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

+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;
+}