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

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Date:   Mon, 14 Oct 2019 05:46:02 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Zhiyuan Hou <zhiyuan2048@...ux.alibaba.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S . Miller" <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: sched: act_mirred: drop skb's dst_entry in
 ingress redirection



On 10/14/19 12:07 AM, Zhiyuan Hou wrote:
> 
> On 2019/10/12 6:59 下午, Eric Dumazet wrote:
>>
>> 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@...ux.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 ?
> When send a skb from local to external, the dst->dst_input will be
> assigned dst_discard after routing decision. So if we redirect these
> skbs to ingress stack, it will be dropped.
> 
> For ipvlan l2 mode or macvlan, clsact egress filters on master deivce
> may also meet these skbs even if they came from slave device. Ingress
> redirection on these skbs may drop them on l3 stack.

Can you please add a test, so that we can see what you are trying to do exactly ?




>> 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 )
> No, this is a different issue ^_^.

Please add a Fixes: tag then.

Thanks.