netdev - Re: [PATCH net-next v3 1/5] tc/act: user space can't use TC_ACT

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Message-ID: <f65cbf3d99529ff8b820788ebe09466560d19c14.camel@redhat.com>
Date:   Wed, 25 Jul 2018 17:48:23 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>, netdev@...r.kernel.org,
        Cong Wang <xiyou.wangcong@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Eyal Birger <eyal.birger@...il.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next v3 1/5] tc/act: user space can't use
 TC_ACT_REDIRECT directly

On Wed, 2018-07-25 at 15:03 +0200, Jiri Pirko wrote:
> Wed, Jul 25, 2018 at 02:54:04PM CEST, pabeni@...hat.com wrote:
> > Hi,
> > 
> > On Wed, 2018-07-25 at 13:56 +0200, Jiri Pirko wrote:
> > > Tue, Jul 24, 2018 at 10:06:39PM CEST, pabeni@...hat.com wrote:
> > > > Only cls_bpf and act_bpf can safely use such value. If a generic
> > > > action is configured by user space to return TC_ACT_REDIRECT,
> > > > the usually visible behavior is passing the skb up the stack - as
> > > > for unknown action, but, with complex configuration, more random
> > > > results can be obtained.
> > > > 
> > > > This patch forcefully converts TC_ACT_REDIRECT to TC_ACT_UNSPEC
> > > > at action init time, making the kernel behavior more consistent.
> > > > 
> > > > v1 -> v3: use TC_ACT_UNSPEC instead of a newly definied act value
> > > > 
> > > > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > > > ---
> > > > net/sched/act_api.c | 5 +++++
> > > > 1 file changed, 5 insertions(+)
> > > > 
> > > > diff --git a/net/sched/act_api.c b/net/sched/act_api.c
> > > > index 148a89ab789b..24b5534967fe 100644
> > > > --- a/net/sched/act_api.c
> > > > +++ b/net/sched/act_api.c
> > > > @@ -895,6 +895,11 @@ struct tc_action *tcf_action_init_1(struct net *net, struct tcf_proto *tp,
> > > > 		}
> > > > 	}
> > > > 
> > > > +	if (a->tcfa_action == TC_ACT_REDIRECT) {
> > > > +		net_warn_ratelimited("TC_ACT_REDIRECT can't be used directly");
> > > 
> > > Can't you push this warning through extack?
> > > 
> > > But, wouldn't it be more appropriate to fail here? User is passing
> > > invalid configuration....
> > 
> > Jiri, Jamal, thank you for the feedback.
> > 
> > Please allow me to answer both of you here, since you raised similar
> > concers.
> > 
> > I thought about rejecting the action, but that change of behavior could
> > break some users, as currently most kind of invalid tcfa_action values
> > are simply accepted.
> > 
> > If there is consensus about it, I can simply fail.
> 
> Well it was obviously wrong to expose TC_ACT_REDIRECT to uapi and it
> really has no meaning for anyone to use it throughout its whole history.
> I would vote for "fail", yet I admit that I am usually alone in opinion
> about similar uapi changes :)

Since even Jamal suggested the same, unless someone else voice some
opposition soon, in v4 I'll opt for rejecting actions using
TC_ACT_REDIRECT.

Thanks,

Paolo