usbnet: Fix tx_bytes statistic running backward in cdc_ncm

cdc_ncm disagrees with usbnet about how much framing overhead should
be counted in the tx_bytes statistics, and tries 'fix' this by
decrementing tx_bytes on the transmit path. But statistics must never
be decremented except due to roll-over; this will thoroughly confuse
user-space. Also, tx_bytes is only incremented by usbnet in the
completion path.

Fix this by requiring drivers that set FLAG_MULTI_FRAME to set a
tx_bytes delta along with the tx_packets count.

Fixes: beeecd42c3b4 ("net: cdc_ncm/cdc_mbim: adding NCM protocol statistics")
Signed-off-by: Ben Hutchings <[e-mail]>
Signed-off-by: Bjørn Mork <[e-mail]>

drivers/net/usb/asix_common.c

···
 		skb_put(skb, sizeof(padbytes));
 	}
 
-	usbnet_set_skb_tx_stats(skb, 1);
+	usbnet_set_skb_tx_stats(skb, 1, 0);
 	return skb;
 }
 

drivers/net/usb/cdc_ncm.c

···
 	ctx->tx_overhead += skb_out->len - ctx->tx_curr_frame_payload;
 	ctx->tx_ntbs++;
 
-	/* usbnet has already counted all the framing overhead.
+	/* usbnet will count all the framing overhead by default.
 	 * Adjust the stats so that the tx_bytes counter show real
 	 * payload data instead.
 	 */
-	dev->net->stats.tx_bytes -= skb_out->len - ctx->tx_curr_frame_payload;
-
-	usbnet_set_skb_tx_stats(skb_out, n);
+	usbnet_set_skb_tx_stats(skb_out, n,
+				ctx->tx_curr_frame_payload - skb_out->len);
 
 	return skb_out;
 

drivers/net/usb/sr9800.c

···
 		skb_put(skb, sizeof(padbytes));
 	}
 
-	usbnet_set_skb_tx_stats(skb, 1);
+	usbnet_set_skb_tx_stats(skb, 1, 0);
 	return skb;
 }
 

drivers/net/usb/usbnet.c

···
 		} else
 			urb->transfer_flags |= URB_ZERO_PACKET;
 	}
-	entry->length = urb->transfer_buffer_length = length;
-	if (!(info->flags & FLAG_MULTI_PACKET))
-		usbnet_set_skb_tx_stats(skb, 1);
+	urb->transfer_buffer_length = length;
+
+	if (info->flags & FLAG_MULTI_PACKET) {
+		/* Driver has set number of packets and a length delta.
+		 * Calculate the complete length and ensure that it's
+		 * positive.
+		 */
+		entry->length += length;
+		if (WARN_ON_ONCE(entry->length <= 0))
+			entry->length = length;
+	} else {
+		usbnet_set_skb_tx_stats(skb, 1, length);
+	}
 
 	spin_lock_irqsave(&dev->txq.lock, flags);
 	retval = usb_autopm_get_interface_async(dev->intf);

include/linux/usb/usbnet.h

···
 	struct urb		*urb;
 	struct usbnet		*dev;
 	enum skb_state		state;
-	size_t			length;
+	long			length;
 	unsigned long		packets;
 };
 
···
  * tx_fixup method before returning an skb.
  */
 static inline void
-usbnet_set_skb_tx_stats(struct sk_buff *skb, unsigned long packets)
+usbnet_set_skb_tx_stats(struct sk_buff *skb,
+			unsigned long packets, long bytes_delta)
 {
 	struct skb_data *entry = (struct skb_data *) skb->cb;
 
 	entry->packets = packets;
+	entry->length = bytes_delta;
 }
 
 extern int usbnet_open(struct net_device *net);