What causes "invalid label prefix file: prefix was empty" in cilium/cilium?

Fill in a non-empty "prefix" for every entry in the labelPrefixes array

invalid label prefix file: prefix was empty

Error message

invalid label prefix file: prefix was empty

What it means

Every entry in the label prefix file's labelPrefixes array must carry a non-empty 'prefix' string; an entry with an empty prefix would match nothing meaningful, so readLabelPrefixCfgFrom rejects the whole file with this error.

Source

Thrown at pkg/labelsfilter/filter.go:285

	}

	f, err := os.Open(fileName)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	lpc := labelPrefixCfg{}
	err = json.NewDecoder(f).Decode(&lpc)
	if err != nil {
		return nil, err
	}
	if lpc.Version != LPCfgFileVersion {
		return nil, fmt.Errorf("unsupported version %d", lpc.Version)
	}
	for _, lp := range lpc.LabelPrefixes {
		if lp.Prefix == "" {
			return nil, fmt.Errorf("invalid label prefix file: prefix was empty")
		}
		if lp.Source == "" {
			return nil, fmt.Errorf("invalid label prefix file: source was empty")
		}
		if !lp.Ignore {
			lpc.whitelist = true
		}
	}
	return &lpc, nil
}

func (cfg *labelPrefixCfg) filterLabels(lbls labels.Labels) (identityLabels, informationLabels labels.Labels) {
	if len(lbls) == 0 {
		return nil, nil
	}

	validLabelPrefixesMU.RLock()
	defer validLabelPrefixesMU.RUnlock()

View on GitHub (pinned to ac7b90affa)

Solutions

Fill in a non-empty "prefix" for every entry in the labelPrefixes array
Remove empty/null placeholder entries from the array
Check the Helm/templating pipeline that generates the file for empty values

Example fix

// before
{"version":1,"labelPrefixes":[{"prefix":"","source":"k8s"}]}
// after
{"version":1,"labelPrefixes":[{"prefix":"k8s:","source":"k8s"}]}

Defensive patterns

Strategy: validation

Validate before calling

func validatePrefixEntries(data []byte) error {
    var cfg struct{ LabelPrefixes []struct{ Prefix string `json:"prefix"` } `json:"labelPrefixes"` }
    if err := json.Unmarshal(data, &cfg); err != nil { return err }
    for i, lp := range cfg.LabelPrefixes {
        if lp.Prefix == "" { return fmt.Errorf("entry %d: empty prefix", i) }
    }
    return nil
}

Prevention

Remove null/empty placeholder entries from labelPrefixes
Check rendered Helm templates for empty prefix values
Validate the whole file with a JSON schema before applying

When it happens

Trigger: A JSON file parsed by readLabelPrefixCfgFrom containing an element in "labelPrefixes" with "prefix":"" or a missing prefix field, e.g. {"version":1,"labelPrefixes":[{"source":"k8s"}]}.

Common situations: Empty Helm template values rendering into the ConfigMap, trailing commas creating null/empty entries, or copy-paste of an entry where the prefix was deleted but the entry kept.

Related errors

AI-assisted analysis of cilium/cilium@ac7b90affa (2026-08-31). Data as JSON: /api/errors/6292c3e9df93b7f1. Report an issue: GitHub.