From 6b70f90f441ee8762a75ad9d783c1e35647b52e4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?B=C3=A9n=C3=A9dikt=20Tran?=
 <10796600+picnixz@users.noreply.github.com>
Date: Sun, 9 Nov 2025 13:45:38 +0100
Subject: [PATCH] gh-100218: correctly set `errno` when
 `socket.if_{nametoindex,indextoname}` raise `OSError` (GH-140905)
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Previously, socket.if_nametoindex() and socket.if_indextoname() could raise
an `OSError` with a `None` errno. Now, the errno from libc is propagated.
(cherry picked from commit 3ce2d57b2f02030353af314d89c5f6215d2f5c96)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
---
 Lib/test/test_socket.py                                | 10 ++++++++--
 .../2025-11-02-11-46-00.gh-issue-100218.9Ezfdq.rst     |  3 +++
 Modules/socketmodule.c                                 |  5 +++--
 3 files changed, 14 insertions(+), 4 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-11-02-11-46-00.gh-issue-100218.9Ezfdq.rst

diff --git a/Lib/test/test_socket.py b/Lib/test/test_socket.py
index cfb9619341b324..e66160c5236261 100644
--- a/Lib/test/test_socket.py
+++ b/Lib/test/test_socket.py
@@ -1174,7 +1174,10 @@ def testInterfaceNameIndex(self):
                          'socket.if_indextoname() not available.')
     @support.skip_android_selinux('if_indextoname')
     def testInvalidInterfaceIndexToName(self):
-        self.assertRaises(OSError, socket.if_indextoname, 0)
+        with self.assertRaises(OSError) as cm:
+            socket.if_indextoname(0)
+        self.assertIsNotNone(cm.exception.errno)
+
         self.assertRaises(ValueError, socket.if_indextoname, -1)
         self.assertRaises(OverflowError, socket.if_indextoname, 2**1000)
         self.assertRaises(TypeError, socket.if_indextoname, '_DEADBEEF')
@@ -1194,8 +1197,11 @@ def testInvalidInterfaceIndexToName(self):
                          'socket.if_nametoindex() not available.')
     @support.skip_android_selinux('if_nametoindex')
     def testInvalidInterfaceNameToIndex(self):
+        with self.assertRaises(OSError) as cm:
+            socket.if_nametoindex("_DEADBEEF")
+        self.assertIsNotNone(cm.exception.errno)
+
         self.assertRaises(TypeError, socket.if_nametoindex, 0)
-        self.assertRaises(OSError, socket.if_nametoindex, '_DEADBEEF')
 
     @unittest.skipUnless(hasattr(sys, 'getrefcount'),
                          'test needs sys.getrefcount()')
diff --git a/Misc/NEWS.d/next/Library/2025-11-02-11-46-00.gh-issue-100218.9Ezfdq.rst b/Misc/NEWS.d/next/Library/2025-11-02-11-46-00.gh-issue-100218.9Ezfdq.rst
new file mode 100644
index 00000000000000..2f7500d295578b
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-11-02-11-46-00.gh-issue-100218.9Ezfdq.rst
@@ -0,0 +1,3 @@
+Correctly set :attr:`~OSError.errno` when :func:`socket.if_nametoindex` or
+:func:`socket.if_indextoname` raise an :exc:`OSError`. Patch by Bénédikt
+Tran.
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
index c90b693dd2d62b..1fbd6eb9294cfa 100644
--- a/Modules/socketmodule.c
+++ b/Modules/socketmodule.c
@@ -7277,10 +7277,10 @@ _socket_if_nametoindex_impl(PyObject *module, PyObject *oname)
     unsigned long index;
 #endif
 
+    errno = ENODEV;  // in case 'if_nametoindex' does not set errno
     index = if_nametoindex(PyBytes_AS_STRING(oname));
     if (index == 0) {
-        /* if_nametoindex() doesn't set errno */
-        PyErr_SetString(PyExc_OSError, "no interface with this name");
+        PyErr_SetFromErrno(PyExc_OSError);
         return NULL;
     }
 
@@ -7300,6 +7300,7 @@ static PyObject *
 _socket_if_indextoname_impl(PyObject *module, NET_IFINDEX index)
 /*[clinic end generated code: output=e48bc324993052e0 input=c93f753d0cf6d7d1]*/
 {
+    errno = ENXIO;  // in case 'if_indextoname' does not set errno
     char name[IF_NAMESIZE + 1];
     if (if_indextoname(index, name) == NULL) {
         PyErr_SetFromErrno(PyExc_OSError);