Re: [PATCH bpf-next v4 4/6] bpf/helpers: mark the callback of bpf_timer_set_sleepable_cb() as sleepable

[Benjamin Tissoires]

On Tue, Mar 19, 2024 at 12:54 AM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
>
> On Fri, 2024-03-15 at 15:29 +0100, Benjamin Tissoires wrote:
> [...]
>
> > @@ -5279,7 +5281,8 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
> >
> >  static bool in_sleepable(struct bpf_verifier_env *env)
> >  {
> > -     return env->prog->sleepable;
> > +     return env->prog->sleepable ||
> > +            (env->cur_state && env->cur_state->in_sleepable);
> >  }
>
> I was curious why 'env->cur_state &&' check was needed and found that
> removing it caused an error in the following fragment:
>
> static int do_misc_fixups(struct bpf_verifier_env *env)
> {
>                 ...
>                 if (is_storage_get_function(insn->imm)) {
>                         if (!in_sleepable(env) ||
>                             env->insn_aux_data[i + delta].storage_get_func_atomic)
>                                 insn_buf[0] = BPF_MOV64_IMM(BPF_REG_5, (__force __s32)GFP_ATOMIC);
>                         else
>                                 insn_buf[0] = BPF_MOV64_IMM(BPF_REG_5, (__force __s32)GFP_KERNEL);
>                         ...
>                 }
>                 ...
> }
>
> When do_misc_fixups() is done env->cur_state is NULL.
> Current implementation would use GFP_ATOMIC allocation even for
> sleepable callbacks, where GFP_KERNEL is sufficient.
> Is this is something we want to address?

I honestly have no idea of the impact there.

AFAICT, if env->cur_state is not set, we don't even know if the
callback will be sleepable or not, so if there is a small penalty,
then it's the safest option, no?

Cheers,
Benjamin

>
> >
> >  /* The non-sleepable programs and sleepable programs with explicit bpf_rcu_read_lock()
>