netdev - Re: [PATCH net] net/sched: flower: fix infinite loop in fl

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Date:   Thu, 27 Jun 2019 18:24:08 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Davide Caratti <dcaratti@...hat.com>
Cc:     Vlad Buslov <vladbu@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Lucas Bates <lucasb@...atatu.com>
Subject: Re: [PATCH net] net/sched: flower: fix infinite loop in fl_walk()

On Thu, Jun 27, 2019 at 3:10 PM Davide Caratti <dcaratti@...hat.com> wrote:
>
> On Wed, 2019-06-26 at 14:15 -0700, Cong Wang wrote:
> > Hi, Davide
> >
> > On Tue, Jun 25, 2019 at 12:29 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> > > It should handle this overflow case more gracefully, I hope.
> > >
> >
> > Please try this attached one and let me know if it works.
> > Hope I get it right this time.
> >
> > Thanks!
>
> hello Cong, and thanks a lot for the patch!
> I see it uses
>
>     (tmp <= id)
>
> as the condition to detect the overflow, and at each iteration it does
>
>     tmp = id, ++id
>
> so that 'tmp' contains the last IDR found in the tree and 'id' is the next
> tentative value to be searched for. When 'id' overflows, (tmp <= id)
> becomes false, and the 'for' loop exits.

Yes, thanks for testing it with tc actions.


> I tested it successfully with TC actions having the highest possible
> index: 'tc actions show' doesn't loop anymore. But with cls_flower (that
> uses idr_for_each_entry_continue_ul() ) I still see the infinite loop:
> even when idr_for_each_entry_continue_ul() is used, fl_get_next_filter()
> never returns NULL, because
>
>     (tmp <= id) && (((entry) = idr_get_next_ul(idr, &(id))) != NULL)
>
> calls idr_get_next_ul(idr, &(id)) at least once. So, even if
> idr_for_each_entry_continue_ul() detected the overflow of 'id' after the
> first iteration, and bailouts the for loop, fl_get_next_filter()
> repeatedly returns a pointer to the idr slot with index equal to
> 0xffffffff. Because of that, the while() loop in fl_walk() keeps dumping
> the same rule.


Good catch, it is actually the arg->cookie++ which causes the trouble
here.


> In my original patch I found easier to check for the overflow of
> arg->cookie in fl_walk(), before the self-increment, so I was sure that
>
>     arg->fn(tp, f, arg)
>
> was already called once when 'f' was the slot having the highest possible
> IDR. Now, I didn't check it, but I guess
>
>     refcount_inc_not_zero(&f->refcnt))
>
> in fl_get_next_filter() is always true during my test, so the inner
> while() loop is not endless, even when the idr has a slot with id equal to
> ULONG_MAX. Probably, to stay on the safe side, cls_flower needs both tests
> to be in place, what do you think?

I think we can just fold the nested loops into one for cls_flower and remove
the arg->cookie++.

What's more, arg->cookie could overflow too,seems we have to switch back to
arg->skip. I am not sure, if this is really a problem we can fix it separately.

Thanks.