fuzzing: Add simple fuzz test for g_string_replace()

Now that the implementation of it is significantly more complex,
involving pointer arithmetic, it should probably be fuzzed. It’s not an
API which is obviously used to handle untrusted input, but some users of
GLib might do so.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

fuzzing/fuzz_string.c

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2024 GNOME Foundation, Inc.
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "fuzz.h"
+
+int
+LLVMFuzzerTestOneInput (const unsigned char *data, size_t size)
+{
+  unsigned char *nul_terminated_data = NULL;
+  char **args = NULL;
+  size_t n_args;
+  const char *init, *find, *replace;
+  GString *string = NULL;
+
+  fuzz_set_logging_func ();
+
+  /* ignore @size (none of the functions support it); ensure @data is nul-terminated */
+  nul_terminated_data = (unsigned char *) g_strndup ((const gchar *) data, size);
+
+  /* Split the data into three arguments. */
+  args = g_strsplit ((char *) nul_terminated_data, "|", 3);
+  n_args = g_strv_length (args);
+  init = (n_args > 0) ? args[0] : "";
+  find = (n_args > 1) ? args[1] : "";
+  replace = (n_args > 2) ? args[2] : "";
+
+  /* Test g_string_replace() and see if it crashes. */
+  string = g_string_new (init);
+  g_string_replace (string, find, replace, 0);
+  g_string_free (string, TRUE);
+
+  g_strfreev (args);
+  g_free (nul_terminated_data);
+
+  return 0;
+}

fuzzing/meson.build

@@ -30,6 +30,7 @@ fuzz_targets = [
   'fuzz_network_address_parse_uri',
   'fuzz_paths',
   'fuzz_resolver',
+  'fuzz_string',
   'fuzz_uri_escape',
   'fuzz_uri_parse',
   'fuzz_uri_parse_params',