 0);
+	libinput_path_remove_device(device);
+	libinput_dispatch(li);
+	litest_drain_events(li);
+
+	/* Device is removed, but fd is still open. Queue an event, add a
+	 * new device with the same fd, the queued event must be discarded
+	 * by libinput */
+	libevdev_uinput_write_event(uinput, EV_REL, REL_Y, 1);
+	libevdev_uinput_write_event(uinput, EV_SYN, SYN_REPORT, 0);
+	libinput_dispatch(li);
+
+	libinput_path_add_device(li, path);
+	libinput_dispatch(li);
+	event = libinput_get_event(li);
+	ck_assert_int_eq(libinput_event_get_type(event),
+			 LIBINPUT_EVENT_DEVICE_ADDED);
+	libinput_event_destroy(event);
+
+	litest_assert_empty_queue(li);
+
+	close(fd);
+	libinput_unref(li);
+	libevdev_uinput_destroy(uinput);
+}
+END_TEST
+
 void
 litest_setup_tests(void)
 {
@@ -714,4 +795,6 @@ litest_setup_tests(void)
 	litest_add_no_device("misc:parser", trackpoint_accel_parser);
 	litest_add_no_device("misc:parser", dimension_prop_parser);
 	litest_add_no_device("misc:time", time_conversion);
+
+	litest_add_no_device("misc:fd", fd_no_event_leak);
 }