netdev - Re: [RFC PATCH 15/17] selftests/bpf: Add verifier tests for dequeue prog

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Message-ID: <CAP01T77U28HTwW2c=DEanZs09z1bFO0A+iSnAUAN+Z5r0efNew@mail.gmail.com>
Date:   Fri, 15 Jul 2022 13:11:36 +0200
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Mykola Lysenko <mykolal@...com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Freysteinn Alfredsson <freysteinn.alfredsson@....se>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [RFC PATCH 15/17] selftests/bpf: Add verifier tests for dequeue prog

On Thu, 14 Jul 2022 at 20:54, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
>
> On Wed, Jul 13, 2022 at 11:45 PM Kumar Kartikeya Dwivedi
> <memxor@...il.com> wrote:
> >
> > On Thu, 14 Jul 2022 at 07:38, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> > >
> > > On Wed, Jul 13, 2022 at 4:15 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> > > >
> > > > From: Kumar Kartikeya Dwivedi <memxor@...il.com>
> > > >
> > > > Test various cases of direct packet access (proper range propagation,
> > > > comparison of packet pointers pointing into separate xdp_frames, and
> > > > correct invalidation on packet drop (so that multiple packet pointers
> > > > are usable safely in a dequeue program)).
> > > >
> > > > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> > > > Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> > > > ---
> > >
> > > Consider writing these tests as plain C BPF code and put them in
> > > test_progs, is there anything you can't express in C and thus requires
> > > test_verifier?
> >
> > Not really, but in general I like test_verifier because it stays
> > immune to compiler shenanigans.
>
> In general I dislike them because they are almost incomprehensible. So
> unless there is a very particular sequence of low-level BPF assembly
> instructions one needs to test, I'd always opt for test_progs as more
> maintainable solution.
>
> Things like making sure that verifier rejects invalid use of
> particular objects or helpers doesn't seem to rely much on particular
> assembly sequence and can and should be expressed with plain C.
>
>
> > So going forward should test_verifier tests be avoided, and normal C
> > tests (using SEC("?...")) be preferred for these cases?
>
> In my opinion, yes, unless absolutely requiring low-level assembly to
> express conditions which are otherwise hard to express reliably in C.
>

Ok, fair point. I will replace these with C tests in the next version.

> >
> > >
> > > >  tools/testing/selftests/bpf/test_verifier.c   |  29 +++-
> > > >  .../testing/selftests/bpf/verifier/dequeue.c  | 160 ++++++++++++++++++
> > > >  2 files changed, 180 insertions(+), 9 deletions(-)
> > > >  create mode 100644 tools/testing/selftests/bpf/verifier/dequeue.c
> > > >
> > >
> > > [...]