On Tue, Apr 18, 2017 at 03:29:16PM -0400, David Miller wrote:
> From: David Miller <davem@xxxxxxxxxxxxx>
> Date: Tue, 18 Apr 2017 15:07:08 -0400 (EDT)
>
> > From: Andy Gospodarek <andy@xxxxxxxxxxxxx>
> > 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.
I completely agree -- I just noted that the offset would be -20 in the
tx_tunnel case and it's easy to get confused since a positive int for
the second arg in skb_pull() does go 'back' with a positive value and
was just rebuilding and testing just this case with:
- off = xdp.data - orig_data;
+ /* note that offset is negative */
+ off = orig_data - xdp.data;
> 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);
This should do it. I'll give that run, too.
> If that doesn't work try adding:
>
> __skb_cow(skb, XDP_PACKET_HEADROOM, 0);
>
> right after the skb_linearize() call in that same function.
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