Re: [PATCH v9 bpf-next 06/14] bpf: Remove recursion call in btf_struct_access

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Aug 04, 2020 at 11:12:49PM -0700, Andrii Nakryiko wrote:
> On Sat, Aug 1, 2020 at 10:04 AM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > Andrii suggested we can simply jump to again label
> > instead of making recursion call.
> >
> > Suggested-by: Andrii Nakryiko <andriin@fb.com>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  kernel/bpf/btf.c | 11 +++++------
> >  1 file changed, 5 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index bc05a24f7361..0f995038b589 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -3931,14 +3931,13 @@ int btf_struct_access(struct bpf_verifier_log *log,
> >                 /* Only allow structure for now, can be relaxed for
> >                  * other types later.
> >                  */
> > -               elem_type = btf_type_skip_modifiers(btf_vmlinux,
> > -                                                   array_elem->type, NULL);
> > -               if (!btf_type_is_struct(elem_type))
> > +               t = btf_type_skip_modifiers(btf_vmlinux, array_elem->type,
> > +                                           NULL);
> > +               if (!btf_type_is_struct(t))
> >                         goto error;
> >
> > -               off = (off - moff) % elem_type->size;
> > -               return btf_struct_access(log, elem_type, off, size, atype,
> > -                                        next_btf_id);
> > +               off = (off - moff) % t->size;
> > +               goto again;
> 
> Transformation looks good, thanks. So:
> 
> Acked-by: Andrii Nakryiko <andriin@fb.com>
> 
> But this '% t->size' makes me wonder what will happen when we have an
> array of zero-sized structs or multi-dimensional arrays with
> dimensions of size 0... I.e.:
> 
> struct {} arr[123];
> 
> or
> 
> int arr[0][0]0];
> 
> We should probably be more careful with division here.

right, definitely..  I'll send follow up patch for that

thanks,
jirka