[PATCH][next] USB: serial: garmin_gps: Use struct_size() and flex_array_size() helpers in pkt_add()

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

Make use of the struct_size() and flex_array_size() helpers instead of
an open-coded version, in order to avoid any potential type mistakes
or integer overflows that, in the worst scenario, could lead to heap
overflows.

Also, address the following sparse warnings:
drivers/usb/serial/garmin_gps.c:270:31: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/160
Link: https://github.com/KSPP/linux/issues/174
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 drivers/usb/serial/garmin_gps.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/serial/garmin_gps.c b/drivers/usb/serial/garmin_gps.c
index e5c75944ebb7..1d806c108efb 100644
--- a/drivers/usb/serial/garmin_gps.c
+++ b/drivers/usb/serial/garmin_gps.c
@@ -267,13 +267,12 @@ static int pkt_add(struct garmin_data *garmin_data_p,
 
 	/* process only packets containing data ... */
 	if (data_length) {
-		pkt = kmalloc(sizeof(struct garmin_packet)+data_length,
-								GFP_ATOMIC);
+		pkt = kmalloc(struct_size(pkt, data, data_length), GFP_ATOMIC);
 		if (!pkt)
 			return 0;
 
 		pkt->size = data_length;
-		memcpy(pkt->data, data, data_length);
+		memcpy(pkt->data, data, flex_array_size(pkt, data, pkt->size));
 
 		spin_lock_irqsave(&garmin_data_p->lock, flags);
 		garmin_data_p->flags |= FLAGS_QUEUING;
-- 
2.27.0