can: do not copy the payload of RTR frames

The actual payload length of the CAN Remote Transmission Request (RTR)
frames is always 0, i.e. no payload is transmitted on the wire.
However, those RTR frames still use the DLC to indicate the length of
the requested frame.

For this reason, it is incorrect to copy the payload of RTR frames
(the payload buffer would only contain garbage data). This patch
encapsulates the payload copy in a check toward the RTR flag.

Link: https://lore.kernel.org/all/20211207121531.42941-4-mailhol.vincent@wanadoo.fr
Cc: Yasushi SHOJI <yashi@spacecubics.com>
Tested-by: Yasushi SHOJI <yashi@spacecubics.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/pch_can.c

@@ -677,17 +677,18 @@ static int pch_can_rx_normal(struct net_device *ndev, u32 obj_num, int quota)
 			cf->can_id = id;
 		}
 
-		if (id2 & PCH_ID2_DIR)
-			cf->can_id |= CAN_RTR_FLAG;
-
 		cf->len = can_cc_dlc2len((ioread32(&priv->regs->
 						    ifregs[0].mcont)) & 0xF);
 
+		if (id2 & PCH_ID2_DIR) {
+			cf->can_id |= CAN_RTR_FLAG;
+		} else {
 			for (i = 0; i < cf->len; i += 2) {
 				data_reg = ioread16(&priv->regs->ifregs[0].data[i / 2]);
 				cf->data[i] = data_reg;
 				cf->data[i + 1] = data_reg >> 8;
 			}
+		}
 
 		rcv_pkts++;
 		stats->rx_packets++;

drivers/net/can/spi/mcp251x.c

@@ -730,6 +730,7 @@ static void mcp251x_hw_rx(struct spi_device *spi, int buf_idx)
 	}
 	/* Data length */
 	frame->len = can_cc_dlc2len(buf[RXBDLC_OFF] & RXBDLC_LEN_MASK);
+	if (!(frame->can_id & CAN_RTR_FLAG))
 		memcpy(frame->data, buf + RXBDAT_OFF, frame->len);
 
 	priv->net->stats.rx_packets++;

drivers/net/can/usb/mcba_usb.c

@@ -450,11 +450,11 @@ static void mcba_usb_process_can(struct mcba_priv *priv,
 		cf->can_id = (sid & 0xffe0) >> 5;
 	}
 
-	if (msg->dlc & MCBA_DLC_RTR_MASK)
-		cf->can_id |= CAN_RTR_FLAG;
-
 	cf->len = can_cc_dlc2len(msg->dlc & MCBA_DLC_MASK);
 
+	if (msg->dlc & MCBA_DLC_RTR_MASK)
+		cf->can_id |= CAN_RTR_FLAG;
+	else
 		memcpy(cf->data, msg->data, cf->len);
 
 	stats->rx_packets++;