netdev - Re: [PATCH net] net: sched: act_mirred: drop skb's dst

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Message-ID: <e16cfafe-059c-3106-835e-d32b7bb5ba61@linux.alibaba.com>
Date:   Fri, 18 Oct 2019 00:33:53 +0800
From:   Zhiyuan Hou <zhiyuan2048@...ux.alibaba.com>
To:     Eyal Birger <eyal.birger@...il.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S . Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, shmulik.ladkani@...il.com
Subject: Re: [PATCH net] net: sched: act_mirred: drop skb's dst_entry in
 ingress redirection


On 2019/10/16 8:13 下午, Eyal Birger wrote:
> Hi,
>
> On Wed, 16 Oct 2019 01:22:01 +0800
> Zhiyuan Hou <zhiyuan2048@...ux.alibaba.com> wrote:
>
>> On 2019/10/15 1:57 上午, Cong Wang wrote:
>>> On Sat, Oct 12, 2019 at 12:16 AM Zhiyuan Hou
>>> <zhiyuan2048@...ux.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!
> Indeed! Thanks for fixing this!
>
>>> 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.
> On the other hand, removing the dst on ingress-to-ingress redirection
> may remove LWT information on incoming packets, which may be undesired.
Sorry, I do not understand why lwt information is needed on
ingress-to-ingress redirection. lwt is used on output path, isn't it?
Can you please give more information?
> Eyal.