kubernetes/cmd/kubeadm/app/cmd/phases/upgrade/apply/preflight.go

// NewPreflightPhase returns a new prefight phase.
func NewPreflightPhase() workflow.Phase { … }

func runPreflight(c workflow.RunData) error { … }

// enforceVersionPolicies makes sure that the version the user specified is valid to upgrade to.
// It handles both fatal and skippable (with --force) errors.
func enforceVersionPolicies(newK8sVersionStr string, newK8sVersion *version.Version, allowExperimentalUpgrades, allowRCUpgrades, force bool, versionGetter upgrade.VersionGetter) error { … }