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

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Date: Fri, 15 Sep 2023 12:03:42 -0300
From: Pedro Tammela <pctammela@...atatu.com>
To: Eric Dumazet <edumazet@...gle.com>, Ma Ke <make_ruc2021@....com>
Cc: 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 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 = {};