On Mon, Jun 14, 2021 at 9:51 AM Yonghong Song <yhs@xxxxxx> 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.
