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

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Message-ID: <15b57fa8-670a-a7a3-a5f5-0685207d054f@iogearbox.net>
Date:   Wed, 25 Jul 2018 18:29:54 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Paolo Abeni <pabeni@...hat.com>, Jiri Pirko <jiri@...nulli.us>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>, netdev@...r.kernel.org,
        Cong Wang <xiyou.wangcong@...il.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Eyal Birger <eyal.birger@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        alexei.starovoitov@...il.com
Subject: Re: [PATCH net-next v3 1/5] tc/act: user space can't use
 TC_ACT_REDIRECT directly

On 07/25/2018 05:48 PM, Paolo Abeni wrote:
> 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:
>>> 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.

That claim is completely wrong.

>> 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.

You should probably leave out act_bpf from that rejection as there may be
a small chance that users could potentially use it as default action.

Thanks,
Daniel