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    SubjectRe: [PATCH bpf-next v3 06/17] HID: allow to change the report descriptor from an eBPF program
    On Mon, Mar 28, 2022 at 11:35 PM Andrii Nakryiko
    <andrii.nakryiko@gmail.com> wrote:
    >
    > On Sun, Mar 27, 2022 at 11:57 PM Benjamin Tissoires
    > <benjamin.tissoires@redhat.com> wrote:
    > >
    > > 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.
    > >
    >
    > Great! Unfortunately I personally have a conflict this week and won't
    > be able to attend, so I'll have to catch up somehow through word of
    > mouth :( Next week's BPF office hours would be best, but I don't want
    > to delay discussions just because of me.

    OK. FWIW, I'll have slides publicly available once I'll do a final
    roundup on them. Hopefully that will give you enough context on HID to
    understand the problem.
    If there are too many conflicts we can surely delay by a week, but I
    would rather have the discussion happening sooner :/

    Cheers,
    Benjamin

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

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