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, 4 insertions, 2 deletions

diff --git a/include/linux/usb/usbnet.h b/include/linux/usb/usbnet.h
index ff3fb2bd0e90..6e0ce8c7b8cb 100644
--- a/include/linux/usb/usbnet.h
+++ b/include/linux/usb/usbnet.h
@@ -227,7 +227,7 @@ struct skb_data {	/* skb->cb is one of these */
         struct urb              *urb;
         struct usbnet           *dev;
         enum skb_state          state;
-        size_t                  length;
+        long                    length;
         unsigned long           packets;
 };
 
@@ -235,11 +235,13 @@ struct skb_data {	/* skb->cb is one of these */
  * 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);