Fix stats prometheus label escaping

lib/stats/stats-prometheus.c

@@ -1,5 +1,6 @@
 /*
- * Copyright (c) 2023 László Várady
+ * Copyright (c) 2024 Axoflow
+ * Copyright (c) 2023-2024 László Várady
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -25,16 +26,56 @@
 #include "stats/stats-cluster.h"
 #include "stats/stats-counter.h"
 #include "timeutils/unixtime.h"
-#include "utf8utils.h"
+#include "str-utils.h"
 #include "scratch-buffers.h"
 
 #include <string.h>
 
+
+/* Exposition format:
+ *
+ * A label value can be any sequence of UTF-8 characters, but the
+ * backslash (\), double-quote ("), and line feed (\n) characters have to be
+ * escaped as \\, \", and \n, respectively.
+ */
 static gchar *
 stats_format_prometheus_sanitize_label_value(const gchar *value)
 {
   GString *sanitized_value = scratch_buffers_alloc();
-  append_unsafe_utf8_as_escaped_binary(sanitized_value, value, -1, "\"");
+
+  const gchar *value_end = value + strlen(value);
+
+  while (value < value_end)
+    {
+      gunichar uchar = g_utf8_get_char_validated(value, value_end - value);
+
+      if (G_UNLIKELY(uchar == (gunichar) -1 || uchar == (gunichar) -2))
+        {
+          /* double backslash to conform to the format */
+          g_string_append_printf(sanitized_value, "\\\\x%02x", *(guint8 *) value);
+          value++;
+          continue;
+        }
+
+      switch (uchar)
+        {
+        case '\\':
+          g_string_append(sanitized_value, "\\\\");
+          break;
+        case '"':
+          g_string_append(sanitized_value, "\\\"");
+          break;
+        case '\n':
+          g_string_append(sanitized_value, "\\n");
+          break;
+        default:
+          g_string_append_unichar_optimized(sanitized_value, uchar);
+          break;
+        }
+
+      value = g_utf8_next_char(value);
+    }
+
   return sanitized_value->str;
 }
 

lib/stats/tests/test_stats_prometheus.c

@@ -1,5 +1,6 @@
 /*
- * Copyright (c) 2023 László Várady
+ * Copyright (c) 2024 Axoflow
+ * Copyright (c) 2023-2024 László Várady
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -164,6 +165,33 @@ Test(stats_prometheus, test_prometheus_format_sanitize)
   stats_cluster_free(cluster);
 }
 
+Test(stats_prometheus, test_prometheus_format_label_escaping)
+{
+  /* Exposition format:
+   *
+   * A label value can be any sequence of UTF-8 characters, but the
+   * backslash (\), double-quote ("), and line feed (\n) characters have to be
+   * escaped as \\, \", and \n, respectively.
+   */
+
+  StatsClusterLabel labels[] =
+  {
+    stats_cluster_label("control_chars", "a\a\tb\nc"),
+    stats_cluster_label("backslashes", "a\\a\\t\\nb"),
+    stats_cluster_label("quotes", "\"\"hello\""),
+    stats_cluster_label("invalid_utf8", "a\xfa"),
+  };
+  StatsCluster *cluster = test_single_cluster("test_name", labels, G_N_ELEMENTS(labels));
+
+  assert_prometheus_format(cluster, SC_TYPE_SINGLE_VALUE,
+                           "syslogng_test_name{control_chars=\"a\a\tb\\nc\","
+                           "backslashes=\"a\\\\a\\\\t\\\\nb\","
+                           "quotes=\"\\\"\\\"hello\\\"\","
+                           "invalid_utf8=\"a\\\\xfa\"} 0\n");
+
+  stats_cluster_free(cluster);
+}
+
 gchar *stats_format_prometheus_format_value(const StatsClusterKey *key, const StatsCounterItem *counter);
 
 Test(stats_prometheus, test_prometheus_format_value)

news/bugfix-224.md

@@ -0,0 +1,4 @@
+`syslog-ng-ctl`: fix escaping of `stats prometheus`
+
+Metric labels (for example, the ones produced by `metrics-probe()`) may contain control characters, invalid UTF-8 or `\`
+characters. In those specific rare cases, the escaping of the `stats prometheus` output was incorrect.