net: detect unstable network names and trigger a settle if needed

The cloud-init-local.service expects that any network device name changes
have already been completed by the kernel or udev daemon.

In some situations we've found that the renaming of interfaces from kernel
names (eth0, eth1, etc) to their persistent names (eno1, ens3, enp0s1,
etc) may happen after cloud-init-local has started where it reads values
from sysfs about what network devices are present, and which device to use
as a fallback nic.

Subsequently, cloud-init-local would write out network configuration for a
kernel device name which would no longer be present by the time that
networking services start to bring up the devices. The result is that the
instance does not get networking configured. Prior to use of
systemd-networkd, the Ubuntu 'networking.service' unit included a call to
udevadm settle which is why this race is not seen on a Xenial system.

This change adds the ability to detect if an interface has a stable name,
if if we find one without stable names and stable names have not been
disabled (net.ifnames=0 in /proc/cmdline), then cloud-init will invoke
udevadm settle.

LP: #1766287

1 files changed, 26 insertions, 0 deletions

diff --git a/cloudinit/net/__init__.py b/cloudinit/net/__init__.py
index 80054546..43226bd0 100644
--- a/cloudinit/net/__init__.py
+++ b/cloudinit/net/__init__.py
@@ -107,6 +107,21 @@ def is_bond(devname):
     return os.path.exists(sys_dev_path(devname, "bonding"))
 
 
+def is_renamed(devname):
+    """
+    /* interface name assignment types (sysfs name_assign_type attribute) */
+    #define NET_NAME_UNKNOWN	0	/* unknown origin (not exposed to user) */
+    #define NET_NAME_ENUM		1	/* enumerated by kernel */
+    #define NET_NAME_PREDICTABLE	2	/* predictably named by the kernel */
+    #define NET_NAME_USER		3	/* provided by user-space */
+    #define NET_NAME_RENAMED	4	/* renamed by user-space */
+    """
+    name_assign_type = read_sys_net_safe(devname, 'name_assign_type')
+    if name_assign_type and name_assign_type in ['3', '4']:
+        return True
+    return False
+
+
 def is_vlan(devname):
     uevent = str(read_sys_net_safe(devname, "uevent"))
     return 'DEVTYPE=vlan' in uevent.splitlines()
@@ -180,6 +195,17 @@ def find_fallback_nic(blacklist_drivers=None):
     if not blacklist_drivers:
         blacklist_drivers = []
 
+    if 'net.ifnames=0' in util.get_cmdline():
+        LOG.debug('Stable ifnames disabled by net.ifnames=0 in /proc/cmdline')
+    else:
+        unstable = [device for device in get_devicelist()
+                    if device != 'lo' and not is_renamed(device)]
+        if len(unstable):
+            LOG.debug('Found unstable nic names: %s; calling udevadm settle',
+                      unstable)
+            msg = 'Waiting for udev events to settle'
+            util.log_time(LOG.debug, msg, func=util.udevadm_settle)
+
     # get list of interfaces that could have connections
     invalid_interfaces = set(['lo'])
     potential_interfaces = set([device for device in get_devicelist()