fuzzing: Add fuzz tests for GDataInputStream’s complex read methods

While reading a single byte or uint16 from an input stream is fairly
simple and uncontroversial, the code to read a line or read up to any of
a set of stop characters is not so trivial. People may be using
`GDataInputStream` to parse untrusted input like this, so we should
probably test that it’s robust against a variety of input conditions.

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

fuzzing/fuzz_data_input_stream_read_line.c

@@ -0,0 +1,44 @@
+/*
+ * 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)
+{
+  GInputStream *base_stream = NULL;
+  GDataInputStream *input_stream = NULL;
+  char *line = NULL;
+  size_t line_length = 0;
+
+  fuzz_set_logging_func ();
+
+  base_stream = g_memory_input_stream_new_from_data (data, size, NULL);
+  input_stream = g_data_input_stream_new (base_stream);
+
+  line = g_data_input_stream_read_line (input_stream, &line_length, NULL, NULL);
+  g_assert (line != NULL || line_length == 0);
+  g_assert (line_length <= size);
+  g_free (line);
+
+  g_clear_object (&input_stream);
+  g_clear_object (&base_stream);
+
+  return 0;
+}