Inheritance of File Descriptors

Each operating system handles the inheritance of file descriptors differently. Windows creates non-inheritable handles by default, whereas UNIX and the POSIX API on Windows create inheritable file descriptors by default. Python prefers the POSIX API over the native Windows API, to have a single code base and to use the same type for file descriptors, and so it creates inheritable file descriptors.

There is one exception: os.pipe() creates non-inheritable pipes on Windows, whereas it creates inheritable pipes on UNIX. The reason is an implementation artifact: os.pipe() calls CreatePipe() on Windows (native API), whereas it calls pipe() on UNIX (POSIX API). The call to CreatePipe() was added in Python in 1994, before the introduction of pipe() in the POSIX API in Windows 98. The issue #4708 proposes to change os.pipe() on Windows to create inheritable pipes.

Inheritance of File Descriptors on Windows

On Windows, the native type of file objects is handles (C type HANDLE). These handles have a HANDLE_FLAG_INHERIT flag which defines if a handle can be inherited in a child process or not. For the POSIX API, the C runtime (CRT) also provides file descriptors (C type int). The handle of a file descriptor can be retrieve using the function _get_osfhandle(fd). A file descriptor can be created from a handle using the function _open_osfhandle(handle).

Using CreateProcess(), handles are only inherited if their inheritable flag (HANDLE_FLAG_INHERIT) is set and the bInheritHandles parameter of CreateProcess() is TRUE; all file descriptors except standard streams (0, 1, 2) are closed in the child process, even if bInheritHandles is TRUE. Using the spawnv() function, all inheritable handles and all inheritable file descriptors are inherited in the child process. This function uses the undocumented fields cbReserved2 and lpReserved2 of the STARTUPINFO structure to pass an array of file descriptors.

To replace standard streams (stdin, stdout, stderr) using CreateProcess(), the STARTF_USESTDHANDLES flag must be set in the dwFlags field of the STARTUPINFO structure and the bInheritHandles parameter of CreateProcess() must be set to TRUE. So when at least one standard stream is replaced, all inheritable handles are inherited by the child process.

The default value of the close_fds parameter of subprocess process is True (bInheritHandles=FALSE) if stdin, stdout and stderr parameters are None, False (bInheritHandles=TRUE) otherwise.

See also:

• Handle Inheritance
• Stackoverflow: Can TCP SOCKET handles be set not inheritable?

Only Inherit Some Handles on Windows

Since Windows Vista, CreateProcess() supports an extension of the STARTUPINFO structure: the STARTUPINFOEX structure. Using this new structure, it is possible to specify a list of handles to inherit: PROC_THREAD_ATTRIBUTE_HANDLE_LIST. Read Programmatically controlling which handles are inherited by new processes in Win32 (Raymond Chen, Dec 2011) for more information.

Before Windows Vista, it is possible to make handles inheritable and call CreateProcess() with bInheritHandles=TRUE. This option works if all other handles are non-inheritable. There is a race condition: if another thread calls CreateProcess() with bInheritHandles=TRUE, handles will also be inherited in the second process.

Microsoft suggests to use a lock to avoid the race condition: read Q315939: PRB: Child Inherits Unintended Handles During CreateProcess Call (last review: November 2006). The Python issue #16500 “Add an atfork module” proposes to add such lock, it can be used to make handles non-inheritable without the race condition. Such lock only protects against a race condition between Python threads; C threads are not protected.

Another option is to duplicate handles that must be inherited, passing the values of the duplicated handles to the child process, so the child process can steal duplicated handles using DuplicateHandle() with DUPLICATE_CLOSE_SOURCE. Handle values change between the parent and the child process because the handles are duplicated (twice); the parent and/or the child process must be adapted to handle this change. If the child program cannot be modified, an intermediate program can be used to steal handles from the parent process before spawning the final child program. The intermediate program has to pass the handle from the child process to the parent process. The parent may have to close duplicated handles if all handles were not stolen, for example if the intermediate process fails. If the command line is used to pass the handle values, the command line must be modified when handles are duplicated, because their values are modified.

This PEP does not include a solution to this problem because there is no perfect solution working on all Windows versions. This point is deferred until use cases relying on handle or file descriptor inheritance on Windows are well known, so we can choose the best solution and carefully test its implementation.

Inheritance of File Descriptors on UNIX

POSIX provides a close-on-exec flag on file descriptors to automatically close a file descriptor when the C function execv() is called. File descriptors with the close-on-exec flag cleared are inherited in the child process, file descriptors with the flag set are closed in the child process.

The flag can be set in two syscalls (one to get current flags, a second to set new flags) using fcntl():

int flags, res;
flags = fcntl(fd, F_GETFD);
if (flags == -1) { /* handle the error */ }
flags |= FD_CLOEXEC;
/* or "flags &= ~FD_CLOEXEC;" to clear the flag */
res = fcntl(fd, F_SETFD, flags);
if (res == -1) { /* handle the error */ }

FreeBSD, Linux, Mac OS X, NetBSD, OpenBSD and QNX also support setting the flag in a single syscall using ioctl():

int res;
res = ioctl(fd, FIOCLEX, 0);
if (!res) { /* handle the error */ }

NOTE: The close-on-exec flag has no effect on fork(): all file descriptors are inherited by the child process. The Python issue #16500 “Add an atfork module” proposes to add a new atfork module to execute code at fork, which may be used to automatically close file descriptors.

Issues with Inheritable File Descriptors

Most of the time, inheritable file descriptors “leaked” to child processes are not noticed, because they don’t cause major bugs. It does not mean that these bugs must not be fixed.

Two common issues with inherited file descriptors:

• On Windows, a directory cannot be removed before all file handles open in the directory are closed. The same issue can be seen with files, except if the file was created with the FILE_SHARE_DELETE flag (O_TEMPORARY mode for open()).
• If a listening socket is leaked to a child process, the socket address cannot be reused before the parent and child processes terminated. For example, if a web server spawns a new program to handle a process, and the server restarts while the program is not done, the server cannot start because the TCP port is still in use.

Example of issues in open source projects:

• Mozilla (Firefox): open since 2002-05
• dbus library: fixed in 2008-05 (dbus commit), close file descriptors in the child process
• autofs: fixed in 2009-02, set the CLOEXEC flag
• qemu: fixed in 2009-12 (qemu commit), set CLOEXEC flag
• Tor: fixed in 2010-12, set CLOEXEC flag
• OCaml: open since 2011-04, “PR#5256: Processes opened using Unix.open_process* inherit all opened file descriptors (including sockets)”
• ØMQ: open since 2012-08
• Squid: open since 2012-07

See also: Excuse me son, but your code is leaking !!! (Dan Walsh, March 2012) for SELinux issues with leaked file descriptors.

Security Vulnerability

Leaking sensitive file handles and file descriptors can lead to security vulnerabilities. An untrusted child process might read sensitive data like passwords or take control of the parent process though a leaked file descriptor. With a leaked listening socket, a child process can accept new connections to read sensitive data.

Example of vulnerabilities:

• Hijacking Apache https by mod_php (2003)
  • Apache: Apr should set FD_CLOEXEC if APR_FOPEN_NOCLEANUP is not set: fixed in 2009
  • PHP: system() (and similar) don’t cleanup opened handles of Apache: open since 2006
• CWE-403: Exposure of File Descriptor to Unintended Control Sphere (2008)
• OpenSSH Security Advisory: portable-keysign-rand-helper.adv (2011)

Read also the CERT Secure Coding Standards: FIO42-C. Ensure files are properly closed when they are no longer needed.

Issues fixed in the subprocess module

Inherited file descriptors caused 4 issues in the subprocess module:

• Issue #2320: Race condition in subprocess using stdin (opened in 2008)
• Issue #3006: subprocess.Popen causes socket to remain open after close (opened in 2008)
• Issue #7213: subprocess leaks open file descriptors between Popen instances causing hangs (opened in 2009)
• Issue #12786: subprocess wait() hangs when stdin is closed (opened in 2011)

These issues were fixed in Python 3.2 by 4 different changes in the subprocess module:

• Pipes are now non-inheritable;
• The default value of the close_fds parameter is now True, with one exception on Windows: the default value is False if at least one standard stream is replaced;
• A new pass_fds parameter has been added;
• Creation of a _posixsubprocess module implemented in C.

Atomic Creation of non-inheritable File Descriptors

In a multi-threaded application, an inheritable file descriptor may be created just before a new program is spawned, before the file descriptor is made non-inheritable. In this case, the file descriptor is leaked to the child process. This race condition could be avoided if the file descriptor is created directly non-inheritable.

FreeBSD, Linux, Mac OS X, Windows and many other operating systems support creating non-inheritable file descriptors with the inheritable flag cleared atomically at the creation of the file descriptor.

A new WSA_FLAG_NO_HANDLE_INHERIT flag for WSASocket() was added in Windows 7 SP1 and Windows Server 2008 R2 SP1 to create non-inheritable sockets. If this flag is used on an older Windows version (ex: Windows XP SP3), WSASocket() fails with WSAEPROTOTYPE.

On UNIX, new flags were added for files and sockets:

• O_CLOEXEC: available on Linux (2.6.23), FreeBSD (8.3), Mac OS 10.8, OpenBSD 5.0, Solaris 11, QNX, BeOS, next NetBSD release (6.1?). This flag is part of POSIX.1-2008.
• SOCK_CLOEXEC flag for socket() and socketpair(), 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, Solaris 11. This flag is part of POSIX.1-2008.
• fcntl(): F_DUP2FD_CLOEXEC flag, available on FreeBSD 9.1 and Solaris 11.
• 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 fcntl() must be called to check if the file descriptor is non-inheritable: O_CLOEXEC is not supported if the FD_CLOEXEC flag is missing. On Linux older than 2.6.27, socket() or socketpair() fail with errno set to EINVAL if the SOCK_CLOEXEC flag is set in the socket type.

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)

On Linux older than 2.6.28, accept4() fails with errno set to ENOSYS.

Summary:

Legend:

• “Atomic File”: first version of the operating system supporting creating atomically a non-inheritable file descriptor using open()
• “Atomic Socket”: first version of the operating system supporting creating atomically a non-inheritable socket
• “X”: not supported yet

See also:

• Secure File Descriptor Handling (Ulrich Drepper, 2008)
• Ghosts of Unix past, part 2: Conflated designs (Neil Brown, 2010) explains the history of O_CLOEXEC and O_NONBLOCK flags
• File descriptor handling changes in 2.6.27
• FreeBSD: atomic close on exec

Status of Python 3.3

Python 3.3 creates inheritable file descriptors on all platforms, except os.pipe() which creates non-inheritable file descriptors on Windows.

New constants and functions related to the atomic creation of non-inheritable file descriptors were added to Python 3.3: os.O_CLOEXEC, os.pipe2() and socket.SOCK_CLOEXEC.

On UNIX, the subprocess module closes all file descriptors in the child process by default, except standard streams (0, 1, 2) and file descriptors of the pass_fds parameter. If the close_fds parameter is set to False, all inheritable file descriptors are inherited in the child process.

On Windows, the subprocess closes all handles and file descriptors in the child process by default. If at least one standard stream (stdin, stdout or stderr) is replaced (ex: redirected into a pipe), all inheritable handles and file descriptors 0, 1 and 2 are inherited in the child process.

Using the functions of the os.execv*() and os.spawn*() families, all inheritable handles and all inheritable file descriptors are inherited by the child process.

On UNIX, the multiprocessing module uses os.fork() and so all file descriptors are inherited by child processes.

On Windows, all inheritable handles and file descriptors 0, 1 and 2 are inherited by the child process using the multiprocessing module, all file descriptors except standard streams are closed.

Summary:

Legend:

• “all”: all inheritable file descriptors or handles are inherited in the child process
• “none”: all handles are closed in the child process
• “STD”: only file descriptors 0 (stdin), 1 (stdout) and 2 (stderr) are inherited in the child process
• “pass_fds”: file descriptors of the pass_fds parameter of the subprocess are inherited
• “not applicable”: on UNIX, the multiprocessing uses fork(), so this case is not affected by this PEP.

Closing All Open File Descriptors

On UNIX, the subprocess module closes almost all file descriptors in the child process. This operation requires MAXFD system calls, where MAXFD is the maximum number of file descriptors, even if there are only few open file descriptors. This maximum can be read using: os.sysconf("SC_OPEN_MAX").

The operation can be slow if MAXFD is large. For example, on a FreeBSD buildbot with MAXFD=655,000, the operation took 300 ms: see issue #11284: slow close file descriptors.

On Linux, Python 3.3 gets the list of all open file descriptors from /proc/<PID>/fd/, and so performances depends on the number of open file descriptors, not on MAXFD.

See also:

• Python issue #1663329: subprocess close_fds perform poor if SC_OPEN_MAX is high
• Squid Bug #837033: Squid should set CLOEXEC on opened FDs. “32k+ close() calls in each child process take a long time ([12-56] seconds) in Xen PV guests.”

Non-inheritable File Descriptors

The following functions are modified to make newly created file descriptors non-inheritable by default:

• asyncore.dispatcher.create_socket()
• io.FileIO
• io.open()
• open()
• os.dup()
• os.fdopen()
• os.open()
• os.openpty()
• os.pipe()
• select.devpoll()
• select.epoll()
• select.kqueue()
• socket.socket()
• socket.socket.accept()
• socket.socket.dup()
• socket.socket.fromfd()
• socket.socketpair()

os.dup2() still creates inheritable by default, see below.

When available, atomic flags are used to make file descriptors non-inheritable. The atomicity is not guaranteed because a fallback is required when atomic flags are not available.

New Functions And Methods

New functions available on all platforms:

• os.get_inheritable(fd: int): return True if the file descriptor can be inherited by child processes, False otherwise.
• os.set_inheritable(fd: int, inheritable: bool): set the inheritable flag of the specified file descriptor.

New functions only available on Windows:

• os.get_handle_inheritable(handle: int): return True if the handle can be inherited by child processes, False otherwise.
• os.set_handle_inheritable(handle: int, inheritable: bool): set the inheritable flag of the specified handle.

New methods:

• socket.socket.get_inheritable(): return True if the socket can be inherited by child processes, False otherwise.
• socket.socket.set_inheritable(inheritable: bool): set the inheritable flag of the specified socket.

Other Changes

On UNIX, subprocess makes file descriptors of the pass_fds parameter inheritable. The file descriptor is made inheritable in the child process after the fork() and before execv(), so the inheritable flag of file descriptors is unchanged in the parent process.

os.dup2() has a new optional inheritable parameter: os.dup2(fd, fd2, inheritable=True). fd2 is created inheritable by default, but non-inheritable if inheritable is False.

os.dup2() behaves differently than os.dup() because the most common use case of os.dup2() is to replace the file descriptors of the standard streams: stdin (0), stdout (1) and stderr (2). Standard streams are expected to be inherited by child processes.

Add a new open_noinherit() function

In June 2007, Henning von Bargen proposed on the python-dev mailing list to add a new open_noinherit() function to fix issues of inherited file descriptors in child processes. At this time, the default value of the close_fds parameter of the subprocess module was False.

Read the mail thread: [Python-Dev] Proposal for a new function “open_noinherit” to avoid problems with subprocesses and security risks.

PEP 433

PEP 433, “Easier suppression of file descriptor inheritance”, was a previous attempt proposing various other alternatives, but no consensus could be reached.