netdev - Re: [PATCH] net: sched: drr: dont intepret cls results when asked to drop

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Date: Fri, 15 Sep 2023 18:55:05 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Pedro Tammela <pctammela@...atatu.com>, Ma Ke <make_ruc2021@....com>, 
	xiyou.wangcong@...il.com, jiri@...nulli.us, davem@...emloft.net, 
	kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Victor Nogueira <victor@...atatu.com>
Subject: Re: [PATCH] net: sched: drr: dont intepret cls results when asked to drop

On Fri, Sep 15, 2023 at 11:06 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Fri, Sep 15, 2023 at 5:03 PM Pedro Tammela <pctammela@...atatu.com> wrote:
> >
> > On 15/09/2023 09:55, Eric Dumazet wrote:
> > > On Fri, Sep 15, 2023 at 12:42 PM Ma Ke <make_ruc2021@....com> wrote:
> > >>
> > >> If asked to drop a packet via TC_ACT_SHOT it is unsafe to
> > >> assume res.class contains a valid pointer.
> > >>
> > >> Signed-off-by: Ma Ke <make_ruc2021@....com>
> > >> ---
> > >>   net/sched/sch_drr.c | 2 ++
> > >>   1 file changed, 2 insertions(+)
> > >>
> > >> diff --git a/net/sched/sch_drr.c b/net/sched/sch_drr.c
> > >> index 19901e77cd3b..2b854cb6edf9 100644
> > >> --- a/net/sched/sch_drr.c
> > >> +++ b/net/sched/sch_drr.c
> > >> @@ -309,6 +309,8 @@ static struct drr_class *drr_classify(struct sk_buff *skb, struct Qdisc *sch,
> > >>          *qerr = NET_XMIT_SUCCESS | __NET_XMIT_BYPASS;
> > >>          fl = rcu_dereference_bh(q->filter_list);
> > >>          result = tcf_classify(skb, NULL, fl, &res, false);
> > >> +       if (result == TC_ACT_SHOT)
> > >> +               return NULL;
> > >>          if (result >= 0) {
> > >>   #ifdef CONFIG_NET_CLS_ACT
> > >>                  switch (result) {
> > >> --
> > >> 2.37.2
> > >>
> > >
> > >   I do not see a bug, TC_ACT_SHOT is handled in the switch (result) just fine
> > > at line 320 ?
> >
> > Following the code path (with CONFIG_NET_CLS_ACT=n in mind), it looks
> > like there are a couple of places which return TC_ACT_SHOT before
> > calling any classifiers, which then would cause some qdiscs to look into
> > a uninitialized 'struct tcf_result res'.
> > I could be misreading it... But if it's the problem the author is trying
> > to fix, the obvious way to do it would be:
> >         struct tcf_result res = {};
>
> CONFIG_NET_CLS_ACT=n, how come TC_ACT_SHOT could be used ?
>
> Can we get rid of CONFIG_NET_CLS_ACT, this seems obfuscation to me at
> this point.

The problem is the verdict vs return code are intermixed - not saying
this was fixing anything useful.
We discussed this in the past after/during commit
caa4b35b4317d5147b3ab0fbdc9c075c7d2e9c12
Victor worked on a patch to resolve that. Victor, maybe revive that
patch and post as RFC?


cheers,
jamal