udev-builtin-input_id: use heuristics to detect joysticks

Several keyboard devices are erroneously tagged with ID_INPUT_JOYSTICK
because of random buttons they set. For example, the LiteOn Lenovo
Calliope USB Keyboard sets BTN_TRIGGER, BTN_TOP2, BTN_PINKIE and
BTN_BASE, see libinput issue 745 for details.

ID_INPUT_JOYSTICK triggers the uaccess rules, making those keyboards
easily accessible. That's not a problem in the LiteOn example since that
event node doesn't contain the normal keys and eavesdropping on volume
keys is probably not very interesting.

Improve the joystick detection by adding heuristics similar to what
libinput 1.20 uses: check for some specific set of keys that are common
on keyboards but very unlikely on joysticks. If enough of those are
present (or the device has less than 2 axes or joysticks), don't tag it
as joystick.

libinput also checks for > 10 keyboard keys, but this is not done here
to be more conservative.

1 files changed, 44 insertions, 11 deletions

diff --git a/src/udev/udev-builtin-input_id.c b/src/udev/udev-builtin-input_id.c
index 55f7d3f44d..f04e29b8f8 100644
--- a/src/udev/udev-builtin-input_id.c
+++ b/src/udev/udev-builtin-input_id.c
@@ -156,7 +156,8 @@ static bool test_pointers(sd_device *dev,
         bool has_abs_coordinates = false;
         bool has_rel_coordinates = false;
         bool has_mt_coordinates = false;
-        bool has_joystick_axes_or_buttons = false;
+        size_t num_joystick_axes = 0;
+        size_t num_joystick_buttons = 0;
         bool has_pad_buttons = false;
         bool is_direct = false;
         bool has_touch = false;
@@ -214,15 +215,19 @@ static bool test_pointers(sd_device *dev,
          * Catz Mad Catz M.M.O.TE). Skip those.
          */
         if (!test_bit(BTN_JOYSTICK - 1, bitmask_key)) {
-                for (int button = BTN_JOYSTICK; button < BTN_DIGI && !has_joystick_axes_or_buttons; button++)
-                        has_joystick_axes_or_buttons = test_bit(button, bitmask_key);
-                for (int button = BTN_TRIGGER_HAPPY1; button <= BTN_TRIGGER_HAPPY40 && !has_joystick_axes_or_buttons; button++)
-                        has_joystick_axes_or_buttons = test_bit(button, bitmask_key);
-                for (int button = BTN_DPAD_UP; button <= BTN_DPAD_RIGHT && !has_joystick_axes_or_buttons; button++)
-                        has_joystick_axes_or_buttons = test_bit(button, bitmask_key);
+                for (int button = BTN_JOYSTICK; button < BTN_DIGI; button++)
+                        if (test_bit(button, bitmask_key))
+                                num_joystick_buttons++;
+                for (int button = BTN_TRIGGER_HAPPY1; button <= BTN_TRIGGER_HAPPY40; button++)
+                        if (test_bit(button, bitmask_key))
+                                num_joystick_buttons++;
+                for (int button = BTN_DPAD_UP; button <= BTN_DPAD_RIGHT; button++)
+                        if (test_bit(button, bitmask_key))
+                                num_joystick_buttons++;
         }
-        for (int axis = ABS_RX; axis < ABS_PRESSURE && !has_joystick_axes_or_buttons; axis++)
-                has_joystick_axes_or_buttons = test_bit(axis, bitmask_abs);
+        for (int axis = ABS_RX; axis < ABS_PRESSURE; axis++)
+                if (test_bit(axis, bitmask_abs))
+                        num_joystick_axes++;
 
         if (has_abs_coordinates) {
                 if (has_stylus || has_pen)
@@ -235,9 +240,9 @@ static bool test_pointers(sd_device *dev,
                         is_abs_mouse = true;
                 else if (has_touch || is_direct)
                         is_touchscreen = true;
-                else if (has_joystick_axes_or_buttons)
+                else if (num_joystick_buttons > 0 || num_joystick_axes > 0)
                         is_joystick = true;
-        } else if (has_joystick_axes_or_buttons)
+        } else if (num_joystick_buttons > 0 || num_joystick_axes > 0)
                 is_joystick = true;
 
         if (has_mt_coordinates) {
@@ -262,6 +267,34 @@ static bool test_pointers(sd_device *dev,
         if (is_mouse && id->bustype == BUS_I2C)
                 is_pointing_stick = true;
 
+        /* Joystick un-detection. Some keyboards have random joystick buttons
+         * set. Avoid those being labeled as ID_INPUT_JOYSTICK with some heuristics.
+         * The well-known keys represent a (randomly picked) set of key groups.
+         * A joystick may have one of those but probably not several. And a joystick with less than 2 buttons
+         * or axes is not a joystick either.
+         * libinput uses similar heuristics, any changes here should be added to libinput too.
+         */
+        if (is_joystick) {
+                static const unsigned int well_known_keyboard_keys[] = {
+                        KEY_LEFTCTRL, KEY_CAPSLOCK, KEY_NUMLOCK, KEY_INSERT,
+                        KEY_MUTE, KEY_CALC, KEY_FILE, KEY_MAIL, KEY_PLAYPAUSE,
+                        KEY_BRIGHTNESSDOWN,
+                };
+                size_t num_well_known_keys = 0;
+
+                if (has_keys)
+                        for (size_t i = 0; i < ELEMENTSOF(well_known_keyboard_keys); i++)
+                                if (test_bit(well_known_keyboard_keys[i], bitmask_key))
+                                        num_well_known_keys++;
+
+                if (num_well_known_keys >= 4 || num_joystick_buttons + num_joystick_axes < 2) {
+                        log_device_debug(dev, "Input device has %zd joystick buttons and %zd axes but also %zd keyboard key sets, "
+                                         "assuming this is a keyboard, not a joystick.",
+                                         num_joystick_buttons, num_joystick_axes, num_well_known_keys);
+                        is_joystick = false;
+                }
+        }
+
         if (is_pointing_stick)
                 udev_builtin_add_property(dev, test, "ID_INPUT_POINTINGSTICK", "1");
         if (is_mouse || is_abs_mouse)