netdev - Re: [RFC bpf-next v2 7/9] bpf: when doing BPF_PROG_TEST

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Date:   Wed, 20 Mar 2019 15:08:25 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
Cc:     Stanislav Fomichev <sdf@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf@...r.kernel.org, David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        simon.horman@...ronome.com, Willem de Bruijn <willemb@...gle.com>,
        Petar Penkov <peterpenkov96@...il.com>
Subject: Re: [RFC bpf-next v2 7/9] bpf: when doing BPF_PROG_TEST_RUN for flow
 dissector use no-skb mode

On Wed, Mar 20, 2019 at 3:02 PM Stanislav Fomichev <sdf@...ichev.me> wrote:
>
> On 03/20, Willem de Bruijn wrote:
> > On Wed, Mar 20, 2019 at 12:57 PM Stanislav Fomichev <sdf@...ichev.me> wrote:
> > >
> > > On 03/19, Willem de Bruijn wrote:
> > > > On Tue, Mar 19, 2019 at 6:21 PM Stanislav Fomichev <sdf@...gle.com> wrote:
> > > > >
> > > > > Now that we have __flow_bpf_dissect which works on raw data (by
> > > > > constructing temporary on-stack skb), use it when doing
> > > > > BPF_PROG_TEST_RUN for flow dissector.
> > > > >
> > > > > This should help us catch any possible bugs due to missing shinfo on
> > > > > the per-cpu skb.
> > > > >
> > > > > Note that existing __skb_flow_bpf_dissect swallows L2 headers and returns
> > > > > nhoff=0, we need to preserve the existing behavior.
> > > > >
> > > > > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > > > > ---
> > > > >  net/bpf/test_run.c | 48 ++++++++++++++--------------------------------
> > > > >  1 file changed, 14 insertions(+), 34 deletions(-)
> > > > >
> > > >
> > > > > @@ -300,9 +277,13 @@ int bpf_prog_test_run_flow_dissector(struct bpf_prog *prog,
> > > > >         preempt_disable();
> > > > >         time_start = ktime_get_ns();
> > > > >         for (i = 0; i < repeat; i++) {
> > > > > -               retval = bpf_flow_dissect_skb(prog, skb,
> > > > > -                                             &flow_keys_dissector,
> > > > > -                                             &flow_keys);
> > > > > +               retval = bpf_flow_dissect(prog, data, eth->h_proto, ETH_HLEN,
> > > > > +                                         size, &flow_keys_dissector,
> > > > > +                                         &flow_keys);
> > > > > +               if (flow_keys.nhoff >= ETH_HLEN)
> > > > > +                       flow_keys.nhoff -= ETH_HLEN;
> > > > > +               if (flow_keys.thoff >= ETH_HLEN)
> > > > > +                       flow_keys.thoff -= ETH_HLEN;
> > > >
> > > > why are these conditional?
> > > Hm, I didn't want these to be negative, because bpf flow program can set
> > > them to zero and clamp_flow_keys makes sure they are in a "sensible"
> > > range. For this particular case, I think we need to amend
> > > clamp_flow_keys to make sure that flow_keys.nhoff is in the range of
> > > initial_nhoff..hlen, not 0..hlen (and then we can drop these checks).
> >
> > So, previously eth_type_trans would call with data at the network
> > header. Now it is called with data at the link layer. How would
> > __skb_flow_bpf_dissect "swallows L2 headers and returns nhoff=0"? That
> s/__skb_flow_bpf_dissect/eth_type_trans/, I'll clarify that in the patch
> description.
>
> > sounds incorrect.
> Previously, for skb case, eth_type_trans would pull ETH_HLEN (L2) and
> after that we did skb_reset_network_header. So when later we initialized
> flow keys (flow_keys->nhoff = skb_network_offset(skb)), that would
> yield nhoff == 0.
>
> For example, see:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/prog_tests/flow_dissector.c
>
> Now, we explicitly call bpf_flow_dissect with nhoff=ETH_HLEN and have to
> undo it, otherwise, it breaks those tests.
>
> We could do something like the following instead:
> retval = bpf_flow_dissect(prog, data + ETH_HLEN, eth->h_proto, 0,
>                           size, &flow_keys_dissector,
>                           &flow_keys);
>
> But I wanted to make sure nhoff != 0 works.

Makes sense. Ensuring that nhoff lies within initial_nhoff..hlen
sounds correct to me. But this is a limitation of the test, so should
be in the test logic, not in the generic clamp code. Perhaps just fail
the test if returned nhoff < ETH_HLEN?