Subject: Re: [PATCH bpf-next v2 12/28] bpf/hid: add hid_{get|set}_data helpers

From: Greg KH <[email-masked]>
Date: Sat, 5 Mar 2022 11:47:17 +0100

On Sat, Mar 05, 2022 at 11:33:07AM +0100, Benjamin Tissoires wrote:
> On Fri, Mar 4, 2022 at 7:41 PM Greg KH <[email-masked]> wrote:
> >
> > On Fri, Mar 04, 2022 at 06:28:36PM +0100, Benjamin Tissoires wrote:
> > > When we process an incoming HID report, it is common to have to account
> > > for fields that are not aligned in the report. HID is using 2 helpers
> > > hid_field_extract() and implement() to pick up any data at any offset
> > > within the report.
> > >
> > > Export those 2 helpers in BPF programs so users can also rely on them.
> > > The second net worth advantage of those helpers is that now we can
> > > fetch data anywhere in the report without knowing at compile time the
> > > location of it. The boundary checks are done in hid-bpf.c, to prevent
> > > a memory leak.
> > >
> > > Signed-off-by: Benjamin Tissoires <[email-masked]>
> > >
> > > ---
> > >
> > > changes in v2:
> > > - split the patch with libbpf and HID left outside.
> > > ---
> > >  include/linux/bpf-hid.h        |  4 +++
> > >  include/uapi/linux/bpf.h       | 32 ++++++++++++++++++++
> > >  kernel/bpf/hid.c               | 53 ++++++++++++++++++++++++++++++++++
> > >  tools/include/uapi/linux/bpf.h | 32 ++++++++++++++++++++
> > >  4 files changed, 121 insertions(+)
> > >
> > > diff --git a/include/linux/bpf-hid.h b/include/linux/bpf-hid.h
> > > index 0c5000b28b20..69bb28523ceb 100644
> > > --- a/include/linux/bpf-hid.h
> > > +++ b/include/linux/bpf-hid.h
> > > @@ -93,6 +93,10 @@ struct bpf_hid_hooks {
> > >       int (*link_attach)(struct hid_device *hdev, enum bpf_hid_attach_type type);
> > >       void (*link_attached)(struct hid_device *hdev, enum bpf_hid_attach_type type);
> > >       void (*array_detached)(struct hid_device *hdev, enum bpf_hid_attach_type type);
> > > +     int (*hid_get_data)(struct hid_device *hdev, u8 *buf, size_t buf_size,
> > > +                         u64 offset, u32 n, u8 *data, u64 data_size);
> > > +     int (*hid_set_data)(struct hid_device *hdev, u8 *buf, size_t buf_size,
> > > +                         u64 offset, u32 n, u8 *data, u64 data_size);
> > >  };
> > >
> > >  #ifdef CONFIG_BPF
> > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > index a7a8d9cfcf24..4845a20e6f96 100644
> > > --- a/include/uapi/linux/bpf.h
> > > +++ b/include/uapi/linux/bpf.h
> > > @@ -5090,6 +5090,36 @@ union bpf_attr {
> > >   *   Return
> > >   *           0 on success, or a negative error in case of failure. On error
> > >   *           *dst* buffer is zeroed out.
> > > + *
> > > + * int bpf_hid_get_data(void *ctx, u64 offset, u32 n, u8 *data, u64 size)
> > > + *   Description
> > > + *           Get the data of size n (in bits) at the given offset (bits) in the
> > > + *           ctx->event.data field and store it into data.
> > > + *
> > > + *           if n is less or equal than 32, we can address with bit precision,
> > > + *           the value in the buffer. However, data must be a pointer to a u32
> > > + *           and size must be 4.
> > > + *
> > > + *           if n is greater than 32, offset and n must be a multiple of 8
> > > + *           and the result is working with a memcpy internally.
> > > + *   Return
> > > + *           The length of data copied into data. On error, a negative value
> > > + *           is returned.
> > > + *
> > > + * int bpf_hid_set_data(void *ctx, u64 offset, u32 n, u8 *data, u64 size)
> > > + *   Description
> > > + *           Set the data of size n (in bits) at the given offset (bits) in the
> > > + *           ctx->event.data field.
> > > + *
> > > + *           if n is less or equal than 32, we can address with bit precision,
> > > + *           the value in the buffer. However, data must be a pointer to a u32
> > > + *           and size must be 4.
> > > + *
> > > + *           if n is greater than 32, offset and n must be a multiple of 8
> > > + *           and the result is working with a memcpy internally.
> > > + *   Return
> > > + *           The length of data copied into ctx->event.data. On error, a negative
> > > + *           value is returned.
> >
> 
> Quick answer on this one (before going deeper with the other remarks next week):
> 
> > Wait, nevermind my reviewed-by previously, see my comment about how this
> > might be split into 4:
> >         bpf_hid_set_bytes()
> >         bpf_hid_get_bytes()
> >         bpf_hid_set_bits()
> >         bpf_hid_get_bits()
> >
> > Should be easier to understand and maintain over time, right?
> 
> Yes, definitively. I thought about adding a `bytes` suffix to the
> function name for n > 32, but not the `bits` one, meaning the API was
> still bunkers in my head.
> 
> And as I mentioned in the commit notes, I knew the API proposed in
> this patch was not correct, simply because while working on the
> selftests it was completely wrong :)
> 
> In terms of API usage, does it feel wrong for you to have only a
> subset of the array available "for free" and enforce the rest to be
> used through these helpers?

It does feel "odd" but:

> That's the point I am not sure but I feel like 1024 (or slightly more)
> would be enough for most use cases, and when we are dealing with
> bigger data sets the helpers can be justified.

How often are hid reports that big?  If providing access to the first
1024 or so bytes for free would handle the huge majority of fixup cases,
and only if you have crazy devices with large reports would you have to
use the functions, it might be fine.

The problem is, only time will tell, and then, it's too late to change
the api :)

So I agree with this for now, stick with what you have (if you split
this into 4 functions), and let's see what happens.

thanks,

greg k-h