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

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Date:   Sat, 8 Oct 2016 23:13:31 -0700
From:   Krister Johansen <kjlx@...pleofstupid.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Krister Johansen <kjlx@...pleofstupid.com>,
        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.

Hi Cong,
Thanks for the follow-up.

On Thu, Oct 06, 2016 at 12:01:15PM -0700, Cong Wang wrote:
> On Wed, Oct 5, 2016 at 11:11 PM, Krister Johansen
> > pernet_operations pointer.  The code in register_pernet_subsys() makes
> > no attempt to check for duplicates.  If we add a pointer that's already
> > in the list, and subsequently call unregister, the results seem
> > undefined.  It looks like we'll remove the pernet_operations for the
> > existing action, assuming we don't corrupt the list in the process.
> >
> > Is this actually safe?  If so, what corner case is the act->type /
> > act->kind protecting us from?
> 
> ops->type and ops->kind should be unique too, user-space already
> relies on this (tc action ls action xxx). The code exists probably just
> for sanity check.

With that in mind, would it make sense to change the check to a WARN/BUG
or some kind of assertion?  I mistakenly inferred that it was possible to
legtimately end up in this scenario.

> So please give that patch a try, let's see if we miss any other problem.

Will do.  I have not forgotten.  I hope to have results for you in a few
days.

> > Part of the desire to inhibit extra modprobe calls is that if hundreds
> > of these all start at once on boot, it's really unnecessary to have all
> > of the rest of them wait while lots of extra modprobe calls are forked
> > by the kernel.
> 
> You can tell systemd to load these modules before starting these
> containers to avoid blocking, no?

That was exactly what I did to work around the panic until I was able to
get a patch together.  The preload of the modules is still occurring,
but I was hoping to excise that workaround entirely.

Thanks,

-K