From c9d4689e0b943b9cd6984844e88c4c7abcac175d Mon Sep 17 00:00:00 2001
From: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>
Date: Mon, 20 Oct 2025 13:54:31 +0200
Subject: [PATCH] relabeling: Fix labelmap action validation with legacy metric
 name scheme

Fixes #17370

In Prometheus v3.7.0, using labelmap actions with replacement patterns
containing regex variables (e.g., `$1`, `${1}`) would fail validation
when `metric_name_validation_scheme` was set to `legacy`, causing
Prometheus to fail at startup with:
  "$1" is invalid 'replacement' for labelmap action

This was a regression as the same configuration worked in v3.6.0.

The issue was in the validation logic: while UTF-8 validation correctly
allowed `$` characters, legacy validation incorrectly used
`IsValidLabelName` which rejects `$` characters. The fix ensures legacy
validation uses `relabelTargetLegacy` regex which explicitly supports
regex template variables.

Added test cases to verify labelmap validation works with both `$1` and
`${1}` replacement patterns under legacy validation scheme.

Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>
---
 model/relabel/relabel.go      |  8 +++++++-
 model/relabel/relabel_test.go | 16 ++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/model/relabel/relabel.go b/model/relabel/relabel.go
index c7c5439d54..c15049b2b0 100644
--- a/model/relabel/relabel.go
+++ b/model/relabel/relabel.go
@@ -145,7 +145,13 @@ func (c *Config) Validate(nameValidationScheme model.ValidationScheme) error {
 		// UTF-8 allows ${} characters, so standard validation allow $variables by default.
 		// TODO(bwplotka): Relabelling users cannot put $ and ${<...>} characters in metric names or values.
 		// Design escaping mechanism to allow that, once valid use case appears.
-		return c.NameValidationScheme.IsValidLabelName(value)
+		switch c.NameValidationScheme {
+		case model.UTF8Validation:
+			return c.NameValidationScheme.IsValidLabelName(value)
+		default:
+			// For legacy validation, use the legacy regex that allows $variables.
+			return relabelTargetLegacy.MatchString(value)
+		}
 	}
 	if c.Action == Replace && varInRegexTemplate(c.TargetLabel) && !isValidLabelNameWithRegexVarFn(c.TargetLabel) {
 		return fmt.Errorf("%q is invalid 'target_label' for %s action", c.TargetLabel, c.Action)
diff --git a/model/relabel/relabel_test.go b/model/relabel/relabel_test.go
index 79730055cb..99a4e09f56 100644
--- a/model/relabel/relabel_test.go
+++ b/model/relabel/relabel_test.go
@@ -831,6 +831,22 @@ func TestRelabelValidate(t *testing.T) {
 				NameValidationScheme: model.UTF8Validation,
 			},
 		},
+		{
+			config: Config{
+				Regex:                MustNewRegexp("__meta_kubernetes_pod_label_(strimzi_io_.+)"),
+				Action:               LabelMap,
+				Replacement:          "$1",
+				NameValidationScheme: model.LegacyValidation,
+			},
+		},
+		{
+			config: Config{
+				Regex:                MustNewRegexp("__meta_(.+)"),
+				Action:               LabelMap,
+				Replacement:          "${1}",
+				NameValidationScheme: model.LegacyValidation,
+			},
+		},
 	}
 	for i, test := range tests {
 		t.Run(strconv.Itoa(i), func(t *testing.T) {