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

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Date:   Fri, 27 Jul 2018 04:48:51 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jiri Pirko <jiri@...nulli.us>
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

On 07/26/2018 09:43 AM, Jiri Pirko wrote:
> 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?

BPF programs return TC_ACT_* as a verdict which is then further processed,
including TC_ACT_REDIRECT. Hence, it's a contract for them similarly as e.g.
helper enums (BPF_FUNC_*) and other things they make use of.

Cheers,
Daniel