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

[Prashant Bhole <prashantbhole.linux@gmail.com>]

On 12/18/19 7:07 PM, Jesper Dangaard Brouer wrote:
> On Wed, 18 Dec 2019 17:10:47 +0900
> Prashant Bhole <prashantbhole.linux@gmail.com> 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.

Got it. I will take care of it next time.

> 
> 
>> +		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".

You are right, I will fix it next time.

Thanks!

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