bpo-31512: Add non-elevated symlink support for Windows (GH-3652)

Doc/library/os.rst

@@ -2699,19 +2699,15 @@ features:
    as a directory if *target_is_directory* is ``True`` or a file symlink (the
    default) otherwise.  On non-Windows platforms, *target_is_directory* is ignored.
 
-   Symbolic link support was introduced in Windows 6.0 (Vista).  :func:`symlink`
-   will raise a :exc:`NotImplementedError` on Windows versions earlier than 6.0.
-
    This function can support :ref:`paths relative to directory descriptors
    <dir_fd>`.
 
    .. note::
 
-      On Windows, the *SeCreateSymbolicLinkPrivilege* is required in order to
-      successfully create symlinks. This privilege is not typically granted to
-      regular users but is available to accounts which can escalate privileges
-      to the administrator level. Either obtaining the privilege or running your
-      application as an administrator are ways to successfully create symlinks.
+      On newer versions of Windows 10, unprivileged accounts can create symlinks
+      if Developer Mode is enabled. When Developer Mode is not available/enabled,
+      the *SeCreateSymbolicLinkPrivilege* privilege is required, or the process
+      must be run as an administrator.
 
 
       :exc:`OSError` is raised when the function is called by an unprivileged
@@ -2729,6 +2725,9 @@ features:
    .. versionchanged:: 3.6
       Accepts a :term:`path-like object` for *src* and *dst*.
 
+   .. versionchanged:: 3.8
+      Added support for unelevated symlinks on Windows with Developer Mode.
+
 
 .. function:: sync()
 

Misc/NEWS.d/next/Windows/2017-10-04-12-40-45.bpo-31512.YQeBt2.rst

+With the Windows 10 Creators Update, non-elevated users can now create
+symlinks as long as the computer has Developer Mode enabled.

Modules/posixmodule.c

@@ -284,10 +284,7 @@ extern char        *ctermid_r(char *);
 #include <windows.h>
 #include <shellapi.h>   /* for ShellExecute() */
 #include <lmcons.h>     /* for UNLEN */
-#ifdef SE_CREATE_SYMBOLIC_LINK_NAME /* Available starting with Vista */
 #define HAVE_SYMLINK
-static int win32_can_symlink = 0;
-#endif
 #endif /* _MSC_VER */
 
 #ifndef MAXPATHLEN
@@ -7755,26 +7752,6 @@ os_readlink_impl(PyObject *module, path_t *path, int dir_fd)
 
 #if defined(MS_WINDOWS)
 
-/* Grab CreateSymbolicLinkW dynamically from kernel32 */
-static BOOLEAN (CALLBACK *Py_CreateSymbolicLinkW)(LPCWSTR, LPCWSTR, DWORD) = NULL;
-
-static int
-check_CreateSymbolicLink(void)
-{
-    HINSTANCE hKernel32;
-    /* only recheck */
-    if (Py_CreateSymbolicLinkW)
-        return 1;
-
-    Py_BEGIN_ALLOW_THREADS
-    hKernel32 = GetModuleHandleW(L"KERNEL32");
-    *(FARPROC*)&Py_CreateSymbolicLinkW = GetProcAddress(hKernel32,
-                                                        "CreateSymbolicLinkW");
-    Py_END_ALLOW_THREADS
-
-    return Py_CreateSymbolicLinkW != NULL;
-}
-
 /* Remove the last portion of the path - return 0 on success */
 static int
 _dirnameW(WCHAR *path)
@@ -7878,33 +7855,57 @@ os_symlink_impl(PyObject *module, path_t *src, path_t *dst,
 {
 #ifdef MS_WINDOWS
     DWORD result;
+    DWORD flags = 0;
+
+    /* Assumed true, set to false if detected to not be available. */
+    static int windows_has_symlink_unprivileged_flag = TRUE;
 #else
     int result;
 #endif
 
 #ifdef MS_WINDOWS
-    if (!check_CreateSymbolicLink()) {
-        PyErr_SetString(PyExc_NotImplementedError,
-            "CreateSymbolicLink functions not found");
-        return NULL;
-        }
-    if (!win32_can_symlink) {
-        PyErr_SetString(PyExc_OSError, "symbolic link privilege not held");
-        return NULL;
-        }
-#endif
 
-#ifdef MS_WINDOWS
+    if (windows_has_symlink_unprivileged_flag) {
+        /* Allow non-admin symlinks if system allows it. */
+        flags |= SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE;
+    }
 
     Py_BEGIN_ALLOW_THREADS
     _Py_BEGIN_SUPPRESS_IPH
-    /* if src is a directory, ensure target_is_directory==1 */
-    target_is_directory |= _check_dirW(src->wide, dst->wide);
-    result = Py_CreateSymbolicLinkW(dst->wide, src->wide,
-                                    target_is_directory);
+    /* if src is a directory, ensure flags==1 (target_is_directory bit) */
+    if (target_is_directory || _check_dirW(src->wide, dst->wide)) {
+        flags |= SYMBOLIC_LINK_FLAG_DIRECTORY;
+    }
+
+    result = CreateSymbolicLinkW(dst->wide, src->wide, flags);
     _Py_END_SUPPRESS_IPH
     Py_END_ALLOW_THREADS
 
+    if (windows_has_symlink_unprivileged_flag && !result &&
+        ERROR_INVALID_PARAMETER == GetLastError()) {
+
+        Py_BEGIN_ALLOW_THREADS
+        _Py_BEGIN_SUPPRESS_IPH
+        /* This error might be caused by
+        SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE not being supported.
+        Try again, and update windows_has_symlink_unprivileged_flag if we
+        are successful this time.
+
+        NOTE: There is a risk of a race condition here if there are other
+        conditions than the flag causing ERROR_INVALID_PARAMETER, and
+        another process (or thread) changes that condition in between our
+        calls to CreateSymbolicLink.
+        */
+        flags &= ~(SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE);
+        result = CreateSymbolicLinkW(dst->wide, src->wide, flags);
+        _Py_END_SUPPRESS_IPH
+        Py_END_ALLOW_THREADS
+
+        if (result || ERROR_INVALID_PARAMETER != GetLastError()) {
+            windows_has_symlink_unprivileged_flag = FALSE;
+        }
+    }
+
     if (!result)
         return path_error2(src, dst);
 
@@ -13469,35 +13470,6 @@ static PyMethodDef posix_methods[] = {
     {NULL,              NULL}            /* Sentinel */
 };
 
-
-#if defined(HAVE_SYMLINK) && defined(MS_WINDOWS)
-static int
-enable_symlink()
-{
-    HANDLE tok;
-    TOKEN_PRIVILEGES tok_priv;
-    LUID luid;
-
-    if (!OpenProcessToken(GetCurrentProcess(), TOKEN_ALL_ACCESS, &tok))
-        return 0;
-
-    if (!LookupPrivilegeValue(NULL, SE_CREATE_SYMBOLIC_LINK_NAME, &luid))
-        return 0;
-
-    tok_priv.PrivilegeCount = 1;
-    tok_priv.Privileges[0].Luid = luid;
-    tok_priv.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;
-
-    if (!AdjustTokenPrivileges(tok, FALSE, &tok_priv,
-                               sizeof(TOKEN_PRIVILEGES),
-                               (PTOKEN_PRIVILEGES) NULL, (PDWORD) NULL))
-        return 0;
-
-    /* ERROR_NOT_ALL_ASSIGNED returned when the privilege can't be assigned. */
-    return GetLastError() == ERROR_NOT_ALL_ASSIGNED ? 0 : 1;
-}
-#endif /* defined(HAVE_SYMLINK) && defined(MS_WINDOWS) */
-
 static int
 all_ins(PyObject *m)
 {
@@ -14105,10 +14077,6 @@ INITFUNC(void)
     PyObject *list;
     const char * const *trace;
 
-#if defined(HAVE_SYMLINK) && defined(MS_WINDOWS)
-    win32_can_symlink = enable_symlink();
-#endif
-
     m = PyModule_Create(&posixmodule);
     if (m == NULL)
         return NULL;

Modules/winreparse.h

@@ -45,6 +45,11 @@ typedef struct {
     FIELD_OFFSET(_Py_REPARSE_DATA_BUFFER, GenericReparseBuffer)
 #define _Py_MAXIMUM_REPARSE_DATA_BUFFER_SIZE  ( 16 * 1024 )
 
+// Defined in WinBase.h in 'recent' versions of Windows 10 SDK
+#ifndef SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
+#define SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE 0x2
+#endif
+
 #ifdef __cplusplus
 }
 #endif