[internal/LibFuzzer] Add option to set cwd to BUILD_DIR based on environment

Add optional cwd argument to LibFuzzerRunner class which passes to
parent ProcessRunner class to subprocess.Popen with cwd argument.
When a LibFuzzerRunner is requested with get_runner() the cwd can be set
to BUILD_DIR using FUZZ_TARGET_CWD_IS_BUILD_DIR boolean env var.

Author: notvictorl

src/clusterfuzz/_internal/bot/fuzzers/libfuzzer.py

@@ -299,14 +299,16 @@ class LibFuzzerRunner(new_process.ModifierProcessRunnerMixin,
                       new_process.UnicodeProcessRunner, LibFuzzerCommon):
   """libFuzzer runner (when minijail is not used)."""
 
-  def __init__(self, executable_path, default_args=None):
+  def __init__(self, executable_path, default_args=None, cwd=None):
     """Inits the LibFuzzerRunner.
 
     Args:
       executable_path: Path to the fuzzer executable.
       default_args: Default arguments to always pass to the fuzzer.
+      cwd: Optional current working directory for the process.
     """
-    super().__init__(executable_path=executable_path, default_args=default_args)
+    super().__init__(
+        executable_path=executable_path, default_args=default_args, cwd=cwd)
 
   def fuzz(self,
            corpus_directories,
@@ -1133,6 +1135,10 @@ def get_runner(fuzzer_path, temp_dir=None, use_minijail=None):
     temp_dir = fuzzer_utils.get_temp_dir()
 
   build_dir = environment.get_value('BUILD_DIR')
+
+  cwd = build_dir if environment.get_value(
+      'FUZZ_TARGET_CWD_IS_BUILD_DIR') else None
+
   is_android = environment.is_android()
   is_fuchsia = environment.platform() == 'FUCHSIA'
 
@@ -1141,6 +1147,12 @@ def get_runner(fuzzer_path, temp_dir=None, use_minijail=None):
     os.chmod(fuzzer_path, 0o755)
 
   is_chromeos_system_job = environment.is_chromeos_system_job()
+
+  if cwd and (use_minijail or is_chromeos_system_job or is_fuchsia or
+              is_android):
+    logs.warning('FUZZ_TARGET_CWD_IS_BUILD_DIR is only supported for standard '
+                 'LibFuzzerRunner and will be ignored.')
+
   if is_chromeos_system_job:
     minijail_chroot = minijail.ChromeOSChroot(build_dir)
   elif use_minijail:
@@ -1185,7 +1197,7 @@ def get_runner(fuzzer_path, temp_dir=None, use_minijail=None):
   elif is_android:
     runner = AndroidLibFuzzerRunner(fuzzer_path, build_dir)
   else:
-    runner = LibFuzzerRunner(fuzzer_path)
+    runner = LibFuzzerRunner(fuzzer_path, cwd=cwd)
 
   return runner
 

src/clusterfuzz/_internal/system/new_process.py

@@ -246,12 +246,14 @@ class ProcessRunner:
     executable_path: Path to the executable to be run.
     default_args: An optional sequence of arguments that are always passed to
         the executable when run.
+    cwd: Optional current working directory for the process.
   """
 
-  def __init__(self, executable_path, default_args=None):
+  def __init__(self, executable_path, default_args=None, cwd=None):
     """Inits ProcessRunner."""
     self._executable_path = executable_path
     self._default_args = []
+    self.cwd = cwd
 
     if default_args:
       self.default_args.extend(default_args)
@@ -332,6 +334,10 @@ def run(self,
     if extra_env is not None:
       env.update(extra_env)
 
+    if self.cwd and 'cwd' not in popen_args:
+      logs.info(f'Executing process with custom cwd: {self.cwd}')
+      popen_args['cwd'] = self.cwd
+
     return ChildProcess(
         subprocess.Popen(
             command,

src/clusterfuzz/_internal/tests/core/bot/fuzzers/libFuzzer/libfuzzer_test.py

@@ -17,12 +17,14 @@
 import os
 import shutil
 import unittest
+from unittest import mock
 
 from clusterfuzz._internal.bot.fuzzers import engine_common
 from clusterfuzz._internal.bot.fuzzers import libfuzzer
 from clusterfuzz._internal.bot.fuzzers import strategy_selection
 from clusterfuzz._internal.bot.fuzzers.libFuzzer import fuzzer
 from clusterfuzz._internal.fuzzing import strategy
+from clusterfuzz._internal.system import environment
 from clusterfuzz._internal.tests.test_libs import helpers as test_helpers
 
 TESTDATA_PATH = os.path.join(os.path.dirname(__file__), 'libfuzzer_test_data')
@@ -177,5 +179,31 @@ def do_strategy(self, *args, **kwargs):
         libfuzzer.should_set_fork_flag(existing_arguments, MockPool()))
 
 
+class GetRunnerTest(unittest.TestCase):
+  """Tests for get_runner."""
+
+  def setUp(self):
+    test_helpers.patch_environ(self)
+
+  @mock.patch('os.chmod')
+  def test_cwd_is_build_dir(self, _):
+    """Test that FUZZ_TARGET_CWD_IS_BUILD_DIR causes cwd to be set to BUILD_DIR"""
+    environment.set_value('BUILD_DIR', '/build/dir')
+    environment.set_value('FUZZ_TARGET_CWD_IS_BUILD_DIR', True)
+    environment.set_value('USE_MINIJAIL', False)
+    runner = libfuzzer.get_runner('/fake/path')
+    self.assertIsInstance(runner, libfuzzer.LibFuzzerRunner)
+    self.assertEqual(runner.cwd, '/build/dir')
+
+  @mock.patch('os.chmod')
+  def test_no_default_cwd(self, _):
+    """Test that cwd is None when FUZZ_TARGET_CWD_IS_BUILD_DIR is not set."""
+    environment.set_value('BUILD_DIR', '/build/dir')
+    environment.set_value('USE_MINIJAIL', False)
+    runner = libfuzzer.get_runner('fake/path')
+    self.assertIsInstance(runner, libfuzzer.LibFuzzerRunner)
+    self.assertIsNone(runner.cwd)
+
+
 if __name__ == '__main__':
   unittest.main()

src/clusterfuzz/_internal/tests/core/system/new_process_test.py

@@ -142,6 +142,20 @@ def test_results_timeout(self):
       self.assertLess(abs(results.time_executed - 0.5), self.TIME_ERROR)
       self.assertTrue(results.timed_out)
 
+  def test_cwd(self):
+    """Test that cwd is passed to Popen."""
+    with mock.patch('subprocess.Popen') as mock_popen:
+      runner = new_process.ProcessRunner(
+          '/test/path', default_args=['-arg1'], cwd='/working/dir')
+      runner.run()
+      mock_popen.assert_called_with(
+          ['/test/path', '-arg1'],
+          env=mock.ANY,
+          stdin=mock.ANY,
+          stdout=mock.ANY,
+          stderr=mock.ANY,
+          cwd='/working/dir')
+
   def test_timeout(self):
     """Tests timeout signals."""
     with mock.patch('subprocess.Popen', mock_popen_factory(1.0, '',