Re: [PATCH v4 net-next RFC] net: Generic XDP

[David Miller <davem@davemloft.net>]

From: David Miller <davem@davemloft.net>
Date: Tue, 18 Apr 2017 15:07:08 -0400 (EDT)

> From: Andy Gospodarek <andy@greyhouse.net>
> Date: Tue, 18 Apr 2017 15:05:35 -0400
> 
>> On Fri, Apr 14, 2017 at 05:59:51PM -0700, Alexei Starovoitov wrote:
>>> On Thu, Apr 13, 2017 at 04:23:15PM -0400, David Miller wrote:
>>> > +
>>> > +	switch (act) {
>>> > +	case XDP_TX:
>>> > +		__skb_push(skb, skb->mac_len);
>>> 
>>> s/skb->mac_len/mac_len/
>>> 
>> 
>> I was away from my keyboard for a few days, but was able to get some
>> time to test this today.
>> 
>> When using this change above suggested by Alexei, XDP_DROP and XDP_TX
>> actions appear to work well with xdp1 and xdp2.
>> 
>> I'm seeing some rather odd behavior with xdp_tx_tunnel so it might be
>> good to hold off on committing this just yet.
>> 
>> At first glance, it looks like there is enough headroom for the new
>> frame, but the resulting packet data do not look right and I'm actually
>> seeing some data that may be left on the stack from a previous caller.
> 
> Thanks for testing Andy, I'll take a look and see if I can figure it out.

Andy, I think we might be getting burnt by signedness issues in the
offset handling when the XDP program adjusts the packet data pointer.

In netif_receive_generic_xdp(), try changing the offset handling code to
read something like:

	off = xdp.data - orig_data;
	if (off > 0)
		__skb_pull(skb, off);
	else if (off < 0)
		__skb_push(skb, -off);

If that doesn't work try adding:

	__skb_cow(skb, XDP_PACKET_HEADROOM, 0);

right after the skb_linearize() call in that same function.