netdev - Re: [PATCH net-next 6/9] net-timestamp: add tx OPT_ID

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Message-ID: <CAL+tcoA_HwCYG+_DtdRHNL-L07RYqQfxY+pmT2fUvs-N1HYV9g@mail.gmail.com>
Date: Wed, 9 Oct 2024 07:18:30 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, dsahern@...nel.org, willemb@...gle.com, ast@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev, 
	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 6/9] net-timestamp: add tx OPT_ID_TCP support for
 bpf case

On Wed, Oct 9, 2024 at 2:56 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Jason Xing wrote:
> > From: Jason Xing <kernelxing@...cent.com>
> >
> > We can set OPT_ID|OPT_ID_TCP before we initialize the last skb
> > from each sendmsg. We only set the socket once like how we use
> > setsockopt() with OPT_ID|OPT_ID_TCP flags.
> >
> > Signed-off-by: Jason Xing <kernelxing@...cent.com>
> > ---
> >  net/core/skbuff.c | 16 +++++++++++++---
> >  net/ipv4/tcp.c    | 19 +++++++++++++++----
> >  2 files changed, 28 insertions(+), 7 deletions(-)
> >
>
> > @@ -491,10 +491,21 @@ static u32 bpf_tcp_tx_timestamp(struct sock *sk)
> >       if (!(flags & SOF_TIMESTAMPING_TX_RECORD_MASK))
> >               return 0;
> >
> > +     /* We require users to set both OPT_ID and OPT_ID_TCP flags
> > +      * together here, or else the key might be inaccurate.
> > +      */
> > +     if (flags & SOF_TIMESTAMPING_OPT_ID &&
> > +         flags & SOF_TIMESTAMPING_OPT_ID_TCP &&
> > +         !(sk->sk_tsflags & (SOF_TIMESTAMPING_OPT_ID | SOF_TIMESTAMPING_OPT_ID_TCP))) {
> > +             atomic_set(&sk->sk_tskey, (tcp_sk(sk)->write_seq - copied));
> > +             sk->sk_tsflags |= (SOF_TIMESTAMPING_OPT_ID | SOF_TIMESTAMPING_OPT_ID_TCP);
>
> So user and BPF admin conflict on both sk_tsflags and sktskey?
>
> I think BPF resetting this key, or incrementing it, may break user
> expectations.

Yes, when it comes to OPT_ID and OPT_ID_TCP, conflict could happen.
The reason why I don't use it like BPF_SOCK_OPS_TS_SCHED_OPT_CB flags
(which is set along with each last skb) is that OPT_ID logic is a
little bit complex. If we want to avoid touching sk_tsflags field in
struct sock, we have to re-implement a similiar logic as you've
already done in these years.

Now, this patch is easier but as you said it may "break" users... But
I wonder if we can give the bpf program the first priority like what
TCP_BPF_RTO_MIN does. TCP_BPF_RTO_MIN can override icsk_rto_min field
in struct inet_connection_sock.

Thanks,
Jason