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

On Mon, Jun 14, 2021 at 10:31 PM Andrii Nakryiko
[off-list ref] wrote:

On Mon, Jun 14, 2021 at 8:29 PM Alexei Starovoitov
[off-list ref] wrote:

quoted

On Mon, Jun 14, 2021 at 9:51 AM Yonghong Song [off-list ref] wrote:

quoted

quoted

+     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.

quoted

quoted

+     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.