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From:   Matteo Croce <mcroce@redhat.com>
Date:   Fri, 25 Oct 2019 20:24:20 +0200
Message-ID: <CAGnkfhzN=P+j5n3A2RrRTseHgqMU1-5CsRd8xonZ2mLBtNoJ_g@mail.gmail.com>
Subject: Re: [PATCH net-next 3/4] flow_dissector: extract more ICMP information
To:     Simon Horman <simon.horman@netronome.com>
Cc:     netdev <netdev@vger.kernel.org>,
        Jay Vosburgh <j.vosburgh@gmail.com>,
        Veaceslav Falico <vfalico@gmail.com>,
        Andy Gospodarek <andy@greyhouse.net>,
        "David S . Miller" <davem@davemloft.net>,
        Stanislav Fomichev <sdf@google.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Song Liu <songliubraving@fb.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Paul Blakey <paulb@mellanox.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Oct 25, 2019 at 8:29 AM Simon Horman <simon.horman@netronome.com> w=
rote:
>
> On Fri, Oct 25, 2019 at 02:27:28AM +0200, Matteo Croce wrote:
> > On Wed, Oct 23, 2019 at 7:55 PM Simon Horman <simon.horman@netronome.co=
m> wrote:
> > >
> > > On Wed, Oct 23, 2019 at 12:53:37PM +0200, Matteo Croce wrote:
> > > > On Wed, Oct 23, 2019 at 12:00 PM Simon Horman
> > > > <simon.horman@netronome.com> wrote:
> > > > > On Mon, Oct 21, 2019 at 10:09:47PM +0200, Matteo Croce wrote:
> > > > > > +     switch (ih->type) {
> > > > > > +     case ICMP_ECHO:
> > > > > > +     case ICMP_ECHOREPLY:
> > > > > > +     case ICMP_TIMESTAMP:
> > > > > > +     case ICMP_TIMESTAMPREPLY:
> > > > > > +     case ICMPV6_ECHO_REQUEST:
> > > > > > +     case ICMPV6_ECHO_REPLY:
> > > > > > +             /* As we use 0 to signal that the Id field is not=
 present,
> > > > > > +              * avoid confusion with packets without such fiel=
d
> > > > > > +              */
> > > > > > +             key_icmp->id =3D ih->un.echo.id ? : 1;
> > > > >
> > > > > Its not obvious to me why the kernel should treat id-zero as a sp=
ecial
> > > > > value if it is not special on the wire.
> > > > >
> > > > > Perhaps a caller who needs to know if the id is present can
> > > > > check the ICMP type as this code does, say using a helper.
> > > > >
> > > >
> > > > Hi,
> > > >
> > > > The problem is that the 0-0 Type-Code pair identifies the echo repl=
ies.
> > > > So instead of adding a bool is_present value I hardcoded the info i=
n
> > > > the ID field making it always non null, at the expense of a possibl=
e
> > > > collision, which is harmless.
> > >
> > > Sorry, I feel that I'm missing something here.
> > >
> > > My reading of the code above is that for the cased types above
> > > (echo, echo reply, ...) the id is present. Otherwise it is not.
> > > My idea would be to put a check for those types in a helper.
> > >
> >
> > Something like icmp_has_id(), I like it.
> >
> > > I do agree that the override you have used is harmless enough
> > > in the context of the only user of the id which appears in
> > > the following patch of this series.
> > >
> > >
> > > Some other things I noticed in this patch on a second pass:
> > >
> > > * I think you can remove the icmp field from struct flow_dissector_ke=
y_ports
> > >
> >
> > You mean flow_dissector_key_icmp maybe?
>
> Yes, sorry for the misinformation.
>
> > > * I think that adding icmp to struct flow_keys should be accompanied =
by
> > >   adding ICMP to flow_keys_dissector_symmetric_keys. But I think this=
 is
> > >   not desirable outside of the bonding use-case and rather
> > >   the bonding driver should define its own structures that
> > >   includes the keys it needs - basically copies of struct flow_keys
> > >   and flow_keys_dissector_symmetric_keys with some modifications.
> > >
> >
> > Just flow_keys_dissector_symmetric_keys or flow_keys_dissector_keys too=
?
> > Anyway, it seems that the bonding uses the flow_dissector only when
> > using encap2+3 or encap3+4 hashing, which means decap some known
> > tunnels (mpls and gre and pppoe I think).
>
> That is the use case I noticed.
>
> In that case it uses skb_flow_dissect_flow_keys() which in turn
> uses struct flow_keys and flow_keys_basic_dissector_keys (which is
> assigned to flow_keys_dissector_keys.
>
> Sorry about mentioning flow_keys_dissector_symmetric_keys, I think
> that was a copy-paste-error on my side.
>

np

> In any case, my point is that if you update struct flow_keys then likely
> some corresponding change should also be made to one or more of
> *__dissector_keys. But such a change would have scope outside of bonding,
> which is perhaps undesirable. So it might be better to make local
> structures and call __skb_flow_dissect from within the bonding code.
>

What drawbacks will it have to have the ICMP dissector enabled with
flow_keys_dissector_keys?

I see three options here:
1. add the ICMP key in flow_keys_dissector_keys and change the
flow_dissector behaviour, when dealing with echoes
2. do a local copy in the bonding code
3. leave flow_keys_dissector_keys as is, so the bonding will balance
echoes only when not decapping tunnels

I don't really know if option 1 could be a bug or a feature, sure
option 2 is safer. That can be changed later easily anyway.

>
> As for other use cases, that do not currently use the dissector,
> I think you will need to update them too to get then desired new
> feature introduced in patch 4 for those use-cases, which I assume is
> desired. Perhaps converting those use-cases to use the flow dissector
> is a good way forwards. Perhaps not.
>

I don't really know why the bonding doesn't use the dissector.
Performance? Anyway, maybe converting the bonding to
the flow_dissector would make sense, this can be done in the future.
I have to talk with the bonding maintainers to understand what's
behind this choice.

--=20
Matteo Croce
per aspera ad upstream