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

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

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 ?

>   
>   	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))
>