[PATCH 13/18] HID: logitech-hidpp: store device serial as a byte array

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

Device serial numbers are read from 4-byte protocol payloads only to be
formatted as raw byte sequences with %4phD. Storing them as u32 requires
unaligned pointer casts and dereferences that depend on endian-neutral
memory layout, and the FAP-based serial helper reads from the RAP union
member instead of the matching FAP payload.

Represent device serial numbers as 4-byte arrays, copy the payload bytes
with a fixed serial length constant, and read the FAP device information
response from the FAP payload directly.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
---
 drivers/hid/hid-logitech-hidpp.c | 28 +++++++++++++---------------
 1 file changed, 13 insertions(+), 15 deletions(-)

diff --git a/drivers/hid/hid-logitech-hidpp.c b/drivers/hid/hid-logitech-hidpp.c
index c4ebfc4d66fe..3a3baeb9fbb9 100644
--- a/drivers/hid/hid-logitech-hidpp.c
+++ b/drivers/hid/hid-logitech-hidpp.c
@@ -48,6 +48,7 @@ MODULE_PARM_DESC(disable_tap_to_click,
 #define HIDPP_REPORT_SHORT_LENGTH		7
 #define HIDPP_REPORT_LONG_LENGTH		20
 #define HIDPP_REPORT_VERY_LONG_MAX_LENGTH	64
+#define HIDPP_SERIAL_LENGTH			4
 
 #define HIDPP_REPORT_SHORT_SUPPORTED		BIT(0)
 #define HIDPP_REPORT_LONG_SUPPORTED		BIT(1)
@@ -889,7 +890,8 @@ static char *hidpp_unifying_get_name(struct hidpp_device *hidpp_dev)
 	return name;
 }
 
-static int hidpp_unifying_get_serial(struct hidpp_device *hidpp, u32 *serial)
+static int hidpp_unifying_get_serial(struct hidpp_device *hidpp,
+				     u8 serial[HIDPP_SERIAL_LENGTH])
 {
 	struct hidpp_report response;
 	int ret;
@@ -903,11 +905,7 @@ static int hidpp_unifying_get_serial(struct hidpp_device *hidpp, u32 *serial)
 	if (ret)
 		return ret;
 
-	/*
-	 * We don't care about LE or BE, we will output it as a string
-	 * with %4phD, so we need to keep the order.
-	 */
-	*serial = *((u32 *)&response.rap.params[1]);
+	memcpy(serial, &response.rap.params[1], HIDPP_SERIAL_LENGTH);
 	return 0;
 }
 
@@ -915,14 +913,14 @@ static int hidpp_unifying_init(struct hidpp_device *hidpp)
 {
 	struct hid_device *hdev = hidpp->hid_dev;
 	const char *name;
-	u32 serial;
+	u8 serial[HIDPP_SERIAL_LENGTH];
 	int ret;
 
-	ret = hidpp_unifying_get_serial(hidpp, &serial);
+	ret = hidpp_unifying_get_serial(hidpp, serial);
 	if (ret)
 		return ret;
 
-	snprintf(hdev->uniq, sizeof(hdev->uniq), "%4phD", &serial);
+	snprintf(hdev->uniq, sizeof(hdev->uniq), "%4phD", serial);
 	dbg_hid("HID++ Unifying: Got serial: %s\n", hdev->uniq);
 
 	name = hidpp_unifying_get_name(hidpp);
@@ -1030,7 +1028,8 @@ static int hidpp_root_get_protocol_version(struct hidpp_device *hidpp)
 
 #define CMD_GET_DEVICE_INFO				0x00
 
-static int hidpp_get_serial(struct hidpp_device *hidpp, u32 *serial)
+static int hidpp_get_serial(struct hidpp_device *hidpp,
+			    u8 serial[HIDPP_SERIAL_LENGTH])
 {
 	struct hidpp_report response;
 	u8 feature_index;
@@ -1047,22 +1046,21 @@ static int hidpp_get_serial(struct hidpp_device *hidpp, u32 *serial)
 	if (ret)
 		return ret;
 
-	/* See hidpp_unifying_get_serial() */
-	*serial = *((u32 *)&response.rap.params[1]);
+	memcpy(serial, &response.fap.params[1], HIDPP_SERIAL_LENGTH);
 	return 0;
 }
 
 static int hidpp_serial_init(struct hidpp_device *hidpp)
 {
 	struct hid_device *hdev = hidpp->hid_dev;
-	u32 serial;
+	u8 serial[HIDPP_SERIAL_LENGTH];
 	int ret;
 
-	ret = hidpp_get_serial(hidpp, &serial);
+	ret = hidpp_get_serial(hidpp, serial);
 	if (ret)
 		return ret;
 
-	snprintf(hdev->uniq, sizeof(hdev->uniq), "%4phD", &serial);
+	snprintf(hdev->uniq, sizeof(hdev->uniq), "%4phD", serial);
 	dbg_hid("HID++ DeviceInformation: Got serial: %s\n", hdev->uniq);
 
 	return 0;
-- 
2.56.0.506.g506a7eabc5-goog