Subject: Re: [PATCH v4 bpf-next 1/3] bpf: tcp: Handle BPF SYN Cookie in cookie_v[46]_check().

From: Martin KaFai Lau <[email-masked]>
Date: Tue, 5 Dec 2023 16:19:20 -0800

On 12/4/23 5:34 PM, Kuniyuki Iwashima wrote:
> diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
> index 61f1c96cfe63..0f9c3aed2014 100644
> --- a/net/ipv4/syncookies.c
> +++ b/net/ipv4/syncookies.c
> @@ -304,6 +304,59 @@ static int cookie_tcp_reqsk_init(struct sock *sk, struct sk_buff *skb,
>   	return 0;
>   }
>   
> +#if IS_ENABLED(CONFIG_BPF)
> +struct request_sock *cookie_bpf_check(struct net *net, struct sock *sk,
> +				      struct sk_buff *skb)
> +{
> +	struct request_sock *req = inet_reqsk(skb->sk);
> +	struct inet_request_sock *ireq = inet_rsk(req);
> +	struct tcp_request_sock *treq = tcp_rsk(req);
> +	struct tcp_options_received tcp_opt;
> +	int ret;
> +
> +	skb->sk = NULL;
> +	skb->destructor = NULL;
> +	req->rsk_listener = NULL;
> +
> +	memset(&tcp_opt, 0, sizeof(tcp_opt));
> +	tcp_parse_options(net, skb, &tcp_opt, 0, NULL);

In patch 2, the bpf prog is passing the tcp_opt to the kfunc. The selftest in 
patch 3 is also parsing the tcp-options.

The kernel parses the tcp-option here again to do some checking and req's member 
initialization. Can these checking and initialization be done in the 
bpf_sk_assign_tcp_reqsk() kfunc instead to avoid the double tcp-option parsing?

> +
> +	if (ireq->tstamp_ok ^ tcp_opt.saw_tstamp) {
> +		__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESFAILED);
> +		goto reset;
> +	}
> +
> +	__NET_INC_STATS(net, LINUX_MIB_SYNCOOKIESRECV);
> +
> +	if (ireq->tstamp_ok) {
> +		if (!READ_ONCE(net->ipv4.sysctl_tcp_timestamps))
> +			goto reset;
> +
> +		req->ts_recent = tcp_opt.rcv_tsval;
> +		treq->ts_off = tcp_opt.rcv_tsecr - tcp_ns_to_ts(false, tcp_clock_ns());
> +	}
> +
> +	if (ireq->sack_ok && !READ_ONCE(net->ipv4.sysctl_tcp_sack))
> +		goto reset;
> +
> +	if (ireq->wscale_ok && !READ_ONCE(net->ipv4.sysctl_tcp_window_scaling))
> +		goto reset;
> +
> +	ret = cookie_tcp_reqsk_init(sk, skb, req);
> +	if (ret) {
> +		reqsk_free(req);
> +		req = NULL;
> +	}
> +
> +	return req;
> +
> +reset:
> +	reqsk_free(req);
> +	return ERR_PTR(-EINVAL);
> +}
> +EXPORT_SYMBOL_GPL(cookie_bpf_check);
> +#endif