[PATCH] act_mirred: do not drop packets when fails to mirror it

[PATCH] act_mirred: do not drop packets when fails to mirror it

[Jason Wang]

We drop packet unconditionally when we fail to mirror it. This is not intended
in some cases. Consdier for kvm guest, we may mirror the traffic of the bridge
to a tap device used by a VM. When kernel fails to mirror the packet in
conditions such as when qemu crashes or stop polling the tap, it's hard for the
management software to detect such condition and clean the the mirroring
before. This would lead all packets to the bridge to be dropped and break the
netowrk of other virtual machines.

To solve the issue, the patch does not drop packets when kernel fails to mirror
it, and only drop the redirected packets.

Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 net/sched/act_mirred.c |    9 +++------
 1 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
index fe81cc1..3682951 100644
--- a/net/sched/act_mirred.c
+++ b/net/sched/act_mirred.c
@@ -198,15 +198,12 @@ static int tcf_mirred(struct sk_buff *skb, const struct tc_action *a,
 	err = dev_queue_xmit(skb2);
 
 out:
-	if (err) {
+	if (err)
 		m->tcf_qstats.overlimits++;
-		/* should we be asking for packet to be dropped?
-		 * may make sense for redirect case only
-		 */
+	if (err && m->tcf_action == TC_ACT_STOLEN)
 		retval = TC_ACT_SHOT;
-	} else {
+	else
 		retval = m->tcf_action;
-	}
 	spin_unlock(&m->tcf_lock);
 
 	return retval;

Re: [PATCH] act_mirred: do not drop packets when fails to mirror it

[Jamal Hadi Salim]

On Wed, 2012-08-15 at 17:37 +0800, Jason Wang wrote:
> We drop packet unconditionally when we fail to mirror it. This is not intended
> in some cases.

Hi Jason,
Did you actually notice the behavior you described or were you going by
the XXX comment I had in the code?

cheers,
jamal

Re: [PATCH] act_mirred: do not drop packets when fails to mirror it

[Jason Wang]

On 08/15/2012 08:35 PM, Jamal Hadi Salim wrote:
> On Wed, 2012-08-15 at 17:37 +0800, Jason Wang wrote:
>> We drop packet unconditionally when we fail to mirror it. This is not intended
>> in some cases.
> Hi Jason,
> Did you actually notice the behavior you described or were you going by
> the XXX comment I had in the code?
>
> cheers,
> jamal
Hi Jamal:

I met it actually through the following steps:

- start a kvm guest with tap and make it to be an interface of the bridge
- mirror the ingress traffic of the bridge to the tap
- terminate the qemu process, the tap device is then removed
- all packet goes to bridge would be dropped, so the network of guests 
in the same bridge would be broken

It's hard for the management to forcast the termination of the porcess 
and clean the mirroring before. The realistic way is to remove the 
mirroring after the termination of the process. So, if we drop the 
packets when the mirred device (tap) is removed, in the gap between the 
qemu termitnaion and disabling mirroring, the bridge ( and other VMs 
using the it) would not recevie any packet.

>
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Re: [PATCH] act_mirred: do not drop packets when fails to mirror it

[Jamal Hadi Salim]

On Wed, 2012-08-15 at 21:42 +0800, Jason Wang wrote:

> 
> I met it actually through the following steps:
> 
> - start a kvm guest with tap and make it to be an interface of the bridge
> - mirror the ingress traffic of the bridge to the tap
> - terminate the qemu process, the tap device is then removed
> - all packet goes to bridge would be dropped, so the network of guests 
> in the same bridge would be broken
> 

Makes sense.
Can you please leave the err check braces i.e
        if (err) {
                m->tcf_qstats.overlimits++;
                if (m->tcfm_eaction != TCA_EGRESS_MIRROR)
                        retval = TC_ACT_SHOT;
                else 
                    retval = m->tcf_action;   
        } else {
                retval = m->tcf_action;
        }

Or at least dont use TC_ACT_STOLEN.

cheers,
jamal

Re: [PATCH] act_mirred: do not drop packets when fails to mirror it

[Jason Wang]

On 08/15/2012 11:10 PM, Jamal Hadi Salim wrote:
> On Wed, 2012-08-15 at 21:42 +0800, Jason Wang wrote:
>
>> I met it actually through the following steps:
>>
>> - start a kvm guest with tap and make it to be an interface of the bridge
>> - mirror the ingress traffic of the bridge to the tap
>> - terminate the qemu process, the tap device is then removed
>> - all packet goes to bridge would be dropped, so the network of guests
>> in the same bridge would be broken
>>
> Makes sense.
> Can you please leave the err check braces i.e
>          if (err) {
>                  m->tcf_qstats.overlimits++;
>                  if (m->tcfm_eaction != TCA_EGRESS_MIRROR)
>                          retval = TC_ACT_SHOT;
>                  else
>                      retval = m->tcf_action;
>          } else {
>                  retval = m->tcf_action;
>          }
>
> Or at least dont use TC_ACT_STOLEN.
>
> cheers,
> jamal
>

Sure, will post v2. Thanks.
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