[PATCH] HID: rmi: check sanity of the incoming report

[Benjamin Tissoires <benjamin.tissoires@redhat.com>]

In the Dell XPS 13 9333, it appears that sometimes the bus get confused
and corrupts the incoming data. It fills the input report with the
sentinel value "ff". Synaptics told us that such behavior does not comes
from the touchpad itself, so we filter out such reports here.

Unfortunately, we can not simply discard the incoming data because they
may contain useful information. Most of the time, the misbehavior is
quite near the end of the report, so we can still use the valid part of
it.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=1123584

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
---
 drivers/hid/hid-rmi.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/drivers/hid/hid-rmi.c b/drivers/hid/hid-rmi.c
index 8389e81..db92c3b 100644
--- a/drivers/hid/hid-rmi.c
+++ b/drivers/hid/hid-rmi.c
@@ -320,9 +320,6 @@ static int rmi_f11_input_event(struct hid_device *hdev, u8 irq, u8 *data,
 	int offset;
 	int i;
 
-	if (size < hdata->f11.report_size)
-		return 0;
-
 	if (!(irq & hdata->f11.irq_mask))
 		return 0;
 
@@ -332,9 +329,13 @@ static int rmi_f11_input_event(struct hid_device *hdev, u8 irq, u8 *data,
 		int fs_bit_position = (i & 0x3) << 1;
 		int finger_state = (data[fs_byte_position] >> fs_bit_position) &
 					0x03;
+		int position = offset + 5 * i;
+
+		if (position + 5 > size)
+			/* partial report, go on with what we received */
+			break;
 
-		rmi_f11_process_touch(hdata, i, finger_state,
-				&data[offset + 5 * i]);
+		rmi_f11_process_touch(hdata, i, finger_state, &data[position]);
 	}
 	input_mt_sync_frame(hdata->input);
 	input_sync(hdata->input);
@@ -412,9 +413,29 @@ static int rmi_read_data_event(struct hid_device *hdev, u8 *data, int size)
 	return 1;
 }
 
+static int rmi_check_sanity(struct hid_device *hdev, u8 *data, int size)
+{
+	int valid_size = size;
+	/*
+	 * On the Dell XPS 13 9333, the bus sometimes get confused and fills
+	 * the report with a sentinel value "ff". Synaptics told us that such
+	 * behavior does not comes from the touchpad itself, so we filter out
+	 * such reports here.
+	 */
+
+	while ((data[valid_size - 1] == 0xff) && valid_size > 0)
+		valid_size--;
+
+	return valid_size;
+}
+
 static int rmi_raw_event(struct hid_device *hdev,
 		struct hid_report *report, u8 *data, int size)
 {
+	size = rmi_check_sanity(hdev, data, size);
+	if (size < 2)
+		return 0;
+
 	switch (data[0]) {
 	case RMI_READ_DATA_REPORT_ID:
 		return rmi_read_data_event(hdev, data, size);
-- 
2.1.0