Comment 16 for bug 1997922

One of the solutions we discussed was the ability to (on Sysfs-based environments) validate leader/subordinate (upper/lower) distinctions for devices based on whether the virtual functions are created by the driver once it configures the device. Typically subordinate devices will have either a 'master' or 'upper_<upper_dev_name>' symlink in '/sys/class/net/<lower_devName>/device/'. A leader will have a 'lower_<lower_dev_name>' symlink to indicate that it is the 'upper'.

The problem we have with just relying on the upper/lower relationship being setup is that there is a race present if cloud-init walks /sys/class/net before drivers finish initial configuration and setup of the upper/lower relationships for the network VFs. When cloud-init beats driver setup, those upper_|lower_ symlinks aren't surfaced yet by the kernel.

So, ultimately we still have to rely on driver names to inform us of the unlikely scenarios where a network device or switch may have duplicate MACs for upper and lower functions, but those devices have not yet been fully configured.

A gap can exist as much as 40 seconds between driver detection for a device and the final upper_|lower_ configuration as seen in /sys for this Hyper-V system where duplicate MACs are expected:

ubuntu@hyper-v-timestamp:~$ for dev in eth0 enP62764s1; do echo ---- $dev; ls -l --full-time /sys/class/net/$dev/device/driver; ls -l --full-time /sys/class/net/$dev/ | egrep 'upper|lower|master'; done
---- eth0
lrwxrwxrwx 1 root root 0 2023-01-31 21:25:18.651923500 +0000 /sys/class/net/eth0/device/driver -> ../../../../../../bus/vmbus/drivers/hv_netvsc
lrwxrwxrwx 1 root root 0 2023-01-31 21:25:27.864907346 +0000 lower_enP62764s1 -> ../../../1d6f00a2-f52c-4ea5-9bf1-5cbb5824b1d3/pcif52c:00/f52c:00:02.0/net/enP62764s1
---- enP62764s1
lrwxrwxrwx 1 root root 0 2023-01-31 21:25:19.227923500 +0000 /sys/class/net/enP62764s1/device/driver -> ../../../../../../../../bus/pci/drivers/mlx5_core
lrwxrwxrwx 1 root root 0 2023-01-31 21:25:20.923923500 +0000 master -> ../../../../../000d3a1e-a3af-000d-3a1e-a3af000d3a1e/net/eth0
lrwxrwxrwx 1 root root 0 2023-01-31 21:26:05.957294183 +0000 upper_eth0 -> ../../../../../000d3a1e-a3af-000d-3a1e-a3af000d3a1e/net/eth0