Subject: Re: [PATCH v2 net-next 6/9] bpf: helpers: add bpf_xdp_adjust_mb_header helper

From: Lorenzo Bianconi <[email-masked]>
Date: Fri, 4 Sep 2020 09:50:31 +0200

> On Thu, Sep 03, 2020 at 10:58:50PM +0200, Lorenzo Bianconi wrote:
> > +BPF_CALL_2(bpf_xdp_adjust_mb_header, struct  xdp_buff *, xdp,
> > +	   int, offset)
> > +{
> > +	void *data_hard_end, *data_end;
> > +	struct skb_shared_info *sinfo;
> > +	int frag_offset, frag_len;
> > +	u8 *addr;
> > +
> > +	if (!xdp->mb)
> > +		return -EOPNOTSUPP;
> > +
> > +	sinfo = xdp_get_shared_info_from_buff(xdp);
> > +
> > +	frag_len = skb_frag_size(&sinfo->frags[0]);
> > +	if (offset > frag_len)
> > +		return -EINVAL;
> > +
> > +	frag_offset = skb_frag_off(&sinfo->frags[0]);
> > +	data_end = xdp->data_end + offset;
> > +
> > +	if (offset < 0 && (-offset > frag_offset ||
> > +			   data_end < xdp->data + ETH_HLEN))
> > +		return -EINVAL;
> > +
> > +	data_hard_end = xdp_data_hard_end(xdp); /* use xdp->frame_sz */
> > +	if (data_end > data_hard_end)
> > +		return -EINVAL;
> > +
> > +	addr = page_address(skb_frag_page(&sinfo->frags[0])) + frag_offset;
> > +	if (offset > 0) {
> > +		memcpy(xdp->data_end, addr, offset);
> > +	} else {
> > +		memcpy(addr + offset, xdp->data_end + offset, -offset);
> > +		memset(xdp->data_end + offset, 0, -offset);
> > +	}
> > +
> > +	skb_frag_size_sub(&sinfo->frags[0], offset);
> > +	skb_frag_off_add(&sinfo->frags[0], offset);
> > +	xdp->data_end = data_end;
> > +
> > +	return 0;
> > +}
> 
> wait a sec. Are you saying that multi buffer XDP actually should be skb based?
> If that's what mvneta driver is doing that's fine, but that is not a
> reasonable requirement to put on all other drivers.

I did not got what you mean here. The xdp multi-buffer layout uses the skb_shared_info
at the end of the first buffer to link subsequent frames [0] and we rely on skb_frag*
utilities to set/read offset and length of subsequent buffers.

Regards,
Lorenzo

[0] http://people.redhat.com/lbiancon/conference/NetDevConf2020-0x14/add-xdp-on-driver.html - XDP multi-buffers section