[PATCH v4] HID: wacom: avoid divide exception in dropped-packet calculation

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

The dropped-packet calculation uses a modulo against sequence_size
(logical_maximum - logical_minimum + 1).  When a HID descriptor
declares a sequence field whose logical range spans all 32-bit
values, sequence_size wraps to zero and the modulo triggers a
divide exception.

Replace the division-based calculation with a branch that computes
the forward distance through the sequence range directly, as
suggested by Jason Gerecke.  This keeps the existing int types and
avoids the divide-by-zero without any 64-bit arithmetic.

Fixes: 359673ea3a20 ("HID: wacom: Support sequence numbers smaller than 16-bit")
Cc: stable@vger.kernel.org
Suggested-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@gmail.com>
---

Changes in v4:
- Replace s64 arithmetic with a branch-based helper that computes
  the forward wrapped distance directly, as suggested by Jason Gerecke.
  Keeps sequence_number as int; no type changes needed.

Changes in v3:
- Replace raw s64 modulo with div64_s64()-based remainder to avoid
  __moddi3 link failure on 32-bit architectures (i386, arm32).

Changes in v2:
- Normalize logical_maximum/value as unsigned when logical_minimum >= 0,
  matching HID parser signedness. Store sequence state as s64.

 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..1d3ae94 100644
--- a/drivers/hid/wacom_wac.c
+++ b/drivers/hid/wacom_wac.c
@@ -2426,6 +2426,13 @@ static void wacom_wac_pen_usage_mapping(struct hid_device *hdev,
 	}
 }
 
+static int wacom_hid_wrapped_dist(int after, int before, int min, int max)
+{
+	if (after >= before)
+		return after - before;
+	return (max - before) + (after - min) + 1;
+}
+
 static void wacom_wac_pen_event(struct hid_device *hdev, struct hid_field *field,
 		struct hid_usage *usage, __s32 value)
 {
@@ -2551,8 +2558,11 @@ static void wacom_wac_pen_event(struct hid_device *hdev, struct hid_field *field
 	case WACOM_HID_WD_SEQUENCENUMBER:
 		if (wacom_wac->hid_data.sequence_number != value &&
 		    wacom_wac->hid_data.sequence_number >= 0) {
-			int sequence_size = field->logical_maximum - field->logical_minimum + 1;
-			int drop_count = (value - wacom_wac->hid_data.sequence_number) % sequence_size;
+			int drop_count = wacom_hid_wrapped_dist(
+				value,
+				wacom_wac->hid_data.sequence_number,
+				field->logical_minimum,
+				field->logical_maximum);
 			hid_warn(hdev, "Dropped %d packets", drop_count);
 		}
 		wacom_wac->hid_data.sequence_number = value + 1;
-- 
2.53.0