linux-kernel - Re: [PATCH HID 06/12] HID: bpf: add HID-BPF hooks for hid_hw_output

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Date: Fri, 21 Jun 2024 08:37:15 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Benjamin Tissoires <bentiss@...nel.org>
Cc: Jiri Kosina <jikos@...nel.org>, Alexei Starovoitov <ast@...nel.org>, Shuah Khan <shuah@...nel.org>, 
	Jonathan Corbet <corbet@....net>, "open list:HID CORE LAYER" <linux-input@...r.kernel.org>, 
	LKML <linux-kernel@...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 HID 06/12] HID: bpf: add HID-BPF hooks for hid_hw_output_report

On Fri, Jun 21, 2024 at 1:56 AM Benjamin Tissoires <bentiss@...nel.org> wrote:
>
> Same story than hid_hw_raw_requests:
>
> This allows to intercept and prevent or change the behavior of
> hid_hw_output_report() from a bpf program.
>
> The intent is to solve a couple of use case:
>   - firewalling a HID device: a firewall can monitor who opens the hidraw
>     nodes and then prevent or allow access to write operations on that
>     hidraw node.
>   - change the behavior of a device and emulate a new HID feature request
>
> The hook is allowed to be run as sleepable so it can itself call
> hid_hw_output_report(), which allows to "convert" one feature request into
> another or even call the feature request on a different HID device on the
> same physical device.
>
> Signed-off-by: Benjamin Tissoires <bentiss@...nel.org>
>
> ---
>
> Here checkpatch complains about:
> WARNING: use of RCU tasks trace is incorrect outside BPF or core RCU code
>
> However, we are jumping in BPF code, so I think this is correct, but I'd
> like to have the opinion on the BPF folks.
> ---
>  drivers/hid/bpf/hid_bpf_dispatch.c   | 37 ++++++++++++++++++++++++++++++++----
>  drivers/hid/bpf/hid_bpf_struct_ops.c |  1 +
>  drivers/hid/hid-core.c               | 10 ++++++++--
>  drivers/hid/hidraw.c                 |  2 +-
>  include/linux/hid.h                  |  3 ++-
>  include/linux/hid_bpf.h              | 24 ++++++++++++++++++++++-
>  6 files changed, 68 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/hid/bpf/hid_bpf_dispatch.c b/drivers/hid/bpf/hid_bpf_dispatch.c
> index 8d6e08b7c42f..2a29a0625a3b 100644
> --- a/drivers/hid/bpf/hid_bpf_dispatch.c
> +++ b/drivers/hid/bpf/hid_bpf_dispatch.c
> @@ -111,6 +111,38 @@ int dispatch_hid_bpf_raw_requests(struct hid_device *hdev,
>  }
>  EXPORT_SYMBOL_GPL(dispatch_hid_bpf_raw_requests);
>
> +int dispatch_hid_bpf_output_report(struct hid_device *hdev,
> +                                  __u8 *buf, u32 size, __u64 source,
> +                                  bool from_bpf)
> +{
> +       struct hid_bpf_ctx_kern ctx_kern = {
> +               .ctx = {
> +                       .hid = hdev,
> +                       .allocated_size = size,
> +                       .size = size,
> +               },
> +               .data = buf,
> +               .from_bpf = from_bpf,
> +       };
> +       struct hid_bpf_ops *e;
> +       int ret;
> +
> +       rcu_read_lock_trace();
> +       list_for_each_entry_rcu(e, &hdev->bpf.prog_list, list) {
> +               if (e->hid_hw_output_report) {
> +                       ret = e->hid_hw_output_report(&ctx_kern.ctx, source);
> +                       if (ret)
> +                               goto out;
> +               }
> +       }
> +       ret = 0;
> +
> +out:
> +       rcu_read_unlock_trace();

same question.
What protects prog_list ?
list_for_each_entry_rcu() should be used within RCU CS
if elements of that list are freed via call_rcu().
rcu_read_lock_trace() looks wrong here.