GH-124241: try reading /proc/self/exe to determine sys.executable

Lib/test/test_getpath.py

@@ -497,6 +497,34 @@ def test_symlink_normal_posix(self):
         actual = getpath(ns, expected)
         self.assertEqual(expected, actual)
 
+    def test_bad_argv0_posix(self):
+        """Test that executable resolves correctly if argv0 is not Python and we
+        know real_executable (getpath.c computes it from readlink(/proc/self/exe)).
+        """
+        ns = MockPosixNamespace(
+            PREFIX="/usr",
+            argv0="not-python",
+            real_executable="/usr/bin/python",
+        )
+        ns.add_known_xfile("/usr/bin/python")
+        ns.add_known_file("/usr/lib/python9.8/os.py")
+        ns.add_known_dir("/usr/lib/python9.8/lib-dynload")
+        expected = dict(
+            executable="/usr/bin/python",
+            base_executable="/usr/bin/python",
+            prefix="/usr",
+            exec_prefix="/usr",
+            module_search_paths_set=1,
+            module_search_paths=[
+                "/usr/lib/python98.zip",
+                "/usr/lib/python9.8",
+                "/usr/lib/python9.8/lib-dynload",
+            ],
+        )
+        actual = getpath(ns, expected)
+        self.assertEqual(expected, actual)
+
+
     def test_symlink_buildpath_posix(self):
         """Test an in-build-tree layout on POSIX.
 

Misc/NEWS.d/next/Core_and_Builtins/2026-03-03-21-45-16.gh-issue-124241._GLhVo.rst

@@ -0,0 +1,2 @@
+On POSIX, we now try to read the symlink target from ``/proc/self/exe`` to
+determine :data:`sys.executable`.

Modules/getpath.c

@@ -797,6 +797,11 @@ progname_to_dict(PyObject *dict, const char *key)
         PyMem_RawFree(path);
         break;
     }
+#elif defined(HAVE_READLINK)
+    wchar_t resolved[MAXPATHLEN + 1];
+    if (_Py_wreadlink(L"/proc/self/exe", resolved, Py_ARRAY_LENGTH(resolved)) > 0) {
+        return wchar_to_dict(dict, key, resolved);
+    }
 #endif
     return PyDict_SetItemString(dict, key, Py_None) == 0;
 }

Modules/getpath.py

@@ -280,6 +280,17 @@ def search_up(prefix, *landmarks, test=isfile):
     # whether we are in a build tree. This is true even if the
     # executable path was provided in the config.
     real_executable = executable
+elif os_name == 'posix' and real_executable:
+    # real_executable is more accurate than the value we have computed for
+    # executable, so use it instead if it resolves to a different path
+    # (eg. GH-124241).
+    # If real_executable and executable resolve to the same path, prefer
+    # executable, as that is much more likely to be the path the user is using.
+    try:
+        if realpath(executable) != real_executable:
+            executable = real_executable
+    except OSError:
+        pass
 
 if not executable and program_name and ENV_PATH:
     # Resolve names against PATH.