netdev - Re: [PATCH net-next v2 00/12] net-timestamp: bpf extension to equip applications transparently

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Message-ID: <670dc531710c_2e1742294b4@willemb.c.googlers.com.notmuch>
Date: Mon, 14 Oct 2024 21:28:17 -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, 
 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 v2 00/12] net-timestamp: bpf extension to equip
 applications transparently

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.

Introducing a new sk_tsflags for bpf goes a long way. Though I prefer
a separate sk_tsflags_bpf and not touching existing sk_tsflags over
the array approach of patch 1. Also need to check pahole and maybe
move sk_tsflags_bpf elsewhere in the struct.

Other state is sk_tskey. The current approach can initialize the key
in bpf before the user attempts it for the same socket. Admittedly
unlikely. But hard to reach states creates hard to debug issues.

This field cannot easily be duplicated, because the key is tracked
in skb_shinfo. Where there is not sufficient room for two keys.

The same goes for txflags.

The current approach is to set those flags if either user or bpf
requestss them, then on __skb_tstamp_tx detect if the user did not set
them, and if so skip output to the user. Need to take a closer look,
but seems to work.

So getting closer.