Date:   Thu, 28 Mar 2019 10:14:26 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     si-wei liu <si-wei.liu@...cle.com>
Cc:     Jiri Pirko <jiri@...nulli.us>, mst@...hat.com,
        sridhar.samudrala@...el.com, davem@...emloft.net, kubakici@...pl,
        alexander.duyck@...il.com, netdev@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, liran.alon@...cle.com,
        boris.ostrovsky@...cle.com, vijay.balakrishna@...cle.com
Subject: Re: [PATCH net v3] failover: allow name change on IFF_UP slave
 interfaces

On Wed, 27 Mar 2019 16:44:19 -0700
si-wei liu <si-wei.liu@...cle.com> wrote:

> On 3/27/2019 4:11 AM, Jiri Pirko wrote:
> > Wed, Mar 27, 2019 at 12:48:13AM CET, si-wei.liu@...cle.com 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@...cle.com>
> >> Reviewed-by: Liran Alon <liran.alon@...cle.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)
> >> ---
> >> net/core/dev.c | 37 ++++++++++++++++++++++++++++++++++---
> >> 1 file changed, 34 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/net/core/dev.c b/net/core/dev.c
> >> index 722d50d..3e0cd80 100644
> >> --- a/net/core/dev.c
> >> +++ b/net/core/dev.c
> >> @@ -1171,6 +1171,7 @@ int dev_get_valid_name(struct net *net, struct net_device *dev,
> >> int dev_change_name(struct net_device *dev, const char *newname)
> >> {
> >> 	unsigned char old_assign_type;
> >> +	bool reopen_needed = false;
> >> 	char oldname[IFNAMSIZ];
> >> 	int err = 0;
> >> 	int ret;
> >> @@ -1180,8 +1181,24 @@ 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)
> >> -		return -EBUSY;
> >> +
> >> +	/* 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) {
> >> +		if (likely(!(dev->priv_flags & IFF_FAILOVER_SLAVE)))
> >> +			return -EBUSY;
> >> +		reopen_needed = true;
> >> +	}
> >>
> >> 	write_seqcount_begin(&devnet_rename_seq);
> >>
> >> @@ -1198,6 +1215,9 @@ int dev_change_name(struct net_device *dev, const char *newname)
> >> 		return err;
> >> 	}
> >>
> >> +	if (reopen_needed)
> >> +		dev_close(dev);  
> > Ugh. Don't dev_close/dev_open on name change.  
> See my response to Michael and Stephen. What's your suggestion then?

To a DEV_CHANGE notification instead?


My opinion is that allowing name change is not worth the doing.
Also, the kernel should never do the name change, it is up to userspace.

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