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

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Message-ID: <20200805173658.GB319954@krava>
Date:   Wed, 5 Aug 2020 19:36:58 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Martin KaFai Lau <kafai@...com>,
        David Miller <davem@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Wenbo Zhang <ethercflow@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Brendan Gregg <bgregg@...flix.com>,
        Florent Revest <revest@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v9 bpf-next 06/14] bpf: Remove recursion call in
 btf_struct_access

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@...nel.org> wrote:
> >
> > Andrii suggested we can simply jump to again label
> > instead of making recursion call.
> >
> > Suggested-by: Andrii Nakryiko <andriin@...com>
> > Signed-off-by: Jiri Olsa <jolsa@...nel.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@...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