netdev - Re: [PATCH net-next] net_tstamp: add SOF_TIMESTAMPING_OPT_ID

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Date:   Tue, 6 Dec 2022 16:21:50 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
        pabeni@...hat.com, soheil@...gle.com
Subject: Re: [PATCH net-next] net_tstamp: add SOF_TIMESTAMPING_OPT_ID_TCP

On Tue, Dec 6, 2022 at 3:58 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Tue, 6 Dec 2022 15:46:25 -0500 Willem de Bruijn wrote:
> > > We can't just copy back the value of
> > >
> > >         tcp_sk(sk)->snd_una - tcp_sk(sk)->write_seq
> > >
> > > to the user if the input of setsockopt is large enough (ie. extend the
> > > struct, if len >= sizeof(new struct) -> user is asking to get this?
> > > Or even add a bit somewhere that requests a copy back?
> >
> > We could, but indeed then we first need a way to signal that the
> > kernel is new enough to actually write something meaningful back that
> > is safe to read.
>
> It should be sufficient to init the memory to -1.
> (I guess I'm not helping my own "this is less hacky" argument? :)
>
> > And if we change the kernel API and applications, I find this a
> > somewhat hacky approach: why program the slightly wrong thing and
> > return the offset to patch it up in userspace, if we can just program
> > the right thing to begin with?
>
> Ah, so you'd also switch all your apps to use this new bit?
>
> What wasn't clear to me whether this is a
>  1 - we clearly did the wrong thing
> or
>  2 - there is a legit use case for un-packetized(?) data not being
>      counted
>
> In case of (1) we should make it clearer that the new bit is in fact
> a "fixed" version of the functionality.
> For (2) we can view this as an extension of the existing functionality
> so combining in the same bit with write back seems natural (and TBH
> I like the single syscall probing path more than try-one-then-the-other,
> but that's 100% subjective).
>
> Anyway, don't wanna waste too much of your time. If you prefer to keep
> as is the doc change is good enough for me.

It's definitely 1. I'll be more explicit in the documentation and
commit message about that.

I would have just made the one line 's/snd_una/write_seq/' change if I
could be certain that no existing code relies on the current behavior.
But I already came across it in one test and production. It's too
risky.