netdev - Re: [PATCH net-next] macvlan: fix failure during registration v2

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Date:	Thu, 21 Apr 2016 11:39:45 -0700
From:	Francesco Ruggeri <fruggeri@...sta.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Mahesh Bandewar <maheshb@...gle.com>
Subject: Re: [PATCH net-next] macvlan: fix failure during registration v2

On Thu, Apr 21, 2016 at 10:44 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
<
>> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
>> index 95394ed..e770221 100644
>> --- a/drivers/net/macvtap.c
>> +++ b/drivers/net/macvtap.c
>> @@ -1303,6 +1303,8 @@ static int macvtap_device_event(struct notifier_block *unused,
>>               }
>>               break;
>>       case NETDEV_UNREGISTER:
>> +             if (vlan->minor == 0)
>> +                     break;
>
> I don't understand this bit.  A minor of 0 is never assigned.  That is
> clear from the code.  On what code path can you get here without
> assigning a minor?
>

You can have vlan->minor == 0 if macvtap_device_event(NETDEV_REGISTER)
failed.

macvtap_device_event is invoked in the context of macvtap_newlink and
it can fail if for example a macvtap interface using the same ifindex
already exists in a different namespace. That is how we originally ran
into the port->count issue.
In that case the sequence is

macvtap_newlink
  macvlan_common_newlink
    register_netdevice
      call_netdevice_notifiers(NETDEV_REGISTER, dev)
        macvtap_device_event(NETDEV_REGISTER)
          <fail here, vlan->minor = 0>
      rollback_registered(dev);
        rollback_registered_many
          call_netdevice_notifiers(NETDEV_UNREGISTER, dev);
            macvtap_device_event(NETDEV_UNREGISTER)
              <nothing to do here>

Should this bit go into a separate patch?
Would a comment like this help:

/* We can have vlan->minor == 0 if NETDEV_REGISTER above failed */

Marc Angel posted https://marc.info/?l=linux-netdev&m=146116146925511&w=2
about conflicts if one tries to create macvtap interfaces with the same
ifindex in different namespaces.

Thanks,
Francesco


>
> Eric