netdev - Re: [PATCH bpf v2 1/2] bpf: Forbid bpf_ktime_get_coarse_ns and bpf_timer

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Date:   Mon, 15 Nov 2021 20:44:02 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Dmitrii Banshchikov <me@...que.spb.ru>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        Andrey Ignatov <rdna@...com>,
        syzbot <syzbot+43fd005b5a1b4d10781e@...kaller.appspotmail.com>
Subject: Re: [PATCH bpf v2 1/2] bpf: Forbid bpf_ktime_get_coarse_ns and
 bpf_timer_* in tracing progs

On Mon, Nov 15, 2021 at 6:02 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Dmitrii.
>
> > @@ -4632,6 +4632,9 @@ union bpf_attr {
> >   *           system boot, in nanoseconds. Does not include time the system
> >   *           was suspended.
> >   *
> > + *           Tracing programs cannot use **bpf_ktime_get_coarse_ns**\() (but
> > + *           this may change in the future).
>
> Sorry no. This is a bug fix and there is no place for 'may change in the
> future' nonsense. It's simply not possible right now and unless you have
> a plan to make this work backed up by actual patches this comment is
> worse than wishful thinking.

No. The point of 'may' is that it actually may change.
It's certainly realistic and probable.
But based on the tone of your message it doesn't seem that
you're interested in hearing the arguments in favor of it.
So I just removed this comment to put this matter to rest.

> > + *
> >   *           See: **clock_gettime**\ (**CLOCK_MONOTONIC_COARSE**)
> >   *   Return
> >   *           Current *ktime*.
> > @@ -4804,6 +4807,9 @@ union bpf_attr {
> >   *           All other bits of *flags* are reserved.
> >   *           The verifier will reject the program if *timer* is not from
> >   *           the same *map*.
> > + *
> > + *           Tracing programs cannot use **bpf_timer_init**\() (but this may
> > + *           change in the future).
>
> This is even worse than the above because it cannot happen ever. Please
> stop this nonsensical wishful thinking crap. It does not add any value,
> it just adds confusion.

I respectfully disagree, but I removed this comment as well.
And force pushed the commits.