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Date:   Wed, 16 Oct 2019 15:13:07 +0300
From:   Eyal Birger <eyal.birger@gmail.com>
To:     Zhiyuan Hou <zhiyuan2048@linux.alibaba.com>
Cc:     Cong Wang <xiyou.wangcong@gmail.com>,
        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>, shmulik.ladkani@gmail.com
Subject: Re: [PATCH net] net: sched: act_mirred: drop skb's dst_entry in
 ingress redirection
Message-ID: <20191016151307.40f63896@jimi>
In-Reply-To: <e2bd3004-9f4b-f3ce-1214-2140f0b7cc61@linux.alibaba.com>
References: <20191012071620.8595-1-zhiyuan2048@linux.alibaba.com>
        <CAM_iQpVkTb6Qf9J-PuXJoQTZa5ojN_oun64SMv9Kji7tZkxSyA@mail.gmail.com>
        <e2bd3004-9f4b-f3ce-1214-2140f0b7cc61@linux.alibaba.com>
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Hi,

On Wed, 16 Oct 2019 01:22:01 +0800
Zhiyuan Hou <zhiyuan2048@linux.alibaba.com> wrote:

> On 2019/10/15 1:57 =E4=B8=8A=E5=8D=88, Cong Wang wrote:
> > On Sat, Oct 12, 2019 at 12:16 AM Zhiyuan Hou
> > <zhiyuan2048@linux.alibaba.com> wrote: =20
> >> 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 =3D dev_queue_xmit(skb2);
> >> -       else
> >> +       else {
> >> +               skb_dst_drop(skb2);
> >>                  err =3D netif_receive_skb(skb2);
> >> +       } =20

> > 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? =20
> 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.
>=20
> 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.

Eyal.