Re: [PATCH net-next 2/4] tc/act: remove unneeded RCU lock in action callback

[Daniel Borkmann <daniel@iogearbox.net>]

On 07/13/2018 04:26 PM, Paolo Abeni wrote:
> On Fri, 2018-07-13 at 16:08 +0200, Daniel Borkmann wrote:
>> On 07/13/2018 11:55 AM, Paolo Abeni wrote:
>>> Each lockless action currently does its own RCU locking in ->act().
>>> This is allows using plain RCU accessor, even if the context
>>> is really RCU BH.
>>>
>>> This change drops the per action RCU lock, replace the accessors
>>> with _bh variant, cleans up a bit the surronding code and documents
>>> the RCU status in the relevant header.
>>> No functional nor performance change is intended.
>>>
>>> The goal of this patch is clarifying that the RCU critical section
>>> used by the tc actions extends up to the classifier's caller.
>>>
>>> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
>>
>> [...]
>>> diff --git a/net/sched/act_bpf.c b/net/sched/act_bpf.c
>>> index 06f743d8ed41..ac20266460c0 100644
>>> --- a/net/sched/act_bpf.c
>>> +++ b/net/sched/act_bpf.c
>>> @@ -45,8 +45,7 @@ static int tcf_bpf(struct sk_buff *skb, const struct tc_action *act,
>>>  	tcf_lastuse_update(&prog->tcf_tm);
>>>  	bstats_cpu_update(this_cpu_ptr(prog->common.cpu_bstats), skb);
>>>  
>>> -	rcu_read_lock();
>>> -	filter = rcu_dereference(prog->filter);
>>> +	filter = rcu_dereference_bh(prog->filter);
>>>  	if (at_ingress) {
>>>  		__skb_push(skb, skb->mac_len);
>>>  		bpf_compute_data_pointers(skb);
>>> @@ -56,7 +55,6 @@ static int tcf_bpf(struct sk_buff *skb, const struct tc_action *act,
>>>  		bpf_compute_data_pointers(skb);
>>>  		filter_res = BPF_PROG_RUN(filter, skb);
>>>  	}
>>> -	rcu_read_unlock();
>>
>> This conversion is not correct, BPF itself relies on RCU but not RCU-bh flavor.
>> You might probably see a splat if you do e.g. a map lookup with this change in
>> interpreter mode on tx side.
> 
> Thank you for your review.
> 
> I actually tested with lockdep, and lockdep is happy about it.
> 
> The not so nice fact is that many TC modules already use plain RCU
> primitives in the control path (call_rcu, kfree_rcu, etc.) and
> rcu_derefence_bh() in the datapath (e.g. all the classifiers). AFACS,
> despite the mix, this use is safe.

Hmm, so out of __dev_queue_xmit() we do the RCU-bh read-side. We call
into sch_handle_egress() which calls into tcf_classify() which may be
a matchall one e.g. mall_classify(). It invokes tcf_exts_exec() that
does the a->ops->act() which is the tcf_bpf() from here. If you then
call a helper like bpf_map_lookup_elem(), there's a WARN_ON_ONCE() for
!rcu_read_lock_held() since all of BPF is under normal RCU flavor. Why
would that not trigger? RCU != RCU-bh. Only the rcu_read_lock_bh_held()
would hold true.

Thanks,
Daniel