Re: [PATCH net v2] failover: allow name change on IFF_UP slave interfaces

["Samudrala, Sridhar" <sridhar.samudrala@intel.com>]

On 3/19/2019 10:20 PM, si-wei liu wrote:
> Hi Sridhar,
> 
> Are you fine with leaving the IFF_SLAVE_RENAME_OK flag as is, and thus 
> can provide your Ack-by or Reviewed-by? I can change the code if you 
> feel strong.

My preference would be not to introduce a new flag unless there is any 
usecase where we want a IFF_FAILOVER_SLAVE type of device to support 2 
different behaviors. (rename_ok and rename_not_ok)

Thanks
Sridhar

> 
> Thanks,
> -Siwei
> 
> 
> On 3/6/2019 8:54 PM, si-wei liu wrote:
>>
>>
>> On 3/6/2019 8:13 PM, Samudrala, Sridhar wrote:
>>>
>>> On 3/6/2019 7:08 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 work with the new naming
>>>> behavior of failover slaves. Userspace component interacting with
>>>> slaves should be changed to operate on failover master 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
>>>> all userspace should only deal with 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>
>>>> Acked-by: Michael S. Tsirkin <mst@redhat.com>
>>>>
>>>> ---
>>>> v1 -> v2:
>>>> - Drop configurable module parameter (Sridhar)
>>>>
>>>>
>>>>   include/linux/netdevice.h | 3 +++
>>>>   net/core/dev.c            | 3 ++-
>>>>   net/core/failover.c       | 6 +++---
>>>>   3 files changed, 8 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>>>> index 857f8ab..6d9e4e0 100644
>>>> --- a/include/linux/netdevice.h
>>>> +++ b/include/linux/netdevice.h
>>>> @@ -1487,6 +1487,7 @@ struct net_device_ops {
>>>>    * @IFF_NO_RX_HANDLER: device doesn't support the rx_handler hook
>>>>    * @IFF_FAILOVER: device is a failover master device
>>>>    * @IFF_FAILOVER_SLAVE: device is lower dev of a failover master 
>>>> device
>>>> + * @IFF_SLAVE_RENAME_OK: rename is allowed while slave device is 
>>>> running
>>>>    */
>>>>   enum netdev_priv_flags {
>>>>       IFF_802_1Q_VLAN            = 1<<0,
>>>> @@ -1518,6 +1519,7 @@ enum netdev_priv_flags {
>>>>       IFF_NO_RX_HANDLER        = 1<<26,
>>>>       IFF_FAILOVER            = 1<<27,
>>>>       IFF_FAILOVER_SLAVE        = 1<<28,
>>>> +    IFF_SLAVE_RENAME_OK        = 1<<29,
>>>>   };
>>>>     #define IFF_802_1Q_VLAN            IFF_802_1Q_VLAN
>>>> @@ -1548,6 +1550,7 @@ enum netdev_priv_flags {
>>>>   #define IFF_NO_RX_HANDLER        IFF_NO_RX_HANDLER
>>>>   #define IFF_FAILOVER            IFF_FAILOVER
>>>>   #define IFF_FAILOVER_SLAVE        IFF_FAILOVER_SLAVE
>>>> +#define IFF_SLAVE_RENAME_OK        IFF_SLAVE_RENAME_OK
>>>>     /**
>>>>    *    struct net_device - The DEVICE structure.
>>>> diff --git a/net/core/dev.c b/net/core/dev.c
>>>> index 722d50d..ae070de 100644
>>>> --- a/net/core/dev.c
>>>> +++ b/net/core/dev.c
>>>> @@ -1180,7 +1180,8 @@ 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)
>>>> +    if (dev->flags & IFF_UP &&
>>>> +        !(dev->priv_flags & IFF_SLAVE_RENAME_OK))
>>>>           return -EBUSY;
>>>
>>> Without the configurable module parameter, i think we don't even need
>>> the new SLAVE_RENAME_OK private flag.
>>> Can't we simply check for IFF_FAILOVER_SLAVE ?
>> I'd prefer keeping this flag for now, even though without configurable 
>> module parameter. This has clear semantics that helps decouple 
>> behavior against specific link type, and may benefit other 
>> auto-enslaved netdevs as well.
>>
>> -Siwei
>>
>>>
>>>> write_seqcount_begin(&devnet_rename_seq);
>>>> diff --git a/net/core/failover.c b/net/core/failover.c
>>>> index 4a92a98..34c5c87 100644
>>>> --- a/net/core/failover.c
>>>> +++ b/net/core/failover.c
>>>> @@ -80,14 +80,14 @@ static int failover_slave_register(struct 
>>>> net_device *slave_dev)
>>>>           goto err_upper_link;
>>>>       }
>>>>   -    slave_dev->priv_flags |= IFF_FAILOVER_SLAVE;
>>>> +    slave_dev->priv_flags |= (IFF_FAILOVER_SLAVE | 
>>>> IFF_SLAVE_RENAME_OK);
>>>>         if (fops && fops->slave_register &&
>>>>           !fops->slave_register(slave_dev, failover_dev))
>>>>           return NOTIFY_OK;
>>>>         netdev_upper_dev_unlink(slave_dev, failover_dev);
>>>> -    slave_dev->priv_flags &= ~IFF_FAILOVER_SLAVE;
>>>> +    slave_dev->priv_flags &= ~(IFF_FAILOVER_SLAVE | 
>>>> IFF_SLAVE_RENAME_OK);
>>>>   err_upper_link:
>>>>       netdev_rx_handler_unregister(slave_dev);
>>>>   done:
>>>> @@ -121,7 +121,7 @@ int failover_slave_unregister(struct net_device 
>>>> *slave_dev)
>>>>         netdev_rx_handler_unregister(slave_dev);
>>>>       netdev_upper_dev_unlink(slave_dev, failover_dev);
>>>> -    slave_dev->priv_flags &= ~IFF_FAILOVER_SLAVE;
>>>> +    slave_dev->priv_flags &= ~(IFF_FAILOVER_SLAVE | 
>>>> IFF_SLAVE_RENAME_OK);
>>>>         if (fops && fops->slave_unregister &&
>>>>           !fops->slave_unregister(slave_dev, failover_dev))
>>>>
>>
> 
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