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

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Date:   Tue, 22 Mar 2022 15:51:21 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Jiri Kosina <jikos@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Shuah Khan <shuah@...nel.org>,
        Dave Marchevsky <davemarchevsky@...com>,
        Joe Stringer <joe@...ium.io>, Jonathan Corbet <corbet@....net>,
        Tero Kristo <tero.kristo@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-input@...r.kernel.org,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 06/17] HID: allow to change the report
 descriptor from an eBPF program

On Fri, Mar 18, 2022 at 9:16 AM Benjamin Tissoires
<benjamin.tissoires@...hat.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.
We suggest to use __weak global functions and unstable kfunc helpers
to achieve the same goal.
This way HID side and bpf side can evolve without introducing
stable uapi burden.
For example this particular patch can be compressed to:
__weak int hid_bpf_report_fixup(struct hid_device *hdev, u8 *rdesc,
unsigned int *size)
{
   return 0;
}
ALLOW_ERROR_INJECTION(ALLOW_ERROR_INJECTION, ERRNO);

- buf = kmemdup(start, size, GFP_KERNEL);
+ if (!hid_bpf_report_fixup(device, start, &size))
+   buf = kmemdup(start, size, GFP_KERNEL);

Then bpf program can replace hid_bpf_report_fixup function and adjust its
return value while reading args.

Similar approach can be done with all other hooks.

Once api between HID and bpf stabilizes we can replace nop functions
with writeable tracepoints to make things a bit more stable
while still allowing for change of the interface in the future.

The amount of bpf specific code in HID core will be close to zero
while bpf can be used to flexibly tweak it.

kfunc is a corresponding mechanism to introduce unstable api
from bpf into the kernel instead of stable helpers.
Just whitelist some functions as unstable kfunc helpers and call them
from bpf progs.
See net/bpf/test_run.c and bpf_kfunc_call* for inspiration.