HID: wacom: generic: Treat serial number and related fields as unsigned

The HID descriptors for most Wacom devices oddly declare the serial
number and other related fields as signed integers. When these numbers
are ingested by the HID subsystem, they are automatically sign-extended
into 32-bit integers. We treat the fields as unsigned elsewhere in the
kernel and userspace, however, so this sign-extension causes problems.
In particular, the sign-extended tool ID sent to userspace as ABS_MISC
does not properly match unsigned IDs used by xf86-input-wacom and libwacom.

We introduce a function 'wacom_s32tou' that can undo the automatic sign
extension performed by 'hid_snto32'. We call this function when processing
the serial number and related fields to ensure that we are dealing with
and reporting the unsigned form. We opt to use this method rather than
adding a descriptor fixup in 'wacom_hid_usage_quirk' since it should be
more robust in the face of future devices.

Ref: https://github.com/linuxwacom/input-wacom/issues/134
Fixes: f85c9dc6 ("HID: wacom: generic: Support tool ID and additional tool types")
CC: <stable@vger.kernel.org> # v4.10+
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Aaron Armstrong Skomra <aaron.skomra@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/wacom.h

@@ -202,6 +202,21 @@ static inline void wacom_schedule_work(struct wacom_wac *wacom_wac,
 	}
 }
 
+/*
+ * Convert a signed 32-bit integer to an unsigned n-bit integer. Undoes
+ * the normally-helpful work of 'hid_snto32' for fields that use signed
+ * ranges for questionable reasons.
+ */
+static inline __u32 wacom_s32tou(s32 value, __u8 n)
+{
+	switch (n) {
+	case 8:  return ((__u8)value);
+	case 16: return ((__u16)value);
+	case 32: return ((__u32)value);
+	}
+	return value & (1 << (n - 1)) ? value & (~(~0U << n)) : value;
+}
+
 extern const struct hid_device_id wacom_ids[];
 
 void wacom_wac_irq(struct wacom_wac *wacom_wac, size_t len);

drivers/hid/wacom_wac.c

@@ -2303,7 +2303,7 @@ static void wacom_wac_pen_event(struct hid_device *hdev, struct hid_field *field
 	case HID_DG_TOOLSERIALNUMBER:
 		if (value) {
 			wacom_wac->serial[0] = (wacom_wac->serial[0] & ~0xFFFFFFFFULL);
-			wacom_wac->serial[0] |= (__u32)value;
+			wacom_wac->serial[0] |= wacom_s32tou(value, field->report_size);
 		}
 		return;
 	case HID_DG_TWIST:
@@ -2319,15 +2319,17 @@ static void wacom_wac_pen_event(struct hid_device *hdev, struct hid_field *field
 		return;
 	case WACOM_HID_WD_SERIALHI:
 		if (value) {
+			__u32 raw_value = wacom_s32tou(value, field->report_size);
+
 			wacom_wac->serial[0] = (wacom_wac->serial[0] & 0xFFFFFFFF);
-			wacom_wac->serial[0] |= ((__u64)value) << 32;
+			wacom_wac->serial[0] |= ((__u64)raw_value) << 32;
 			/*
 			 * Non-USI EMR devices may contain additional tool type
 			 * information here. See WACOM_HID_WD_TOOLTYPE case for
 			 * more details.
 			 */
 			if (value >> 20 == 1) {
-				wacom_wac->id[0] |= value & 0xFFFFF;
+				wacom_wac->id[0] |= raw_value & 0xFFFFF;
 			}
 		}
 		return;
@@ -2339,7 +2341,7 @@ static void wacom_wac_pen_event(struct hid_device *hdev, struct hid_field *field
 		 * bitwise OR so the complete value can be built
 		 * up over time :(
 		 */
-		wacom_wac->id[0] |= value;
+		wacom_wac->id[0] |= wacom_s32tou(value, field->report_size);
 		return;
 	case WACOM_HID_WD_OFFSETLEFT:
 		if (features->offset_left && value != features->offset_left)