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

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Date:   Thu, 26 Jul 2018 09:43:00 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        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

Wed, Jul 25, 2018 at 06:29:54PM CEST, daniel@...earbox.net wrote:
>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.

Why? Does addition of TC_ACT_REDIRECT to uapi have any meaning?


>
>>> 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