Linux Virtualization - Mar 2019 - [RFC PATCH net-next] failover: allow name change on IFF_UP slave interfaces

On Mon, Mar 04, 2019 at 07:50:59PM -0500, 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.
> 
> For that to work, now introduce a module-level tunable,
> "slave_rename_ok" that allows users to lift up the rename
restriction on
> failover slave which is already UP. Although it's possible this change
> 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 the failover slave. 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. The default
> for the "slave_rename_ok" is set to true(1). If userspace
doesn't have
> the right support in place meanwhile users don't care about reliable
> userspace naming, the value can be set to false(0).
> 
> Signed-off-by: Si-Wei.Liu at oracle.com
> Reviewed-by: Liran Alon <liran.alon at oracle.com>
Not sure which of the versions I should reply to.

I have a vague idea: would it work to *not* set
IFF_UP on slave devices at all?

Would this reduce the chances of existing scripts such as dracut being
confused?

And this leaves open the option for scripts to address
slaves by checking some custom attribute.
> ---
>  include/linux/netdevice.h |  3 +++
>  net/core/dev.c            |  3 ++-
>  net/core/failover.c       | 11 +++++++++--
>  3 files changed, 14 insertions(+), 3 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;
>  
>  	write_seqcount_begin(&devnet_rename_seq);
> diff --git a/net/core/failover.c b/net/core/failover.c
> index 4a92a98..1fd8bbb 100644
> --- a/net/core/failover.c
> +++ b/net/core/failover.c
> @@ -16,6 +16,11 @@
>  
>  static LIST_HEAD(failover_list);
>  static DEFINE_SPINLOCK(failover_lock);
> +static bool slave_rename_ok = true;
> +
> +module_param(slave_rename_ok, bool, (S_IRUGO | S_IWUSR));
> +MODULE_PARM_DESC(slave_rename_ok,
> +		 "If set allow renaming the slave when failover master is
up");
>  
>  static struct net_device *failover_get_bymac(u8 *mac, struct failover_ops
**ops)
>  {
> @@ -81,13 +86,15 @@ static int failover_slave_register(struct net_device
*slave_dev)
>  	}
>  
>  	slave_dev->priv_flags |= IFF_FAILOVER_SLAVE;
> +	if (slave_rename_ok)
> +		slave_dev->priv_flags |= 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 +128,7 @@ 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))
> -- 
> 1.8.3.1