Subject: Re: [PATCH v3 bpf-next 1/8] bpf: Introduce bpf timers.

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 01 Jul 2021 13:51:04 +0200

Alexei Starovoitov <[email-masked]> writes:

> On Wed, Jun 23, 2021 at 7:25 PM Alexei Starovoitov
> <[email-masked]> wrote:
>>
>> The bpf_timer_init() helper is receiving hidden 'map' argument and
> ...
>> +               if (insn->imm == BPF_FUNC_timer_init) {
>> +                       aux = &env->insn_aux_data[i + delta];
>> +                       if (bpf_map_ptr_poisoned(aux)) {
>> +                               verbose(env, "bpf_timer_init abusing map_ptr\n");
>> +                               return -EINVAL;
>> +                       }
>> +                       map_ptr = BPF_MAP_PTR(aux->map_ptr_state);
>> +                       {
>> +                               struct bpf_insn ld_addrs[2] = {
>> +                                       BPF_LD_IMM64(BPF_REG_3, (long)map_ptr),
>> +                               };
>
> After a couple of hours of ohh so painful debugging I realized that this
> approach doesn't work for inner maps. Duh.
> For inner maps it remembers inner_map_meta which is a template
> of inner map.
> Then bpf_timer_cb() passes map ptr into timer callback and if it tries
> to do map operations on it the inner_map_meta->ops will be valid,
> but the struct bpf_map doesn't have the actual data.
> So to support map-in-map we need to require users to pass map pointer
> explicitly into bpf_timer_init().
> Unfortunately the verifier cannot guarantee that bpf timer field inside
> map element is from the same map that is passed as a map ptr.
> The verifier can check that they're equivalent from safety pov
> via bpf_map_meta_equal(), so common user mistakes will be caught by it.
> Still not pretty that it's partially on the user to do:
> bpf_timer_init(timer, CLOCK, map);
> with 'timer' matching the 'map'.

The implication being that if they don't match, the callback will just
get a different argument and it'll be up to the developer to deal with
any bugs arising from that?

> Another option is to drop 'map' arg from timer callback,
> but the usability of the callback will suffer. The inner maps
> will be quite painful to use from it.
> Anyway I'm going with explicit 'map' arg in the next respin.
> Other ideas?

So the problem here is that the inner map pointer is not known at
verification time but only at runtime? Could the verifier inject code to
always spill inner map pointers to a known area of the stack after a
map-in-map lookup, and then just load them back from there when needed?
Not sure that would be worth the complexity (and overhead!), though;
having to supply an explicit callback arg is not that uncommon a pattern
after all...

-Toke