Re: [PATCH hid v12 03/15] HID: initial BPF implementation

[Benjamin Tissoires]

On Tue, Nov 29, 2022 at 7:00 PM Florent Revest <revest@xxxxxxxxxxxx> wrote:
>
> On Thu, Nov 24, 2022 at 4:50 PM Benjamin Tissoires
> <benjamin.tissoires@xxxxxxxxxx> wrote:
> >
> > On Wed, Nov 23, 2022 at 9:14 PM Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> wrote:
> > >
> > > On Wed, Nov 23, 2022 at 6:53 AM Benjamin Tissoires
> > > <benjamin.tissoires@xxxxxxxxxx> wrote:
> > > >
> > > > Hi Jon,
> > > >
> > > > On Wed, Nov 23, 2022 at 2:25 PM Jon Hunter <jonathanh@xxxxxxxxxx> wrote:
> > > > >
> > > > > We have a kernel test that checks for new warning and error messages on
> > > > > boot and with this change I am now seeing the following error message on
> > > > > our Tegra platforms ...
> > > > >
> > > > >   WARNING KERN hid_bpf: error while preloading HID BPF dispatcher: -13
> > > > >
> > > > > I have a quick look at the code, but I can't say I am familiar with
> > > > > this. So I wanted to ask if a way to fix this or avoid this? I see the
> > > > > code returns 0, so one option would be to make this an informational or
> > > > > debug print.
> > > >
> > > > I am not in favor of debug in that case, because I suspect it'll hide
> > > > too much when getting a bug report. Informational could do, yes.
> > > >
> > > > However, before that, I'd like to dig a little bit more on why it is
> > > > failing. I thought arm64 now has support of tracing bpf programs, so I
> > > > would not expect this to fail.
>
> We have BPF trampolines on arm64 already but no ftrace direct calls
> right now. (so trampolines get jitted but don't get called eheh :)) So
> indeed BPF tracing programs (fentry/fexit/fmod_ret) do not work on
> arm64 at the moment.

Oh, OK. Thanks for the explanation.

>
> > > Unfortunately the patches to add support for such tracing bpf progs got stuck.
> > > Florent/Mark can probably share the latest status.
>
> We are working on an implementation of ftrace direct calls that would
> fit within the constraints of arm64 and play nice with the other users
> of the ftrace call site. Hopefully we have a patch to share in the
> next couple of weeks I'd say!

yay!

>
> > Oh... I noticed Jon's config was lacking CONFIG_FTRACE. So should I
> > also add a depends on CONFIG_FTRACE to enable HID-BPF?
>
> If HID-BPF fundamentally depends on a fmod_ret program being attached
> to function, it seems to me that it should depend on both:
>     CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS (CONFIG_FTRACE or even
> CONFIG_DYNAMIC_FTRACE aren't enough, there can be architectures that
> do not support direct calls. here you noticed it on arm64 which
> luckily should get fixed someday soon but there could be other
> architectures with that issue too)
> and

OK

>     CONFIG_FUNCTION_ERROR_INJECTION (since [1] error injection needs
> to be explicitly opted-in, it's easy to miss it and fail to attach
> fmod_ret programs in mysterious ways)

Ok as well.

>
> I'm thinking that maybe encoding these two dependencies in the
> CONFIG_HID_BPF is leaking too much of the bpf tracing abstraction to
> the user. Maybe the BPF Kconfig could provide "proxy" configs like
> HAVE_BPF_FENTRY_FEXIT, HAVE_BPF_FMOD_RET (naming is hard) to expose
> these dependencies better ?

That would be nice, but requires some sort of synchronization between
our 2 trees, so I'll take the 2 configs in the HID tree, and try to
submit a patch for the bpf tree with the macro. Then I can reattach to
that macro when it hits Linus'.

>
> 1: https://lore.kernel.org/lkml/20221121104403.1545f9b5@xxxxxxxxxxxxxxxxxx/
>

That thread is a little bit worrying to me. HID-BPF relies on
CONFIG_FUNCTION_ERROR_INJECTION, and I would definitely like to see
HID-BPF in production kernels. I don't really care about cloud
servers, but chromebooks are something I'd like to have enabled. We'll
see how this thread goes I guess.

Anyway, thanks a lot for the deep explanation and understanding of my code :)

Cheers,
Benjamin