BPF_CALL_3(bpf_skb_change_tail, struct sk_buff *, skb, u32, new_len,
	   u64, flags)
{
	u32 max_len = __bpf_skb_max_len(skb);
	u32 min_len = __bpf_skb_min_len(skb);
	int ret;

	if (unlikely(flags || new_len > max_len || new_len < min_len))
		return -EINVAL;
	if (skb->encapsulation)
		return -ENOTSUPP;

	/* The basic idea of this helper is that it's performing the
	 * needed work to either grow or trim an skb, and eBPF program
	 * rewrites the rest via helpers like bpf_skb_store_bytes(),
	 * bpf_lX_csum_replace() and others rather than passing a raw
	 * buffer here. This one is a slow path helper and intended
	 * for replies with control messages.
	 *
	 * Like in bpf_skb_change_proto(), we want to keep this rather
	 * minimal and without protocol specifics so that we are able
	 * to separate concerns as in bpf_skb_store_bytes() should only
	 * be the one responsible for writing buffers.
	 *
	 * It's really expected to be a slow path operation here for
	 * control message replies, so we're implicitly linearizing,
	 * uncloning and drop offloads from the skb by this.
	 */
	ret = __bpf_try_make_writable(skb, skb->len);
	if (!ret) {
		if (new_len > skb->len)
			ret = bpf_skb_grow_rcsum(skb, new_len);
		else if (new_len < skb->len)
			ret = bpf_skb_trim_rcsum(skb, new_len);
		if (!ret && skb_is_gso(skb))
			skb_gso_reset(skb);
	}

	bpf_compute_data_pointers(skb);
	return ret;
}
Unknown