Re: [PATCH v2 bpf-next 1/3] bpf: Introduce bpf_timer

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, Jun 14, 2021 at 10:31 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Mon, Jun 14, 2021 at 8:29 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Mon, Jun 14, 2021 at 9:51 AM Yonghong Song <yhs@fb.com> wrote:
> > > > +     ret = BPF_CAST_CALL(t->callback_fn)((u64)(long)map,
> > > > +                                         (u64)(long)key,
> > > > +                                         (u64)(long)t->value, 0, 0);
> > > > +     WARN_ON(ret != 0); /* Next patch disallows 1 in the verifier */
> > >
> > > I didn't find that next patch disallows callback return value 1 in the
> > > verifier. If we indeed disallows return value 1 in the verifier. We
> > > don't need WARN_ON here. Did I miss anything?
> >
> > Ohh. I forgot to address this bit in the verifier. Will fix.
> >
> > > > +     if (!hrtimer_active(&t->timer) || hrtimer_callback_running(&t->timer))
> > > > +             /* If the timer wasn't active or callback already executing
> > > > +              * bump the prog refcnt to keep it alive until
> > > > +              * callback is invoked (again).
> > > > +              */
> > > > +             bpf_prog_inc(t->prog);
> > >
> > > I am not 100% sure. But could we have race condition here?
> > >     cpu 1: running bpf_timer_start() helper call
> > >     cpu 2: doing hrtimer work (calling callback etc.)
> > >
> > > Is it possible that
> > >    !hrtimer_active(&t->timer) || hrtimer_callback_running(&t->timer)
> > > may be true and then right before bpf_prog_inc(t->prog), it becomes
> > > true? If hrtimer_callback_running() is called, it is possible that
> > > callback function could have dropped the reference count for t->prog,
> > > so we could already go into the body of the function
> > > __bpf_prog_put()?
> >
> > you're correct. Indeed there is a race.
> > Circular dependency is a never ending headache.
> > That's the same design mistake as with tail_calls.
> > It felt that this case would be simpler than tail_calls and a bpf program
> > pinning itself with bpf_prog_inc can be made to work... nope.
> > I'll get rid of this and switch to something 'obviously correct'.
> > Probably a link list with a lock to keep a set of init-ed timers and
> > auto-cancel them on prog refcnt going to zero.
> > To do 'bpf daemon' the prog would need to be pinned.
>
> Hm.. wouldn't this eliminate that race:
>
> switch (hrtimer_try_to_cancel(&t->timer))
> {
> case 0:
>     /* nothing was queued */
>     bpf_prog_inc(t->prog);
>     break;
> case 1:
>     /* already have refcnt and it won't be bpf_prog_put by callback */
>     break;
> case -1:
>     /* callback is running and will bpf_prog_put, so we need to take
> another refcnt */
>     bpf_prog_inc(t->prog);
>     break;
> }
> hrtimer_start(&t->timer, ns_to_ktime(nsecs), HRTIMER_MODE_REL_SOFT);
>
> So instead of guessing (racily) whether there is a queued callback or
> not, try to cancel just in case there is. Then rely on the nice
> guarantees that hrtimer cancellation API provides.

I haven't thought it through yet, but the above approach could
indeed solve this particular race. Unfortunately there are other races.
There is an issue with bpf_timer_init. Since it doesn't take refcnt
another program might do lookup and bpf_timer_start
while the first prog got to refcnt=0 and got freed.
Adding refcnt to bpf_timer_init() makes the prog self pinned
and no callback might ever be executed (if there were no bpf_timer_start),
so that will cause a high chance of bpf prog stuck in the kernel.
There could be ref+uref schemes similar to tail_calls to address all that,
but it gets ugly quickly.
imo all these issues and races is a sign that such self pinning
shouldn't be allowed.