Message-ID: <670dda9437147_2e6c4029461@willemb.c.googlers.com.notmuch>
Date: Mon, 14 Oct 2024 22:59:32 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jason Xing <kerneljasonxing@...il.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, 
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 dsahern@...nel.org, 
 willemb@...gle.com, 
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 daniel@...earbox.net, 
 andrii@...nel.org, 
 martin.lau@...ux.dev, 
 eddyz87@...il.com, 
 song@...nel.org, 
 yonghong.song@...ux.dev, 
 john.fastabend@...il.com, 
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 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 v2 00/12] net-timestamp: bpf extension to equip
 applications transparently

Jason Xing wrote:
> On Tue, Oct 15, 2024 at 9:28 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > Jason Xing wrote:
> > > On Sun, Oct 13, 2024 at 1:48 AM Willem de Bruijn
> > > <willemdebruijn.kernel@...il.com> wrote:
> > > >
> > > > Jason Xing wrote:
> > > > > From: Jason Xing <kernelxing@...cent.com>
> > > > >
> > > > > A few weeks ago, I planned to extend SO_TIMESTMAMPING feature by using
> > > > > tracepoint to print information (say, tstamp) so that we can
> > > > > transparently equip applications with this feature and require no
> > > > > modification in user side.
> > > > >
> > > > > Later, we discussed at netconf and agreed that we can use bpf for better
> > > > > extension, which is mainly suggested by John Fastabend and Willem de
> > > > > Bruijn. Many thanks here! So I post this series to see if we have a
> > > > > better solution to extend. My feeling is BPF is a good place to provide
> > > > > a way to add timestamping by administrators, without having to rebuild
> > > > > applications.
> > > > >
> > > > > This approach mostly relies on existing SO_TIMESTAMPING feature, users
> > > > > only needs to pass certain flags through bpf_setsocktop() to a separate
> > > > > tsflags. For TX timestamps, they will be printed during generation
> > > > > phase. For RX timestamps, we will wait for the moment when recvmsg() is
> > > > > called.
> > > > >
> > > > > After this series, we could step by step implement more advanced
> > > > > functions/flags already in SO_TIMESTAMPING feature for bpf extension.
> > > > >
> > > > > In this series, I only support TCP protocol which is widely used in
> > > > > SO_TIMESTAMPING feature.
> > > > >
> > > > > ---
> > > > > V2
> > > > > Link: https://lore.kernel.org/all/20241008095109.99918-1-kerneljasonxing@gmail.com/
> > > > > 1. Introduce tsflag requestors so that we are able to extend more in the
> > > > > future. Besides, it enables TX flags for bpf extension feature separately
> > > > > without breaking users. It is suggested by Vadim Fedorenko.
> > > > > 2. introduce a static key to control the whole feature. (Willem)
> > > > > 3. Open the gate of bpf_setsockopt for the SO_TIMESTAMPING feature in
> > > > > some TX/RX cases, not all the cases.
> > > > >
> > > > > Note:
> > > > > The main concern we've discussion in V1 thread is how to deal with the
> > > > > applications using SO_TIMESTAMPING feature? In this series, I allow both
> > > > > cases to happen at the same time, which indicates that even one
> > > > > applications setting SO_TIMESTAMPING can still be traced through BPF
> > > > > program. Please see patch [04/12].
> > > >
> > > > This revision does not address the main concern.
> > > >
> > > > An administrator installed BPF program can affect results of a process
> > > > using SO_TIMESTAMPING in ways that break it.
> > >
> > > Sorry, I didn't get it. How the following code snippet would break users?
> >
> > The state between user and bpf timestamping needs to be separate to
> > avoid interference.
> 
> Do you agree that we will use this method as following, only allow
> either of them to work?
> 
> void __skb_tstamp_tx(struct sk_buff *orig_skb,
>                      const struct sk_buff *ack_skb,
>                      struct skb_shared_hwtstamps *hwtstamps,
>                      struct sock *sk, int tstype)
> {
>         if (!sk)
>                 return;
> 
>        ret = skb_tstamp_tx_output(orig_skb, ack_skb, hwtstamps, sk, tstype);
>        if (ret)
>                /* Apps does set the SO_TIMESTAMPING flag, return directly */
>                return;
> 
>        if (static_branch_unlikely(&bpf_tstamp_control))
>                 bpf_skb_tstamp_tx_output(sk, orig_skb, tstype, hwtstamps);
> }
> 
> which means if the apps using non-bpf method, we will not see the
> output even if we load bpf program.

Could the bpf setsockopt fail hard in that case?

Your current patch tries to make them work at the same time. It mostly
does work. There are just a few concerning edge cases that may result
in hard to understand bugs.

Making only one method work per socket and fail hard if both try it is
crude, but at least the failure will be clear: the setsockopt fails.

I think that's safer. And in practice, the use cases for BPF
timestamping probably are exactly when application timestamping is
missing?

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