Subject: Re: [PATCH net] net: sched: act_mirred: drop skb's dst_entry in ingress redirection

From: Eyal Birger <[email-masked]>
Date: Wed, 16 Oct 2019 15:13:07 +0300

Hi,

On Wed, 16 Oct 2019 01:22:01 +0800
Zhiyuan Hou <[email-masked]> wrote:

> On 2019/10/15 1:57 上午, Cong Wang wrote:
> > On Sat, Oct 12, 2019 at 12:16 AM Zhiyuan Hou
> > <[email-masked]> 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.

Eyal.