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

[Zhiyuan Hou]

On 2019/10/16 8:13 äå, Eyal Birger wrote:
> Hi,
>
> On Wed, 16 Oct 2019 01:22:01 +0800
> Zhiyuan Hou <zhiyuan2048@xxxxxxxxxxxxxxxxx> wrote:
>
> > On 2019/10/15 1:57 äå, Cong Wang wrote:
> > > On Sat, Oct 12, 2019 at 12:16 AM Zhiyuan Hou
> > > <zhiyuan2048@xxxxxxxxxxxxxxxxx> 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.