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 <ben.hutchings@codethink.co.uk>
Signed-off-by: Bjørn Mork <bjorn@mork.no>

1 files changed, 13 insertions, 3 deletions

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 0f3ff285f6a1..777757ae1973 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -1346,9 +1346,19 @@ netdev_tx_t usbnet_start_xmit (struct sk_buff *skb,
 		} 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);