[PATCH net-next v4 4/5] selftests: net: Add python context manager for netns entering

[Xiao Liang <shaw.leon@gmail.com>]

Change netns of current thread and switch back on context exit.
For example:

    with NetNSEnter("ns1"):
        ip("link add dummy0 type dummy")

The command be executed in netns "ns1".

Signed-off-by: Xiao Liang <shaw.leon@gmail.com>
---
 tools/testing/selftests/net/lib/py/__init__.py |  2 +-
 tools/testing/selftests/net/lib/py/netns.py    | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/net/lib/py/__init__.py b/tools/testing/selftests/net/lib/py/__init__.py
index 54d8f5eba810..e2d6c7b63019 100644
--- a/tools/testing/selftests/net/lib/py/__init__.py
+++ b/tools/testing/selftests/net/lib/py/__init__.py
@@ -2,7 +2,7 @@
 
 from .consts import KSRC
 from .ksft import *
-from .netns import NetNS
+from .netns import NetNS, NetNSEnter
 from .nsim import *
 from .utils import *
 from .ynl import NlError, YnlFamily, EthtoolFamily, NetdevFamily, RtnlFamily
diff --git a/tools/testing/selftests/net/lib/py/netns.py b/tools/testing/selftests/net/lib/py/netns.py
index ecff85f9074f..8e9317044eef 100644
--- a/tools/testing/selftests/net/lib/py/netns.py
+++ b/tools/testing/selftests/net/lib/py/netns.py
@@ -1,9 +1,12 @@
 # SPDX-License-Identifier: GPL-2.0
 
 from .utils import ip
+import ctypes
 import random
 import string
 
+libc = ctypes.cdll.LoadLibrary('libc.so.6')
+
 
 class NetNS:
     def __init__(self, name=None):
@@ -29,3 +32,18 @@ class NetNS:
 
     def __repr__(self):
         return f"NetNS({self.name})"
+
+
+class NetNSEnter:
+    def __init__(self, ns_name):
+        self.ns_path = f"/run/netns/{ns_name}"
+
+    def __enter__(self):
+        self.saved = open("/proc/thread-self/ns/net")
+        with open(self.ns_path) as ns_file:
+            libc.setns(ns_file.fileno(), 0)
+        return self
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        libc.setns(self.saved.fileno(), 0)
+        self.saved.close()
-- 
2.47.0