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>

4 files changed, 18 insertions, 9 deletions

diff --git a/drivers/net/usb/asix_common.c b/drivers/net/usb/asix_common.c
index 724a9b50df7a..75d6f26729a3 100644
--- a/drivers/net/usb/asix_common.c
+++ b/drivers/net/usb/asix_common.c
@@ -189,7 +189,7 @@ struct sk_buff *asix_tx_fixup(struct usbnet *dev, struct sk_buff *skb,
                 skb_put(skb, sizeof(padbytes));
         }
 
-        usbnet_set_skb_tx_stats(skb, 1);
+        usbnet_set_skb_tx_stats(skb, 1, 0);
         return skb;
 }
 
diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
index 70cbea551139..c3e4da9e79ca 100644
--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -1177,13 +1177,12 @@ cdc_ncm_fill_tx_frame(struct usbnet *dev, struct sk_buff *skb, __le32 sign)
         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;
 
diff --git a/drivers/net/usb/sr9800.c b/drivers/net/usb/sr9800.c
index 7650cdc8fe6b..953de13267df 100644
--- a/drivers/net/usb/sr9800.c
+++ b/drivers/net/usb/sr9800.c
@@ -144,7 +144,7 @@ static struct sk_buff *sr_tx_fixup(struct usbnet *dev, struct sk_buff *skb,
                 skb_put(skb, sizeof(padbytes));
         }
 
-        usbnet_set_skb_tx_stats(skb, 1);
+        usbnet_set_skb_tx_stats(skb, 1, 0);
         return skb;
 }
 
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);