Re: [PATCH v8 bpf-next 07/13] bpf: Add btf_struct_ids_match function

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Jul 29, 2020 at 9:04 AM Jiri Olsa <jolsa@redhat.com> wrote:
>
> On Tue, Jul 28, 2020 at 04:35:16PM -0700, Andrii Nakryiko wrote:
>
> SNIP
>
> > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > > index bae557ff2da8..c981e258fed3 100644
> > > --- a/include/linux/bpf.h
> > > +++ b/include/linux/bpf.h
> > > @@ -1306,6 +1306,8 @@ int btf_struct_access(struct bpf_verifier_log *log,
> > >                       const struct btf_type *t, int off, int size,
> > >                       enum bpf_access_type atype,
> > >                       u32 *next_btf_id);
> > > +bool btf_struct_ids_match(struct bpf_verifier_log *log,
> > > +                         int off, u32 id, u32 mid);
> > >  int btf_resolve_helper_id(struct bpf_verifier_log *log,
> > >                           const struct bpf_func_proto *fn, int);
> > >
> > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > > index 1ab5fd5bf992..562d4453fad3 100644
> > > --- a/kernel/bpf/btf.c
> > > +++ b/kernel/bpf/btf.c
> > > @@ -4140,6 +4140,35 @@ int btf_struct_access(struct bpf_verifier_log *log,
> > >         return -EINVAL;
> > >  }
> > >
> > > +bool btf_struct_ids_match(struct bpf_verifier_log *log,
> > > +                         int off, u32 id, u32 mid)

just realized that if id == mid and off == 0, btf_struct_ids_match()
will return false. Right now verifier is careful to not call
btf_struct_ids_match in such case, but I wonder if it's better to make
that (common) case also work?

> > > +{
> > > +       const struct btf_type *type;
> > > +       u32 nid;
> > > +       int err;
> > > +
> >
> > mid and nid are terrible names, especially as an input argument name.
> > mid == need_type_id? nid == cur_type_id or something along those
> > lines?
>
> 'mid' was for matching id, 'nid' for nested id ;-)
> need_type_id/cur_type_id sound good

nested I guessed, mid was a mystery to me :))

>
> >
> > > +       do {
> > > +               type = btf_type_by_id(btf_vmlinux, id);
> > > +               if (!type)
> > > +                       return false;
> > > +               err = btf_struct_walk(log, type, off, 1, &nid);
> > > +               if (err < 0)
> > > +                       return false;
> > > +
> > > +               /* We found nested struct object. If it matches
> > > +                * the requested ID, we're done. Otherwise let's
> > > +                * continue the search with offset 0 in the new
> > > +                * type.
> > > +                */
> > > +               if (err == walk_struct && mid == nid)
> > > +                       return true;
> > > +               off = 0;
> > > +               id = nid;
> > > +       } while (err == walk_struct);
> >
> > This seems like a slightly more obvious control flow:
> >
> > again:
> >
> >    ...
> >
> >    if (err != walk_struct)
> >       return false;
>
> ok, and perhaps use in here the switch(err) as in the previous patch?

I think straightforward if is better than switch here, because
anything but walk_struct is not what we expect.

>
> thanks,
> jirka
>
> >
> >    if (mid != nid) {
> >       off = 0;
> >       id = nid;
> >       goto again;
> >    }
> >
> >    return true;
> >
> > > +
> > > +       return false;
> > > +}
> > > +
> > >  int btf_resolve_helper_id(struct bpf_verifier_log *log,
> > >                           const struct bpf_func_proto *fn, int arg)
> > >  {
> >
> > [...]
> >
>