Re: [PATCH net v4] failover: allow name change on IFF_UP slave interfaces

[si-wei liu <si-wei.liu@oracle.com>]

On 3/28/2019 10:55 PM, Samudrala, Sridhar wrote:
>
>
> On 3/28/2019 4:47 PM, Si-Wei Liu wrote:
>> When a netdev appears through hot plug then gets enslaved by a failover
>> master that is already up and running, the slave will be opened
>> right away after getting enslaved. Today there's a race that userspace
>> (udev) may fail to rename the slave if the kernel (net_failover)
>> opens the slave earlier than when the userspace rename happens.
>> Unlike bond or team, the primary slave of failover can't be renamed by
>> userspace ahead of time, since the kernel initiated auto-enslavement is
>> unable to, or rather, is never meant to be synchronized with the rename
>> request from userspace.
>>
>> As the failover slave interfaces are not designed to be operated
>> directly by userspace apps: IP configuration, filter rules with
>> regard to network traffic passing and etc., should all be done on master
>> interface. In general, userspace apps only care about the
>> name of master interface, while slave names are less important as long
>> as admin users can see reliable names that may carry
>> other information describing the netdev. For e.g., they can infer that
>> "ens3nsby" is a standby slave of "ens3", while for a
>> name like "eth0" they can't tell which master it belongs to.
>>
>> Historically the name of IFF_UP interface can't be changed because
>> there might be admin script or management software that is already
>> relying on such behavior and assumes that the slave name can't be
>> changed once UP. But failover is special: with the in-kernel
>> auto-enslavement mechanism, the userspace expectation for device
>> enumeration and bring-up order is already broken. Previously initramfs
>> and various userspace config tools were modified to bypass failover
>> slaves because of auto-enslavement and duplicate MAC address. Similarly,
>> in case that users care about seeing reliable slave name, the new type
>> of failover slaves needs to be taken care of specifically in userspace
>> anyway.
>>
>> It's less risky to lift up the rename restriction on failover slave
>> which is already UP. Although it's possible this change may potentially
>> break userspace component (most likely configuration scripts or
>> management software) that assumes slave name can't be changed while
>> UP, it's relatively a limited and controllable set among all userspace
>> components, which can be fixed specifically to listen for the rename
>> and/or link down/up events on failover slaves. Userspace component
>> interacting with slaves is expected to be changed to operate on failover
>> master interface instead, as the failover slave is dynamic in nature
>> which may come and go at any point.  The goal is to make the role of
>> failover slaves less relevant, and userspace components should only
>> deal with failover master in the long run.
>>
>> Fixes: 30c8bd5aa8b2 ("net: Introduce generic failover module")
>> Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
>> Reviewed-by: Liran Alon <liran.alon@oracle.com>
>>
>> -- 
>> v1 -> v2:
>> - Drop configurable module parameter (Sridhar)
>>
>> v2 -> v3:
>> - Drop additional IFF_SLAVE_RENAME_OK flag (Sridhar)
>> - Send down and up events around rename (Michael S. Tsirkin)
>>
>> v3 -> v4:
>> - Simplify notification to be sent (Stephen Hemminger)
>> ---
>>   net/core/dev.c | 24 +++++++++++++++++++++++-
>>   1 file changed, 23 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index 722d50d..6ae5874 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -1180,7 +1180,21 @@ int dev_change_name(struct net_device *dev, 
>> const char *newname)
>>       BUG_ON(!dev_net(dev));
>>         net = dev_net(dev);
>> -    if (dev->flags & IFF_UP)
>> +
>> +    /* Allow failover slave to rename even when
>> +     * it is up and running.
>> +     *
>> +     * Failover slaves are special, since userspace
>> +     * might rename the slave after the interface
>> +     * has been brought up and running due to
>> +     * auto-enslavement.
>> +     *
>> +     * Failover users don't actually care about slave
>> +     * name change, as they are only expected to operate
>> +     * on master interface directly.
>> +     */
>> +    if (dev->flags & IFF_UP &&
>> +        likely(!(dev->priv_flags & IFF_FAILOVER_SLAVE)))
>>           return -EBUSY;
>>         write_seqcount_begin(&devnet_rename_seq);
>> @@ -1227,6 +1241,14 @@ int dev_change_name(struct net_device *dev, 
>> const char *newname)
>>       hlist_add_head_rcu(&dev->name_hlist, dev_name_hash(net, 
>> dev->name));
>>       write_unlock_bh(&dev_base_lock);
>>   +    if (unlikely(dev->flags & IFF_UP)) {
>> +        struct netdev_notifier_change_info change_info;
>> +
>> +        change_info.flags_changed = 0;
>> +        call_netdevice_notifiers_info(NETDEV_CHANGE, dev,
>> +                          &change_info.info);
>
> This function no longer takes the dev parameter in the net-next kernel.
OK. Will get my patch updated with the latest net-next tree.

>
> Did you consider calling netdev_state_change() although it does send a 
> RTM_NEWLINK message too. May be just fixing the notifier call should 
> be fine.
I did look at it and the extra RTM_NEWLINK message was indeed the reason 
I got it rewritten. You see, how the way of initialization is inherited.

Thanks,
-Siwei

>
>> +    }
>> +
>>       ret = call_netdevice_notifiers(NETDEV_CHANGENAME, dev);
>>       ret = notifier_to_errno(ret);
>>