netdev - Re: [PATCH net-next v4 10/11] net-timestamp: export the tskey for TCP bpf extension

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Message-ID: <CAL+tcoCSrBBaW3Rg1hD0mBAGu_ZTCTfjVBGe_7B=_JB+uJTuYA@mail.gmail.com>
Date: Wed, 8 Jan 2025 12:21:17 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, dsahern@...nel.org, willemdebruijn.kernel@...il.com, 
	willemb@...gle.com, ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, 
	eddyz87@...il.com, song@...nel.org, yonghong.song@...ux.dev, 
	john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me, 
	haoluo@...gle.com, jolsa@...nel.org, bpf@...r.kernel.org, 
	netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v4 10/11] net-timestamp: export the tskey for TCP
 bpf extension

Hi Martin,

> > -     bpf_skops_tx_timestamping(sk, skb, op, 2, args);
> > +     if (sk_is_tcp(sk))
> > +             args[2] = skb_shinfo(skb)->tskey;
>
> Instead of only passing one info "skb_shinfo(skb)->tskey" of a skb, pass the
> whole skb ptr to the bpf prog. Take a look at bpf_skops_init_skb. Lets start
> with end_offset = 0 for now so that the bpf prog won't use it to read the
> skb->data. It can be revisited later.
>
>         bpf_skops_init_skb(&sock_ops, skb, 0);
>
> The bpf prog can use bpf_cast_to_kern_ctx() and bpf_core_cast() to get to the
> skb_shinfo(skb). Take a look at the md_skb example in type_cast.c.

In recent days, I've been working on this part. It turns out to be
infeasible to pass "struct __sk_buff *skb" as the second parameter in
skops_sockopt() in patch [11/11]. I cannot find a way to acquire the
skb itself, sorry for that :(

IIUC, there are three approaches to fetch the tskey:
1. Like what I wrote in this patchset, passing the tskey to bpf prog
through calling __cgroup_bpf_run_filter_sock_ops() is simple and
enough.
2. Considering future usability, I feel I can add skb_head, skb_end
fields in sock_ops_convert_ctx_access().
3. Only adding a new field tskey like skb_hwtstamp in
sock_ops_convert_ctx_access()

If there is something wrong, please correct me. Thanks!

patch[11/11]: https://lore.kernel.org/all/20241207173803.90744-12-kerneljasonxing@gmail.com/