Subject: Re: [Patch net-next] net_sched: add reverse binding for tc class

From: Daniel Borkmann <[email-masked]>
Date: Thu, 31 Aug 2017 00:22:38 +0200

On 08/31/2017 12:01 AM, Cong Wang wrote:
> On Wed, Aug 30, 2017 at 2:48 PM, Daniel Borkmann <[email-masked]> wrote:
>> On 08/30/2017 11:30 PM, Cong Wang wrote:
>> [...]
>>>
>>> Note, we still can NOT totally get rid of those class lookup in
>>> ->enqueue() because cgroup and flow filters have no way to determine
>>> the classid at setup time, they still have to go through dynamic lookup.
>>
>> [...]
>>>
>>> ---
>>>    include/net/sch_generic.h |  1 +
>>>    net/sched/cls_basic.c     |  9 +++++++
>>>    net/sched/cls_bpf.c       |  9 +++++++
>>
>> Same is for cls_bpf as well, so bind_class wouldn't work there
>> either as we could return dynamic classids. bind_class cannot
>> be added here, too.
>
> I think you are probably right, but the following code is
> misleading there:
>
>          if (tb[TCA_BPF_CLASSID]) {
>                  prog->res.classid = nla_get_u32(tb[TCA_BPF_CLASSID]);
>                  tcf_bind_filter(tp, &prog->res, base);
>          }
>
> If the classid is dynamic, why this tb[TCA_BPF_CLASSID]?

The prog->res.classid is the default one, but can be overridden
later depending on the specified program. cls_bpf_classify() does
after prog return (filter_res holds return code):

	[...]
		if (filter_res == 0)
			continue;
		if (filter_res != -1) {
			res->class   = 0;
			res->classid = filter_res;
		} else {
			*res = prog->res;
		}
	[...]

Meaning in case of a match (-1), we use the default bound one,
but prog may as well return an alternative found classid if it
wants to. So both versions are possible.