Re: [PATCHv2 bpf-next] samples/bpf: add xdp program on egress for xdp_redirect_map

[Jesper Dangaard Brouer]

On Thu, 26 Nov 2020 16:43:25 +0800
Hangbin Liu <liuhang...@gmail.com> wrote:

> Current sample test xdp_redirect_map only count pkts on ingress. But we
> can't know whether the pkts are redirected or dropped. So add a counter
> on egress interface so we could know how many pkts are redirect in fact.

This is not true.

The 2nd devmap XDP-prog will run in the same RX-context, so it doesn't
tell us if the redirect was successful.  I looked up the code, and the
2nd XDP-prog is even allowed to run when the egress driver doesn't
support the NDO to xmit (dev->netdev_ops->ndo_xdp_xmit), which is very
misleading, if you place a output counter here.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer


static inline int __xdp_enqueue(struct net_device *dev, struct xdp_buff *xdp,
                               struct net_device *dev_rx)
{
        struct xdp_frame *xdpf;
        int err;

        if (!dev->netdev_ops->ndo_xdp_xmit)
                return -EOPNOTSUPP;

        err = xdp_ok_fwd_dev(dev, xdp->data_end - xdp->data);
        if (unlikely(err))
                return err;

        xdpf = xdp_convert_buff_to_frame(xdp);
        if (unlikely(!xdpf))
                return -EOVERFLOW;

        bq_enqueue(dev, xdpf, dev_rx);
        return 0;
}

static struct xdp_buff *dev_map_run_prog(struct net_device *dev,
                                         struct xdp_buff *xdp,
                                         struct bpf_prog *xdp_prog)
{
        struct xdp_txq_info txq = { .dev = dev };
        u32 act;

        xdp_set_data_meta_invalid(xdp);
        xdp->txq = &txq;

        act = bpf_prog_run_xdp(xdp_prog, xdp);
        switch (act) {
        case XDP_PASS:
                return xdp;
        case XDP_DROP:
                break;
        default:
                bpf_warn_invalid_xdp_action(act);
                fallthrough;
        case XDP_ABORTED:
                trace_xdp_exception(dev, xdp_prog, act);
                break;
        }

        xdp_return_buff(xdp);
        return NULL;
}

int dev_xdp_enqueue(struct net_device *dev, struct xdp_buff *xdp,
                    struct net_device *dev_rx)
{
        return __xdp_enqueue(dev, xdp, dev_rx);
}

int dev_map_enqueue(struct bpf_dtab_netdev *dst, struct xdp_buff *xdp,
                    struct net_device *dev_rx)
{
        struct net_device *dev = dst->dev;

        if (dst->xdp_prog) {
                xdp = dev_map_run_prog(dev, xdp, dst->xdp_prog);
                if (!xdp)
                        return 0;
        }
        return __xdp_enqueue(dev, xdp, dev_rx);
}