Re: [PATCH RFC bpf-next 0/9] allow HID-BPF to do device IOs

[Alexei Starovoitov]

On Tue, Feb 13, 2024 at 08:51:26PM +0100, Toke Høiland-Jørgensen wrote:
> Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> writes:
> 
> > On Tue, 13 Feb 2024 at 18:46, Benjamin Tissoires <bentiss@xxxxxxxxxx> wrote:
> >>
> >> On Feb 12 2024, Alexei Starovoitov wrote:
> >> > On Mon, Feb 12, 2024 at 10:21 AM Benjamin Tissoires
> >> > <benjamin.tissoires@xxxxxxxxxx> wrote:
> >> > >
> >> > > On Mon, Feb 12, 2024 at 6:46 PM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
> >> > > >
> >> > > > Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx> 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 still can not
> >> figure it out entirely.
> >>
> >> > All the lessons(bugs) learned with bpf_timer don't need to be re-discovered :)
> >> > 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 = true for it (like it does for regular 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_sleepable
> >> > 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 = 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 impression
> >> that the code of the callback is added at the end of the instruction list, meaning
> >> that I do not know where it starts, and which subprog index it corresponds 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 = true, and in_sleepable = 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?

More than just convience.
bpf_set_sleepable_cb() might need to be called from non-sleepable and
there could be no way to hack it around with fake sleepable entry.
bpf_timer_cancel() clears callback_fn.
So if prog wants to bpf_timer_start() and later bpf_timer_cancel()
it would need to bpf_set_sleepable_cb() every time before bpf_timer_start().
And at that time it might be in non-sleepable ctx.