Subject: Re: [RFC PATCH net-next] failover: allow name change on IFF_UP slave interfaces

From: Jiri Pirko <[email-masked]>
Date: Wed, 6 Mar 2019 13:04:30 +0100

Tue, Mar 05, 2019 at 01:50:59AM CET, [email-masked] 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: [email-masked]
>Reviewed-by: Liran Alon <[email-masked]>
>---
> 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");

No module parameters please. If you need to set something do it using
rtnl_link_ops. Thanks.

> 
> 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
>