python-peps/pep-0433.txt

PEP: 433
Title: Add cloexec argument to functions creating file descriptors
Version: $Revision$
Last-Modified: $Date$
Author: Victor Stinner <victor.stinner@gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 10-January-2013
Python-Version: 3.4


Abstract
========

This PEP proposes to add a new optional argument ``cloexec`` on functions
creating file descriptors in the Python standard library. If the argument is
``True``, the close-on-exec flag will be set on the new file descriptor.


Rationale
=========

On UNIX, subprocess closes file descriptors greater than 2 by default since
Python 3.2 [#subprocess_close]_. All file descriptors created by the parent
process are automatically closed. ``xmlrpc.server.SimpleXMLRPCServer`` sets
the close-on-exec flag of the listening socket, the parent class
``socketserver.BaseServer`` does not set this flag.

There are other cases creating a subprocess or executing a new program where
file descriptors are not closed: functions of the os.spawn*() family and third
party modules calling ``exec()`` or ``fork()`` + ``exec()``. In this case, file
descriptors are shared between the parent and the child processes which is
usually unexpected and causes various issues.

This PEP proposes to continue the work started with the change in the
subprocess, to fix the issue in any code, and not just code using subprocess.


Inherited file descriptors issues
---------------------------------

Closing the file descriptor in the parent process does not close the related
resource (file, socket, ...) because it is still open in the child process.

The listening socket of TCPServer is not closed on ``exec()``: the child
process is able to get connection from new clients; if the parent closes the
listening socket and create a new listening socket on the same address, it
would get an "address already is used" error.

Not closing file descriptors can lead to resource exhaustion: even if the
parent closes all files, creating a new file descriptor may fail with "too many
files" because files are still open in the child process.


Security
--------

Leaking file descriptors is a major security vulnerability. An untrusted child
process can read sensitive data like passwords and take control of the parent
process though leaked file descriptors. It is for example a known vulnerability
to escape from a chroot.


Atomicity
---------

Using ``fcntl()`` to set the close-on-exec flag is not safe in a multithreaded
application. If a thread calls ``fork()`` and ``exec()`` between the creation
of the file descriptor and the call to ``fcntl(fd, F_SETFD, new_flags)``: the
file descriptor will be inherited by the child process. Modern operating
systems offer functions to set the flag during the creation of the file
descriptor, which avoids the race condition.


Portability
-----------

Python 3.2 added ``socket.SOCK_CLOEXEC`` flag, Python 3.3 added
``os.O_CLOEXEC`` flag and ``os.pipe2()`` function. It is already possible to
set atomically close-on-exec flag in Python 3.3 when opening a file and
creating a pipe or socket.

The problem is that these flags and functions are not portable: only recent
versions of operating systems support them. ``O_CLOEXEC`` and ``SOCK_CLOEXEC``
flags are ignored by old Linux versions and so ``FD_CLOEXEC`` flag must be
checked using ``fcntl(fd, F_GETFD)``.  If the kernel ignores ``O_CLOEXEC`` or
``SOCK_CLOEXEC`` flag, a call to ``fcntl(fd, F_SETFD, flags)`` is required to
set close-on-exec flag.

Note: OpenBSD older 5.2 does not close the file descriptor with close-on-exec
flag set if ``fork()`` is used before ``exec()``, but it works correctly if
``exec()`` is called without ``fork()``.


Scope
-----

Applications still have to close explicitly file descriptors after a
``fork()``.  The close-on-exec flag only closes file descriptors after
``exec()``, and so after ``fork()`` + ``exec()``.

This PEP only change the close-on-exec flag of file descriptors created by the
Python standard library, or by modules using the standard library.  Third party
modules not using the standard library should be modified to conform to this
PEP. The new ``os.set_cloexec()`` function can be used for example.

Impacted functions:

 * ``os.forkpty()``
 * ``http.server.CGIHTTPRequestHandler.run_cgi()``

Impacted modules:

 * ``multiprocessing``
 * ``socketserver``
 * ``subprocess``
 * ``tempfile``
 * ``xmlrpc.server``
 * Maybe: ``signal``, ``threading``

XXX Should ``subprocess.Popen`` set the close-on-exec flag on file XXX
XXX descriptors of the constructor the ``pass_fds`` argument?      XXX

.. note::
   See `Close file descriptors after fork`_ for a possible solution for
   ``fork()`` without ``exec()``.


Proposal
========

This PEP proposes to add a new optional argument ``cloexec`` on functions
creating file descriptors in the Python standard library. If the argument is
``True``, the close-on-exec flag will be set on the new file descriptor.

Add a new function:

 * ``os.set_cloexec(fd: int, cloexec: bool)``: set or unset the close-on-exec
   flag of a file descriptor

Add a new optional ``cloexec`` argument to:

 * ``open()``: ``os.fdopen()`` is indirectly modified
 * ``os.dup()``, ``os.dup2()``
 * ``os.pipe()``
 * ``socket.socket()``, ``socket.socketpair()`` ``socket.socket.accept()``
 * Maybe also: ``os.open()``, ``os.openpty()``
 * TODO:

   * ``select.devpoll()``
   * ``select.poll()``
   * ``select.epoll()``
   * ``select.kqueue()``
   * ``socket.socket.recvmsg()``: use ``MSG_CMSG_CLOEXEC``, or ``os.set_cloexec()``

The default value of the ``cloexec`` argument is ``False`` to keep the backward
compatibility.

The close-on-exec flag will not be set on file descriptors 0 (stdin), 1
(stdout) and 2 (stderr), because these files are expected to be inherited.  It
would still be possible to set close-on-exec flag explicitly using
``os.set_cloexec()``.

Drawbacks:

 * Many functions of the Python standard library creating file descriptors are
   cannot be changed by this proposal, because adding a ``cloexec`` optional
   argument would be surprising and too many functions would need it. For
   example, ``os.urandom()`` uses a temporary file on UNIX, but it calls a
   function of Windows API on Windows. Adding a ``cloexec`` argument to
   ``os.urandom()`` would not make sense. See `Always set close-on-exec flag`_
   for an incomplete list of functions creating file descriptors.
 * Checking if a module creates file descriptors is difficult. For example,
   ``os.urandom()`` creates a file descriptor on UNIX to read ``/dev/urandom``
   (and closes it at exit), whereas it is implemented using a function call on
   Windows. It is not possible to control close-on-exec flag of the file
   descriptor used by ``os.urandom()``, because ``os.urandom()`` API does not
   allow it.


Alternatives
============

Always set close-on-exec flag
-----------------------------

Always set close-on-exec flag on new file descriptors created by Python. This
alternative just changes the default value of the new ``cloexec`` argument.

If a file must be inherited by child processes, ``cloexec=False`` argument can
be used.

``subprocess.Popen`` constructor has an ``pass_fds`` argument to specify which
file descriptors must be inherited. The close-on-exec flag of these file
descriptors must be changed with ``os.set_cloexec()``.

Example of functions creating file descriptors which will be modified to
set close-on-exec flag:

 * ``os.urandom()`` (on UNIX)
 * ``curses.window.getwin()``, ``curses.window.putwin()``
 * ``mmap.mmap()`` (if  ``MAP_ANONYMOUS`` is not defined)
 * ``oss.open()``
 * ``Modules/main.c``: ``RunStartupFile()``
 * ``Python/pythonrun.c``: ``PyRun_SimpleFileExFlags()``
 * ``Modules/getpath.c``: ``search_for_exec_prefix()``
 * ``Modules/zipimport.c``: ``read_directory()``
 * ``Modules/_ssl.c``: ``load_dh_params()``
 * ``PC/getpathp.c``: ``calculate_path()``
 * ``Python/errors.c``: ``PyErr_ProgramText()``
 * ``Python/import.c``: ``imp_load_dynamic()``
 * TODO: ``PC/_msi.c``

Many functions are impacted indirectly by this alternative. Examples:

 * ``logging.FileHandler``

Advantages of setting close-on-exec flag by default:

 * There are far more programs that are bitten by FD inheritance upon
   exec (see `Inherited file descriptors issues`_ and `Security`_) than
   programs relying on it
   (see `Applications using inherance of file descriptors`_).

Drawbacks of setting close-on-exec flag by default:

 * The os module is written as a thin wrapper to system calls (to functions of
   the C standard library). If atomic flags to set close-on-exec flag are not
   supported (see `Appendix: Operating system support`_), a single Python
   function call may call 2 or 3 system calls (see `Performances`_ section).
 * Extra system calls, if any, may slow down Python: see `Performances`_.
 * It violates the principle of least surprise.  Developers using the os module
   may expect that Python respects the POSIX standard and so that close-on-exec
   flag is not set by default.

Backward compatibility: only a few programs rely on inherance of file
descriptors, and they only pass a few file descriptors, usually just one.
These programs will fail immediatly with ``EBADF`` error, and it will be simple
to fix them: add ``cloexec=False`` argument or use
``os.set_cloexec(fd, False)``.

The ``subprocess`` module will be changed anyway to unset close-on-exec flag on
file descriptors listed in the ``pass_fds`` argument of Popen constructor. So
it possible that these programs will not need any fix if they use the
``subprocess`` module.


Add a function to set close-on-exec flag by default
---------------------------------------------------

An alternative is to add also a function to change globally the default
behaviour. It would be possible to set close-on-exec flag for the whole
application including all modules and the Python standard library.  This
alternative is based on the `Proposal`_ and adds extra changes.

Add new functions:

 * ``sys.getdefaultcloexec() -> bool``: get the default value of the
   close-on-exec flag for new file descriptor
 * ``sys.setdefaultcloexec(cloexec: bool)``: enable or disable close-on-exec
   flag, the state of the flag can be overriden in each function creating a
   file descriptor

The major change is that the default value of the ``cloexec`` argument is
``sys.getdefaultcloexec()``, instead of ``False``.

When ``sys.setdefaultcloexec(True)`` is called to set close-on-exec by default,
we have the same drawbacks than `Always set close-on-exec
flag`_ alternative.

There are additionnal drawbacks of having two behaviours depending on
``sys.getdefaultcloexec()`` value:

 * It is not more possible to know if the close-on-exec flag will be set or not
   just by reading the source code.


Close file descriptors after fork
---------------------------------

This PEP does not fix issues with applications using ``fork()`` without
``exec()``. Python needs a generic process to register callbacks which
would be called after a fork, see `Add an 'afterfork' module`_. Such
registry could be used to close file descriptors just after a ``fork()``.

Drawbacks:

 * This alternative does not solve the problem for programs using ``exec()``
   without ``fork()``.
 * A third party module may call directly the C function ``fork()`` which will
   not call "atfork" callbacks.
 * All functions creating file descriptors must be changed to register a
   callback and then unregister their callback when the file is closed. Or a
   list of *all* open file descriptors must be maintained.
 * The operating system is a better place than Python to close automatically
   file descriptors. For example, it is not easy to avoid a race condition
   between closing the file and unregistering the callback closing the file.


open(): add "e" flag to mode
----------------------------

A new "e" mode would set close-on-exec flag (best-effort).

This alternative only solves the problem for ``open()``. socket.socket() and
os.pipe() do not have a ``mode`` argument for example.

Since its version 2.7, the GNU libc supports ``"e"`` flag for ``fopen()``.  It
uses ``O_CLOEXEC`` if available, or use ``fcntl(fd, F_SETFD, FD_CLOEXEC)``.
With Visual Studio, fopen() accepts a "N" flag which uses ``O_NOINHERIT``.


Applications using inherance of file descriptors
================================================

Most developers don't know that file descriptors are inherited by default. Most
programs do not rely on inherance of file descriptors. For example,
``subprocess.Popen`` was changed in Python 3.2 to close all file descriptors
greater than 2 in the child process by default. No user complained about this
behavior change.

Network servers using fork may want to pass the client socket to the child
process. For example, on UNIX a CGI server pass the socket client through file
descriptors 0 (stdin) and 1 (stdout) using ``dup2()``. This specific case is
not impacted by this PEP because the close-on-exec flag is never set on file
descriptors smaller than 3.

To access a restricted resource like creating a socket listening on a TCP port
lower than 1024 or reading a file containing sensitive data like passwords, a
common practice is: start as the root user, create a file descriptor, create
a child process, pass the file descriptor to the child process and exit.
Security is very important in such use case: leaking another file descriptor
would be a critical security vulnerability (see `Security`_). The root process
may not exit but monitors the child process instead, and restarts a new child
process and pass the same file descriptor if the previous child process
crashed.

Example of programs taking file descriptors from the parent process using a
command line option:

 * gpg: ``--status-fd <fd>``, ``--logger-fd <fd>``, etc.
 * openssl: ``-pass fd:<fd>``
 * qemu: ``-add-fd <fd>``
 * valgrind: ``--log-fd=<fd>``, ``--input-fd=<fd>``, etc.
 * xterm: ``-S <fd>``

On Linux, it is possible to use ``"/dev/fd/<fd>"`` filename to pass a file
descriptor to a program expecting a filename.


Performances
============

Setting close-on-exec flag may require additional system calls for each
creation of new file descriptors. The number of additional system calls
depends on the method used to set the flag:

 * ``O_NOINHERIT``: no additionnal system call
 * ``O_CLOEXEC``: one addition system call, but only at the creation of the
   first file descriptor, to check if the flag is supported. If no, Python has
   to fallback to the next method.
 * ``ioctl(fd, FIOCLEX)``: one addition system call per file descriptor
 * ``fcntl(fd, F_SETFD, flags)``: two addition system calls per file
   descriptor, one to get old flags and one to set new flags

XXX Benchmark the overhead for these 4 methods. XXX


Implementation
==============

os.set_cloexec(fd, cloexec)
---------------------------

Best-effort by definition. Pseudo-code::

    if os.name == 'nt':
        def set_cloexec(fd, cloexec=True):
            SetHandleInformation(fd, HANDLE_FLAG_INHERIT, int(cloexec))
    else:
        fnctl = None
        ioctl = None
        try:
            import ioctl
        except ImportError:
            try:
                import fcntl
            except ImportError:
                pass
        if ioctl is not None and hasattr('FIOCLEX', ioctl):
            def set_cloexec(fd, cloexec=True):
                if cloexec:
                    ioctl.ioctl(fd, ioctl.FIOCLEX)
                else:
                    ioctl.ioctl(fd, ioctl.FIONCLEX)
        elif fnctl is not None:
            def set_cloexec(fd, cloexec=True):
                flags = fcntl.fcntl(fd, fcntl.F_GETFD)
                if cloexec:
                    flags |= FD_CLOEXEC
                else:
                    flags &= ~FD_CLOEXEC
                fcntl.fcntl(fd, fcntl.F_SETFD, flags)
        else:
            def set_cloexec(fd, cloexec=True):
                raise NotImplementedError("close-on-exec flag is not supported on your platform")

ioctl is preferred over fcntl because it requires only one syscall, instead of
two syscalls for fcntl.

Note: ``fcntl(fd, F_SETFD, flags)`` only supports one flag (``FD_CLOEXEC``), so
it would be possible to avoid ``fcntl(fd, F_GETFD)``. But it may drop other
flags in the future, and so it is safer to keep the two functions calls.

open()
------

 * Windows: ``open()`` with ``O_NOINHERIT`` flag [atomic]
 * ``open()`` with ``O_CLOEXEC flag`` [atomic]
 * ``open()`` + ``os.set_cloexec(fd, True)`` [best-effort]

os.dup()
--------

 * ``fcntl(fd, F_DUPFD_CLOEXEC)`` [atomic]
 * ``dup()`` + ``os.set_cloexec(fd, True)`` [best-effort]

os.dup2()
---------

 * ``dup3()`` with ``O_CLOEXEC`` flag [atomic]
 * ``dup2()`` + ``os.set_cloexec(fd, True)`` [best-effort]

os.pipe()
---------

 * Windows: ``_pipe()`` with ``O_NOINHERIT`` flag [atomic]
 * ``pipe2()`` with ``O_CLOEXEC`` flag [atomic]
 * ``pipe()`` + ``os.set_cloexec(fd, True)`` [best-effort]

socket.socket()
---------------

 * ``socket()`` with ``SOCK_CLOEXEC`` flag [atomic]
 * ``socket()`` + ``os.set_cloexec(fd, True)`` [best-effort]

socket.socketpair()
-------------------

 * ``socketpair()`` with ``SOCK_CLOEXEC`` flag [atomic]
 * ``socketpair()`` + ``os.set_cloexec(fd, True)`` [best-effort]

socket.socket.accept()
----------------------

 * ``accept4()`` with ``SOCK_CLOEXEC`` flag [atomic]
 * ``accept()`` + ``os.set_cloexec(fd, True)`` [best-effort]


Backward compatibility
======================

There is no backward incompatible change. The default behaviour is unchanged:
the close-on-exec flag is not set by default.


Appendix: Operating system support
==================================

Windows
-------

Windows has an ``O_NOINHERIT`` flag: "Do not inherit in child processes".

For example, it is supported by ``open()`` and ``_pipe()``.

The value of the flag can be modified using:
``SetHandleInformation(fd, HANDLE_FLAG_INHERIT, 1)``.

``CreateProcess()`` has an ``bInheritHandles`` argument: if it is FALSE, the
handles are not inherited. It is used by ``subprocess.Popen`` with
``close_fds`` option.

fcntl
-----

Functions:

 * ``fcntl(fd, F_GETFD)``
 * ``fcntl(fd, F_SETFD, flags | FD_CLOEXEC)``

Availability: AIX, Digital UNIX, FreeBSD, HP-UX, IRIX, Linux, Mac OS X,
OpenBSD, Solaris, SunOS, Unicos.

ioctl
-----

Functions:

 * ``ioctl(fd, FIOCLEX, 0)`` sets close-on-exec flag
 * ``ioctl(fd, FIONCLEX, 0)`` unsets close-on-exec flag

Availability: Linux, Mac OS X, QNX, NetBSD, OpenBSD, FreeBSD.


Atomic flags
------------

New flags:

 * ``O_CLOEXEC``: available on Linux (2.6.23+), FreeBSD (8.3+), OpenBSD 5.0,
   will be part of the next NetBSD release (6.1?).  This flag is part of
   POSIX.1-2008.
 * ``socket()``: ``SOCK_CLOEXEC`` flag, available on Linux 2.6.27+,
   OpenBSD 5.2, NetBSD 6.0.
 * ``fcntl()``: ``F_DUPFD_CLOEXEC`` flag, available on Linux 2.6.24+,
   OpenBSD 5.0, FreeBSD 9.1, NetBSD 6.0. This flag is part of POSIX.1-2008.
 * ``recvmsg()``: ``MSG_CMSG_CLOEXEC``, available on Linux 2.6.23+, NetBSD 6.0.

On Linux older than 2.6.23, ``O_CLOEXEC`` flag is simply ignored. So we have to
check that the flag is supported by calling ``fcntl()``. If it does not work,
we have to set the flag using ``fcntl()``.

XXX what is the behaviour on Linux older than 2.6.27 with SOCK_CLOEXEC? XXX

New functions:

 * ``dup3()``: available on Linux 2.6.27+ (and glibc 2.9)
 * ``pipe2()``: available on Linux 2.6.27+ (and glibc 2.9)
 * ``accept4()``: available on Linux 2.6.28+ (and glibc 2.10)

If ``accept4()`` is called on Linux older than 2.6.28, ``accept4()`` returns
``-1`` (fail) and errno is set to ``ENOSYS``.


Links
=====

Links:

 * `Secure File Descriptor Handling
   <http://udrepper.livejournal.com/20407.html>`_ (Ulrich Drepper, 2008)
 * `win32_support.py of the Tornado project
   <https://bitbucket.org/pvl/gaeseries-tornado/src/c2671cea1842/tornado/win32_support.py>`_:
   emulate fcntl(fd, F_SETFD, FD_CLOEXEC) using
   ``SetHandleInformation(fd, HANDLE_FLAG_INHERIT, 1)``

Python issues:

 * `open() does not able to set flags, such as O_CLOEXEC
   <http://bugs.python.org/issue12105>`_
 * `Add "e" mode to open(): close-and-exec (O_CLOEXEC) / O_NOINHERIT
   <http://bugs.python.org/issue16850>`_
 * `TCP listening sockets created without FD_CLOEXEC flag
   <http://bugs.python.org/issue12107>`_
 * `Use O_CLOEXEC in the tempfile module
   <http://bugs.python.org/issue16860>`_
 * `Support accept4() for atomic setting of flags at socket creation
   <http://bugs.python.org/issue10115>`_
 * `Add an 'afterfork' module
   <http://bugs.python.org/issue16500>`_

Ruby:

 * `Set FD_CLOEXEC for all fds (except 0, 1, 2)
   <http://bugs.ruby-lang.org/issues/5041>`_
 * `O_CLOEXEC flag missing for Kernel::open
   <http://bugs.ruby-lang.org/issues/1291>`_:
   `commit reverted
   <http://bugs.ruby-lang.org/projects/ruby-trunk/repository/revisions/31643>`_ later

Footnotes
=========

.. [#subprocess_close] On UNIX since Python 3.2, subprocess.Popen() closes all file descriptors by
   default: ``close_fds=True``. It closes file descriptors in range 3 inclusive
   to ``local_max_fd`` exclusive, where ``local_max_fd`` is ``fcntl(0,
   F_MAXFD)`` on NetBSD, or ``sysconf(_SC_OPEN_MAX)`` otherwise. If the error
   pipe has a descriptor smaller than 3, ``ValueError`` is raised.