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

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

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.

> > +     if (val) {
> > +             /* This restriction will be removed in the next patch */
> > +             verbose(env, "bpf_timer field can only be first in the map value element\n");
> > +             return -EINVAL;
> > +     }
> > +     WARN_ON(meta->map_ptr);
>
> Could you explain when this could happen?

Only if there is a verifier bug or new helper is added with arg to timer
and arg to map. I'll switch to verbose() + efault instead.