[PATCH v3] HID: wacom: validate battery range before scaling

[Aldo Ariel Panzardo <qwe.aldo@gmail.com>]

Battery events divide by the difference between the logical maximum
and minimum without validating the range.  A malformed HID descriptor
with equal limits can therefore trigger a divide-by-zero exception.

Move percentage conversion to a helper that rejects non-positive
ranges and accounts for a nonzero logical minimum.  Clamp the result
to [0, 100] so that hardware whose ADC occasionally reports slightly
beyond its declared limits still registers its battery and updates
capacity normally.

Fixes: 37d160193834 ("HID: wacom: generic: Scale battery capacity measurements to percentages")
Cc: stable@vger.kernel.org
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@gmail.com>
---

Thanks for the feedback, Jason -- you are right, the s64 stuff was
overkill here.  Much simpler this way.

Changes in v3:
- Drop s64 widening and div64_s64 as requested by Jason Gerecke.
  Use __s32 range with a <= 0 guard instead. Keep the clamping.

Changes in v2:
- Clamp the computed percentage to [0, 100] instead of returning
  false, so that slightly out-of-range ADC values do not prevent
  battery registration or silently drop updates.

 drivers/hid/wacom_wac.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/hid/wacom_wac.c b/drivers/hid/wacom_wac.c
index 361738b..36bab07 100644
--- a/drivers/hid/wacom_wac.c
+++ b/drivers/hid/wacom_wac.c
@@ -1960,6 +1960,16 @@ static void wacom_wac_battery_usage_mapping(struct hid_device *hdev,
 	return;
 }
 
+static __s32 wacom_wac_battery_percentage(struct hid_field *field, __s32 value)
+{
+	__s32 range = field->logical_maximum - field->logical_minimum;
+
+	if (range <= 0)
+		return 0;
+
+	return clamp_val((value - field->logical_minimum) * 100 / range, 0, 100);
+}
+
 static void wacom_wac_battery_event(struct hid_device *hdev, struct hid_field *field,
 		struct hid_usage *usage, __s32 value)
 {
@@ -1973,7 +1983,7 @@ static void wacom_wac_battery_event(struct hid_device *hdev, struct hid_field *f
 			wacom_wac->hid_data.bat_status = POWER_SUPPLY_STATUS_UNKNOWN;
 		}
 		else {
-			value = value * 100 / (field->logical_maximum - field->logical_minimum);
+			value = wacom_wac_battery_percentage(field, value);
 			wacom_wac->hid_data.battery_capacity = value;
 			wacom_wac->hid_data.bat_connected = 1;
 			wacom_wac->hid_data.bat_status = WACOM_POWER_SUPPLY_STATUS_AUTO;
@@ -1981,7 +1991,7 @@ static void wacom_wac_battery_event(struct hid_device *hdev, struct hid_field *f
 		wacom_wac->features.quirks |= WACOM_QUIRK_BATTERY;
 		break;
 	case WACOM_HID_WD_BATTERY_LEVEL:
-		value = value * 100 / (field->logical_maximum - field->logical_minimum);
+		value = wacom_wac_battery_percentage(field, value);
 		wacom_wac->hid_data.battery_capacity = value;
 		wacom_wac->hid_data.bat_connected = 1;
 		wacom_wac->hid_data.bat_status = WACOM_POWER_SUPPLY_STATUS_AUTO;
-- 
2.53.0