Subject: Re: [Patch bpf-next v1 1/4] tcp: introduce tcp_read_skb()

From: Jakub Kicinski <[email-masked]>
Date: Mon, 25 Apr 2022 12:07:24 -0700

On Sun, 10 Apr 2022 09:10:39 -0700 Cong Wang wrote:
> +int tcp_read_skb(struct sock *sk, read_descriptor_t *desc,
> +		 sk_read_actor_t recv_actor)
> +{
> +	struct sk_buff *skb;
> +	struct tcp_sock *tp = tcp_sk(sk);
> +	u32 seq = tp->copied_seq;
> +	u32 offset;
> +	int copied = 0;
> +
> +	if (sk->sk_state == TCP_LISTEN)
> +		return -ENOTCONN;
> +	while ((skb = tcp_recv_skb(sk, seq, &offset, true)) != NULL) {
> +		if (offset < skb->len) {
> +			int used;
> +			size_t len;
> +
> +			len = skb->len - offset;
> +			used = recv_actor(desc, skb, offset, len);
> +			if (used <= 0) {
> +				if (!copied)
> +					copied = used;
> +				break;
> +			}
> +			if (WARN_ON_ONCE(used > len))
> +				used = len;
> +			seq += used;
> +			copied += used;
> +			offset += used;
> +
> +			if (offset != skb->len)
> +				continue;
> +		}
> +		if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) {
> +			kfree_skb(skb);
> +			++seq;
> +			break;
> +		}
> +		kfree_skb(skb);
> +		if (!desc->count)
> +			break;
> +		WRITE_ONCE(tp->copied_seq, seq);
> +	}
> +	WRITE_ONCE(tp->copied_seq, seq);
> +
> +	tcp_rcv_space_adjust(sk);
> +
> +	/* Clean up data we have read: This will do ACK frames. */
> +	if (copied > 0)
> +		tcp_cleanup_rbuf(sk, copied);
> +
> +	return copied;
> +}
> +EXPORT_SYMBOL(tcp_read_skb);

I started prototyping a similar patch for TLS a while back but I have
two functions - one to get the skb and another to consume it. I thought
that's better for TLS, otherwise skbs stuck in the middle layer are not
counted towards the rbuf. Any thoughts on structuring the API that way?
I guess we can refactor that later, since TLS TCP-only we don't need
proto_ops plumbing there.

Overall 👍 for adding such API.