Subject: Re: [RFC net-next 11/14] tun: run XDP program in tx path

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 18 Dec 2019 11:07:32 +0100

On Wed, 18 Dec 2019 17:10:47 +0900
Prashant Bhole <[email-masked]> wrote:

> +static u32 tun_do_xdp_tx(struct tun_struct *tun, struct tun_file *tfile,
> +			 struct xdp_frame *frame)
> +{
> +	struct bpf_prog *xdp_prog;
> +	struct tun_page tpage;
> +	struct xdp_buff xdp;
> +	u32 act = XDP_PASS;
> +	int flush = 0;
> +
> +	xdp_prog = rcu_dereference(tun->xdp_tx_prog);
> +	if (xdp_prog) {
> +		xdp.data_hard_start = frame->data - frame->headroom;
> +		xdp.data = frame->data;
> +		xdp.data_end = xdp.data + frame->len;
> +		xdp.data_meta = xdp.data - frame->metasize;

You have not configured xdp.rxq, thus a BPF-prog accessing this will crash.

For an XDP TX hook, I want us to provide/give BPF-prog access to some
more information about e.g. the current tx-queue length, or TC-q number.

Question to Daniel or Alexei, can we do this and still keep BPF_PROG_TYPE_XDP?
Or is it better to introduce a new BPF prog type (enum bpf_prog_type)
for XDP TX-hook ?

To Prashant, look at net/core/filter.c in xdp_convert_ctx_access() on
how the BPF instruction rewrites are done, when accessing xdp_rxq_info.

> +		act = bpf_prog_run_xdp(xdp_prog, &xdp);
> +		switch (act) {
> +		case XDP_PASS:
> +			break;
> +		case XDP_TX:
> +			/* fall through */
> +		case XDP_REDIRECT:
> +			/* fall through */
> +		default:
> +			bpf_warn_invalid_xdp_action(act);
> +			/* fall through */
> +		case XDP_ABORTED:
> +			trace_xdp_exception(tun->dev, xdp_prog, act);
> +			/* fall through */
> +		case XDP_DROP:
> +			xdp_return_frame_rx_napi(frame);

I'm not sure that it is safe to use "napi" variant here, as you have to
be under same RX-NAPI processing loop for this to be safe.

Notice the "rx" part of the name "xdp_return_frame_rx_napi". 

> +			break;
> +		}
> +	}
> +
> +	return act;
> +}

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