Re: [PATCH bpf-next v3 06/17] HID: allow to change the report descriptor from an eBPF program

[Benjamin Tissoires <benjamin.tissoires@redhat.com>]

On Fri, Mar 25, 2022 at 6:00 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Wed, Mar 23, 2022 at 9:08 AM Benjamin Tissoires
> <benjamin.tissoires@redhat.com> wrote:
> >
> > Hi Alexei,
> >
> > On Tue, Mar 22, 2022 at 11:51 PM Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> > >
> > > On Fri, Mar 18, 2022 at 9:16 AM Benjamin Tissoires
> > > <benjamin.tissoires@redhat.com> wrote:
> > > >
> > > > +u8 *hid_bpf_report_fixup(struct hid_device *hdev, u8 *rdesc, unsigned int *size)
> > > > +{
> > > > +       int ret;
> > > > +       struct hid_bpf_ctx_kern ctx = {
> > > > +               .type = HID_BPF_RDESC_FIXUP,
> > > > +               .hdev = hdev,
> > > > +               .size = *size,
> > > > +       };
> > > > +
> > > > +       if (bpf_hid_link_empty(&hdev->bpf, BPF_HID_ATTACH_RDESC_FIXUP))
> > > > +               goto ignore_bpf;
> > > > +
> > > > +       ctx.data = kmemdup(rdesc, HID_MAX_DESCRIPTOR_SIZE, GFP_KERNEL);
> > > > +       if (!ctx.data)
> > > > +               goto ignore_bpf;
> > > > +
> > > > +       ctx.allocated_size = HID_MAX_DESCRIPTOR_SIZE;
> > > > +
> > > > +       ret = hid_bpf_run_progs(hdev, &ctx);
> > > > +       if (ret)
> > > > +               goto ignore_bpf;
> > > > +
> > > > +       if (ctx.size > ctx.allocated_size)
> > > > +               goto ignore_bpf;
> > > > +
> > > > +       *size = ctx.size;
> > > > +
> > > > +       if (*size) {
> > > > +               rdesc = krealloc(ctx.data, *size, GFP_KERNEL);
> > > > +       } else {
> > > > +               rdesc = NULL;
> > > > +               kfree(ctx.data);
> > > > +       }
> > > > +
> > > > +       return rdesc;
> > > > +
> > > > + ignore_bpf:
> > > > +       kfree(ctx.data);
> > > > +       return kmemdup(rdesc, *size, GFP_KERNEL);
> > > > +}
> > > > +
> > > >  int __init hid_bpf_module_init(void)
> > > >  {
> > > >         struct bpf_hid_hooks hooks = {
> > > >                 .hdev_from_fd = hid_bpf_fd_to_hdev,
> > > >                 .pre_link_attach = hid_bpf_pre_link_attach,
> > > > +               .post_link_attach = hid_bpf_post_link_attach,
> > > >                 .array_detach = hid_bpf_array_detach,
> > > >         };
> > > >
> > > > diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
> > > > index 937fab7eb9c6..3182c39db006 100644
> > > > --- a/drivers/hid/hid-core.c
> > > > +++ b/drivers/hid/hid-core.c
> > > > @@ -1213,7 +1213,8 @@ int hid_open_report(struct hid_device *device)
> > > >                 return -ENODEV;
> > > >         size = device->dev_rsize;
> > > >
> > > > -       buf = kmemdup(start, size, GFP_KERNEL);
> > > > +       /* hid_bpf_report_fixup() ensures we work on a copy of rdesc */
> > > > +       buf = hid_bpf_report_fixup(device, start, &size);
> > >
> > > Looking at this patch and the majority of other patches...
> > > the code is doing a lot of work to connect HID side with bpf.
> > > At the same time the evolution of the patch series suggests
> > > that these hook points are not quite stable. More hooks and
> > > helpers are being added.
> > > It tells us that it's way too early to introduce a stable
> > > interface between HID and bpf.
> >
> > I understand that you might be under the impression that the interface
> > is changing a lot, but this is mostly due to my poor knowledge of all
> > the arcanes of eBPF.
> > The overall way HID-BPF works is to work on a single array, and we
> > should pretty much be sorted out. There are a couple of helpers to be
> > able to communicate with the device, but the API has been stable in
> > the kernel for those for quite some time now.
> >
> > The variations in the hooks is mostly because I don't know what is the
> > best representation we can use in eBPF for those, and the review
> > process is changing that.
>
> I think such a big feature as this one, especially that most BPF folks
> are (probably) not familiar with the HID subsystem in the kernel,
> would benefit from a bit of live discussion during BPF office hours.
> Do you think you can give a short overview of what you are trying to
> achieve with some background context on HID specifics at one of the
> next BPF office hours? We have a meeting scheduled every week on
> Thursday, 9am Pacific time. But people need to put their topic onto
> the agenda, otherwise the meeting is cancelled. See [0] for
> spreadsheet and links to Zoom meeting, agenda, etc.

This sounds like a good idea. I just added my topic on the agenda and
will prepare some slides.

Cheers,
Benjamin

>
>   [0] https://docs.google.com/spreadsheets/d/1LfrDXZ9-fdhvPEp_LHkxAMYyxxpwBXjywWa0AejEveU
>
> [...]
>