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

[Pedro Tammela]

On 15/09/2023 09:55, Eric Dumazet wrote:
> On Fri, Sep 15, 2023 at 12:42 PM Ma Ke <make_ruc2021@xxxxxxx> 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@xxxxxxx>
> > ---
> >   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 = {};