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

[Jiri Olsa <jolsa@redhat.com>]

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)
> > +{
> > +       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

> 
> > +       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?

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)
> >  {
> 
> [...]
>