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Subject: Re: [PATCH net] net: sched: act_mirred: drop skb's dst_entry in
 ingress redirection
To:     Zhiyuan Hou <zhiyuan2048@linux.alibaba.com>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Jiri Pirko <jiri@resnulli.us>,
        "David S . Miller" <davem@davemloft.net>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20191012071620.8595-1-zhiyuan2048@linux.alibaba.com>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <2d816fb6-befb-aaeb-328b-539507022a22@gmail.com>
Date:   Sat, 12 Oct 2019 03:59:15 -0700
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On 10/12/19 12:16 AM, Zhiyuan Hou wrote:
> In act_mirred's ingress redirection, if the skb's dst_entry is valid
> when call function netif_receive_skb, the fllowing l3 stack process
> (ip_rcv_finish_core) will check dst_entry and skip the routing
> decision. Using the old dst_entry is unexpected and may discard the
> skb in some case. For example dst->dst_input points to dst_discard.
> 
> This patch drops the skb's dst_entry before calling netif_receive_skb
> so that the skb can be made routing decision like a normal ingress
> skb.
> 
> Signed-off-by: Zhiyuan Hou <zhiyuan2048@linux.alibaba.com>
> ---
>  net/sched/act_mirred.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> 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);
> +	}
>  
>  	if (err) {
>  out:
> 

Why is dst_discard used ?

This could actually drop packets, for loopback.

A Fixes: tag would tremendously help, I wonder if you are not working around
the other issue Wei was tracking yesterday ( https://www.spinics.net/lists/netdev/msg604397.html )