netdev - Re: [PATCH net] Panic when tc_lookup_action_n finds a partially initialized action.

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Date:   Wed, 5 Oct 2016 11:07:59 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Krister Johansen <kjlx@...pleofstupid.com>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net] Panic when tc_lookup_action_n finds a partially
 initialized action.

On Wed, Oct 5, 2016 at 11:01 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Tue, Oct 4, 2016 at 11:52 PM, Krister Johansen
> <kjlx@...pleofstupid.com> wrote:
>> On Mon, Oct 03, 2016 at 11:22:33AM -0700, Cong Wang wrote:
>>> Please try the attached patch. I also convert the read path to RCU
>>> to avoid a possible deadlock. A quick test shows no lockdep splat.
>>
>> I tried this patch, but it doesn't solve the problem.  I got a panic on
>> my very first try:
>
> Thanks for testing it.
>
>
>> The problem here is the same as before: by using RCU the race isn't
>> fixed because the module is still discoverable from act_base before the
>> pernet initialization is completed.
>>
>> You can see from the trap frame that the first two arguments to
>> tcf_hash_check were 0.  It couldn't look up the correct per-subsystem
>> pointer because the id hadn't yet been registered.
>
> I thought the problem is that we don't do pernet ops registration and
> action ops registration atomically therefore chose to use mutex+RCU,
> but I was wrong, the problem here is just ordering, we need to finish
> the pernet initialization before making action ops visible.
>
> If so, why not just reorder them? Does the attached patch make any
> sense now? Our pernet init doesn't rely on act_base, so even we have
> some race, the worst case is after we initialize the pernet netns for an
> action but its ops still not visible, which seems fine (at least no crash).

BTW, I should remove the 'if' check for unregister_pernet_subsys() in
tcf_unregister_action()... Otherwise the error path doesn't work. ;-)