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Subject: Re: [PATCH net] net: sched: act_mirred: drop skb's dst_entry in
 ingress redirection
To:     Cong Wang <xiyou.wangcong@gmail.com>
Cc:     Jamal Hadi Salim <jhs@mojatatu.com>, Jiri Pirko <jiri@resnulli.us>,
        "David S . Miller" <davem@davemloft.net>,
        Linux Kernel Network Developers <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <20191012071620.8595-1-zhiyuan2048@linux.alibaba.com>
 <CAM_iQpVkTb6Qf9J-PuXJoQTZa5ojN_oun64SMv9Kji7tZkxSyA@mail.gmail.com>
From:   Zhiyuan Hou <zhiyuan2048@linux.alibaba.com>
Message-ID: <e2bd3004-9f4b-f3ce-1214-2140f0b7cc61@linux.alibaba.com>
Date:   Wed, 16 Oct 2019 01:22:01 +0800
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On 2019/10/15 1:57 上午, Cong Wang wrote:
> On Sat, Oct 12, 2019 at 12:16 AM Zhiyuan Hou
> <zhiyuan2048@linux.alibaba.com> wrote:
>> diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
>> index 9ce073a05414..6108a64c0cd5 100644
>> --- a/net/sched/act_mirred.c
>> +++ b/net/sched/act_mirred.c
>> @@ -18,6 +18,7 @@
>>   #include <linux/gfp.h>
>>   #include <linux/if_arp.h>
>>   #include <net/net_namespace.h>
>> +#include <net/dst.h>
>>   #include <net/netlink.h>
>>   #include <net/pkt_sched.h>
>>   #include <net/pkt_cls.h>
>> @@ -298,8 +299,10 @@ static int tcf_mirred_act(struct sk_buff *skb, const struct tc_action *a,
>>
>>          if (!want_ingress)
>>                  err = dev_queue_xmit(skb2);
>> -       else
>> +       else {
>> +               skb_dst_drop(skb2);
>>                  err = netif_receive_skb(skb2);
>> +       }
> Good catch!
>
> I don't want to be picky, but it seems this is only needed
> when redirecting from egress to ingress, right? That is,
> ingress to ingress, or ingress to egress is okay? If not,
> please fix all the cases while you are on it?
Sure. But I think this patch is also needed when redirecting from
ingress to ingress. Because we cannot assure that a skb has null dst
in ingress redirection path. For example, if redirecting a skb from
loopback's ingress to other device's ingress, the skb will take a
dst.

As commit logs point out, skb with valid dst cannot be made routing
decision in following process. original dst may cause skb loss or
other unexpected behavior.
>
> Thanks.