netdev - Re: [PATCH bpf-next v3 4/4] selftests/bpf: add icmp_send

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Message-ID: <aIiP5l24ihrS2x-u@gmail.com>
Date: Tue, 29 Jul 2025 11:09:58 +0200
From: Mahe Tardy <mahe.tardy@...il.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: alexei.starovoitov@...il.com, andrii@...nel.org, ast@...nel.org,
	bpf@...r.kernel.org, coreteam@...filter.org, daniel@...earbox.net,
	fw@...len.de, john.fastabend@...il.com, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org, oe-kbuild-all@...ts.linux.dev,
	pablo@...filter.org, lkp@...el.com
Subject: Re: [PATCH bpf-next v3 4/4] selftests/bpf: add icmp_send_unreach
 kfunc tests

On Mon, Jul 28, 2025 at 06:18:11PM -0700, Martin KaFai Lau wrote:
> On 7/28/25 2:43 AM, Mahe Tardy wrote:
> > +SEC("cgroup_skb/egress")
> > +int egress(struct __sk_buff *skb)
> > +{
> > +	void *data = (void *)(long)skb->data;
> > +	void *data_end = (void *)(long)skb->data_end;
> > +	struct iphdr *iph;
> > +	struct tcphdr *tcph;
> > +
> > +	iph = data;
> > +	if ((void *)(iph + 1) > data_end || iph->version != 4 ||
> > +	    iph->protocol != IPPROTO_TCP || iph->daddr != bpf_htonl(SERVER_IP))
> > +		return SK_PASS;
> > +
> > +	tcph = (void *)iph + iph->ihl * 4;
> > +	if ((void *)(tcph + 1) > data_end ||
> > +	    tcph->dest != bpf_htons(SERVER_PORT))
> > +		return SK_PASS;
> > +
> > +	kfunc_ret = bpf_icmp_send_unreach(skb, unreach_code);
> > +
> > +	/* returns SK_PASS to execute the test case quicker */
> 
> Do you know why the user space is slower if 0 (SK_DROP) is used?

I tried to write my understanding of this in the commit description:

"Note that the BPF program returns SK_PASS to let the connection being
established to finish the test cases quicker. Otherwise, you have to
wait for the TCP three-way handshake to timeout in the kernel and
retrieve the errno translated from the unreach code set by the ICMP
control message."

I added this comment because I already had some (offline) feedback that
this looked off, maybe I should develop and put this here directly.

> 
> > +	return SK_PASS;
>