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

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Date:   Fri, 15 Sep 2023 17:06:09 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Pedro Tammela <pctammela@...atatu.com>
Cc:     Ma Ke <make_ruc2021@....com>, jhs@...atatu.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
Subject: Re: [PATCH] net: sched: drr: dont intepret cls results when asked to drop

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.