Re: [RFC bpf-next v1 3/7] bpf: Support pulling non-linear xdp data

[Amery Hung <ameryhung@gmail.com>]

On Tue, Aug 26, 2025 at 6:20 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Mon, 25 Aug 2025 22:12:21 -0700 Amery Hung wrote:
> > > > +     data_end = xdp->data + len;
> > > > +     delta = data_end - xdp->data_end;
> > > > +
> > > > +     if (delta <= 0)
> > > > +             return 0;
> > > > +
> > > > +     if (unlikely(data_end > data_hard_end))
> > > > +             return -EINVAL;
> > >
> > > Is this safe against pointers wrapping on 32b systems?
> > >
> >
> > You are right. This may be a problem.
> >
> > > Maybe it's better to do:
> > >
> > >          if (unlikely(data_hard_end - xdp->data_end < delta))
> > >
> > > ?
> >
> > But delta may be negative if the pointer wraps around and then the
> > function will still continue. How about adding data_end < xdp->data
> > check and reorganizing the checks like this?
>
> You already checked that delta is positive in the previous if (),
> so I think it's safe. Admittedly having 3 separate conditions is
> more readable but it's not strictly necessary. Up to you.

Got it. I will change to the new set of checks. The original kfunc
would return 0 when pointer wrapping happens and delta <= 0.