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From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To: Kumar Kartikeya Dwivedi <memxor@gmail.com>, Benjamin Tissoires
 <bentiss@kernel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>, Benjamin Tissoires
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Subject: Re: [PATCH RFC bpf-next 0/9] allow HID-BPF to do device IOs
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Date: Tue, 13 Feb 2024 20:51:26 +0100
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Kumar Kartikeya Dwivedi <memxor@gmail.com> writes:

> On Tue, 13 Feb 2024 at 18:46, Benjamin Tissoires <bentiss@kernel.org> wro=
te:
>>
>> On Feb 12 2024, Alexei Starovoitov wrote:
>> > On Mon, Feb 12, 2024 at 10:21=E2=80=AFAM Benjamin Tissoires
>> > <benjamin.tissoires@redhat.com> wrote:
>> > >
>> > > On Mon, Feb 12, 2024 at 6:46=E2=80=AFPM Toke H=C3=B8iland-J=C3=B8rge=
nsen <toke@redhat.com> wrote:
>> > > >
>> > > > Benjamin Tissoires <benjamin.tissoires@redhat.com> writes:
>> > > >
>> [...]
>> > I agree that workqueue delegation fits into the bpf_timer concept and
>> > a lot of code can and should be shared.
>>
>> Thanks Alexei for the detailed answer. I've given it an attempt but stil=
l can not
>> figure it out entirely.
>>
>> > All the lessons(bugs) learned with bpf_timer don't need to be re-disco=
vered :)
>> > Too bad, bpf_timer_set_callback() doesn't have a flag argument,
>> > so we need a new kfunc to set a sleepable callback.
>> > Maybe
>> > bpf_timer_set_sleepable_cb() ?
>>
>> OK. So I guess I should drop Toke's suggestion with the bpf_timer_ini() =
flag?
>>
>> > The verifier will set is_async_cb =3D true for it (like it does for re=
gular cb-s).
>> > And since prog->aux->sleepable is kinda "global" we need another
>> > per subprog flag:
>> > bool is_sleepable: 1;
>>
>> done (in push_callback_call())
>>
>> >
>> > We can factor out a check "if (prog->aux->sleepable)" into a helper
>> > that will check that "global" flag and another env->cur_state->in_slee=
pable
>> > flag that will work similar to active_rcu_lock.
>>
>> done (I think), cf patch 2 below
>>
>> > Once the verifier starts processing subprog->is_sleepable
>> > it will set cur_state->in_sleepable =3D true;
>> > to make all subprogs called from that cb to be recognized as sleepable=
 too.
>>
>> That's the point I don't know where to put the new code.
>>
>
> I think that would go in the already existing special case for
> push_async_cb where you get the verifier state of the async callback.
> You can make setting the boolean in that verifier state conditional on
> whether it's your kfunc/helper you're processing taking a sleepable
> callback.
>
>> It seems the best place would be in do_check(), but I am under the impre=
ssion
>> that the code of the callback is added at the end of the instruction lis=
t, meaning
>> that I do not know where it starts, and which subprog index it correspon=
ds to.
>>
>> >
>> > A bit of a challenge is what to do with global subprogs,
>> > since they're verified lazily. They can be called from
>> > sleepable and non-sleepable contex. Should be solvable.
>>
>> I must confess this is way over me (and given that I didn't even managed=
 to make
>> the "easy" case working, that might explain things a little :-P )
>>
>
> I think it will be solvable but made somewhat difficult by the fact
> that even if we mark subprog_info of some global_func A as
> in_sleepable, so that we explore it as sleepable during its
> verification, we might encounter later another global_func that calls
> a global func, already explored as non-sleepable, in sleepable
> context. In this case I think we need to redo the verification of that
> global func as sleepable once again. It could be that it is called
> from both non-sleepable and sleepable contexts, so both paths
> (in_sleepable =3D true, and in_sleepable =3D false) need to be explored,
> or we could reject such cases, but it might be a little restrictive.
>
> Some common helper global func unrelated to caller context doing some
> auxiliary work, called from sleepable timer callback and normal main
> subprog might be an example where rejection will be prohibitive.
>
> An approach might be to explore main and global subprogs once as we do
> now, and then keep a list of global subprogs that need to be revisited
> as in_sleepable (due to being called from a sleepable context) and
> trigger do_check_common for them again, this might have to be repeated
> as the list grows on each iteration, but eventually we will have
> explored all of them as in_sleepable if need be, and the loop will
> end. Surely, this trades off logical simplicity of verifier code with
> redoing verification of global subprogs again.
>
> To add items to such a list, for each global subprog we encounter that
> needs to be analyzed as in_sleepable, we will also collect all its
> callee global subprogs by walking its instructions (a bit like
> check_max_stack_depth does).

Sorry if I'm being dense, but why is all this needed if it's already
possible to just define the timer callback from a program type that
allows sleeping, and then set the actual timeout from a different
program that is not sleepable? Isn't the set_sleepable_cb() kfunc just a
convenience then? Or did I misunderstand and it's not actually possible
to mix callback/timer arming from different program types?

-Toke