gh-95023: Added os.setns and os.unshare functions (#95046)

Added os.setns and os.unshare to easily switch between namespaces
on Linux.

Co-authored-by: Christian Heimes <christian@python.org>
Co-authored-by: CAM Gerlach <CAM.Gerlach@Gerlach.CAM>
Co-authored-by: Victor Stinner <vstinner@python.org>

Doc/library/os.rst

@@ -590,6 +590,44 @@ process and user.
       See the documentation for :func:`getgroups` for cases where it may not
       return the same group list set by calling setgroups().
 
+.. function:: setns(fd, nstype=0)
+
+   Reassociate the current thread with a Linux namespace.
+   See the :manpage:`setns(2)` and :manpage:`namespaces(7)` man pages for more
+   details.
+
+   If *fd* refers to a :file:`/proc/{pid}/ns/` link, ``setns()`` reassociates the
+   calling thread with the namespace associated with that link,
+   and *nstype* may be set to one of the
+   :ref:`CLONE_NEW* constants <os-unshare-clone-flags>`
+   to impose constraints on the operation
+   (``0`` means no constraints).
+
+   Since Linux 5.8, *fd* may refer to a PID file descriptor obtained from
+   :func:`~os.pidfd_open`. In this case, ``setns()`` reassociates the calling thread
+   into one or more of the same namespaces as the thread referred to by *fd*.
+   This is subject to any constraints imposed by *nstype*,
+   which is a bit mask combining one or more of the
+   :ref:`CLONE_NEW* constants <os-unshare-clone-flags>`,
+   e.g. ``setns(fd, os.CLONE_NEWUTS | os.CLONE_NEWPID)``.
+   The caller's memberships in unspecified namespaces are left unchanged.
+
+   *fd* can be any object with a :meth:`~io.IOBase.fileno` method, or a raw file descriptor.
+
+   This example reassociates the thread with the ``init`` process's network namespace::
+
+      fd = os.open("/proc/1/ns/net", os.O_RDONLY)
+      os.setns(fd, os.CLONE_NEWNET)
+      os.close(fd)
+
+   .. availability:: Linux >= 3.0 with glibc >= 2.14.
+
+   .. versionadded:: 3.12
+
+   .. seealso::
+
+      The :func:`~os.unshare` function.
+
 .. function:: setpgrp()
 
    Call the system call :c:func:`setpgrp` or ``setpgrp(0, 0)`` depending on
@@ -756,6 +794,49 @@ process and user.
       The function is now always available and is also available on Windows.
 
 
+.. function:: unshare(flags)
+
+   Disassociate parts of the process execution context, and move them into a
+   newly created namespace.
+   See the :manpage:`unshare(2)`
+   man page for more details.
+   The *flags* argument is a bit mask, combining zero or more of the
+   :ref:`CLONE_* constants <os-unshare-clone-flags>`,
+   that specifies which parts of the execution context should be
+   unshared from their existing associations and moved to a new namespace.
+   If the *flags* argument is ``0``, no changes are made to the calling process's
+   execution context.
+
+   .. availability:: Linux >= 2.6.16.
+
+   .. versionadded:: 3.12
+
+   .. seealso::
+
+      The :func:`~os.setns` function.
+
+.. _os-unshare-clone-flags:
+
+Flags to the :func:`unshare` function, if the implementation supports them.
+See :manpage:`unshare(2)` in the Linux manual
+for their exact effect and availability.
+
+.. data:: CLONE_FILES
+          CLONE_FS
+          CLONE_NEWCGROUP
+          CLONE_NEWIPC
+          CLONE_NEWNET
+          CLONE_NEWNS
+          CLONE_NEWPID
+          CLONE_NEWTIME
+          CLONE_NEWUSER
+          CLONE_NEWUTS
+          CLONE_SIGHAND
+          CLONE_SYSVSEM
+          CLONE_THREAD
+          CLONE_VM
+
+
 .. _os-newstreams:
 
 File Object Creation

Include/internal/pycore_global_strings.h

@@ -489,6 +489,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(node_depth)
         STRUCT_FOR_ID(node_offset)
         STRUCT_FOR_ID(ns)
+        STRUCT_FOR_ID(nstype)
         STRUCT_FOR_ID(number)
         STRUCT_FOR_ID(obj)
         STRUCT_FOR_ID(object)

Lib/test/test_posix.py

@@ -2191,6 +2191,53 @@ def test_utime(self):
                 os.utime("path", dir_fd=0)
 
 
+class NamespacesTests(unittest.TestCase):
+    """Tests for os.unshare() and os.setns()."""
+
+    @unittest.skipUnless(hasattr(os, 'unshare'), 'needs os.unshare()')
+    @unittest.skipUnless(hasattr(os, 'setns'), 'needs os.setns()')
+    @unittest.skipUnless(os.path.exists('/proc/self/ns/uts'), 'need /proc/self/ns/uts')
+    @support.requires_linux_version(3, 0, 0)
+    def test_unshare_setns(self):
+        code = """if 1:
+            import errno
+            import os
+            import sys
+            fd = os.open('/proc/self/ns/uts', os.O_RDONLY)
+            try:
+                original = os.readlink('/proc/self/ns/uts')
+                try:
+                    os.unshare(os.CLONE_NEWUTS)
+                except OSError as e:
+                    if e.errno == errno.ENOSPC:
+                        # skip test if limit is exceeded
+                        sys.exit()
+                    raise
+                new = os.readlink('/proc/self/ns/uts')
+                if original == new:
+                    raise Exception('os.unshare failed')
+                os.setns(fd, os.CLONE_NEWUTS)
+                restored = os.readlink('/proc/self/ns/uts')
+                if original != restored:
+                    raise Exception('os.setns failed')
+            except PermissionError:
+                # The calling process did not have the required privileges
+                # for this operation
+                pass
+            except OSError as e:
+                # Skip the test on these errors:
+                # - ENOSYS: syscall not available
+                # - EINVAL: kernel was not configured with the CONFIG_UTS_NS option
+                # - ENOMEM: not enough memory
+                if e.errno not in (errno.ENOSYS, errno.EINVAL, errno.ENOMEM):
+                    raise
+            finally:
+                os.close(fd)
+            """
+
+        assert_python_ok("-c", code)
+
+
 def tearDownModule():
     support.reap_children()
 

Misc/ACKS

@@ -344,6 +344,7 @@ Hervé Coatanhay
 Riccardo Coccioli
 Nick Coghlan
 Josh Cogliati
+Noam Cohen
 Dave Cole
 Terrence Cole
 Benjamin Collar

Misc/NEWS.d/next/Core and Builtins/2022-07-20-09-04-55.gh-issue-95023.bs-xd7.rst

@@ -0,0 +1 @@
+Implement :func:`os.setns` and  :func:`os.unshare` for Linux. Patch by Noam Cohen.