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

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Date:   Mon, 21 Oct 2019 13:50:13 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eyal Birger <eyal.birger@...il.com>
Cc:     Zhiyuan Hou <zhiyuan2048@...ux.alibaba.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 <shmulik.ladkani@...il.com>
Subject: Re: [PATCH net] net: sched: act_mirred: drop skb's dst_entry in
 ingress redirection

On Fri, Oct 18, 2019 at 2:25 PM Eyal Birger <eyal.birger@...il.com> wrote:
>
> Hi,
>
> On Fri, 18 Oct 2019 00:33:53 +0800
> Zhiyuan Hou <zhiyuan2048@...ux.alibaba.com> wrote:
>
> > 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?
>
> On rx path tunnelled packets parameters received on a collect_md tunnel device
> are kept in a metadata dst. See ip_tunnel_rcv() 'tun_dst' parameter.
>
> The rx metadata dst can be matched by a number of mechanisms like routing
> rules, eBPF, OVS, and netfilter.

Should this meta information be kept when redirecting? The dest device
may be a non-tunnel device, so I don't know if it is still useful when
for non-tunnel devices.

Thanks.