Fix #9333. Expose os.symlink on Windows only when usable.

In order to create symlinks on Windows, SeCreateSymbolicLinkPrivilege
is an account privilege that is required to be held by the user. Not only
must the privilege be enabled for the account, the activated privileges for
the currently running application must be adjusted to enable the requested
privilege.

Rather than exposing an additional function to be called prior to the user's
first os.symlink call, we handle the AdjustTokenPrivileges Windows API call
internally and only expose os.symlink when the privilege escalation was
successful.

Due to the change of only exposing os.symlink when it's available, we can
go back to the original test skipping methods of checking via `hasattr`.

1 files changed, 55 insertions, 8 deletions

diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 7267eca..7419862 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -277,6 +277,9 @@ extern int lstat(const char *, struct stat *);
 #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
+#endif
 #endif /* _MSC_VER */
 
 #if defined(PYCC_VACPP) && defined(PYOS_OS2)
@@ -5091,7 +5094,7 @@ posix_readlink(PyObject *self, PyObject *args)
 #endif /* HAVE_READLINK */
 
 
-#ifdef HAVE_SYMLINK
+#if defined(HAVE_SYMLINK) && !defined(MS_WINDOWS)
 PyDoc_STRVAR(posix_symlink__doc__,
 "symlink(src, dst)\n\n\
 Create a symbolic link pointing to src named dst.");
@@ -5179,7 +5182,7 @@ win_readlink(PyObject *self, PyObject *args)
 
 #endif /* !defined(HAVE_READLINK) && defined(MS_WINDOWS) */
 
-#if !defined(HAVE_SYMLINK) && defined(MS_WINDOWS)
+#if defined(HAVE_SYMLINK) && defined(MS_WINDOWS)
 
 /* Grab CreateSymbolicLinkW dynamically from kernel32 */
 static int has_CreateSymbolicLinkW = 0;
@@ -5257,7 +5260,7 @@ win_symlink(PyObject *self, PyObject *args, PyObject *kwargs)
     Py_INCREF(Py_None);
     return Py_None;
 }
-#endif /* !defined(HAVE_SYMLINK) && defined(MS_WINDOWS) */
+#endif /* defined(HAVE_SYMLINK) && defined(MS_WINDOWS) */
 
 #ifdef HAVE_TIMES
 #if defined(PYCC_VACPP) && defined(PYOS_OS2)
@@ -7779,13 +7782,13 @@ static PyMethodDef posix_methods[] = {
     {"rmdir",           posix_rmdir, METH_VARARGS, posix_rmdir__doc__},
     {"stat",            posix_stat, METH_VARARGS, posix_stat__doc__},
     {"stat_float_times", stat_float_times, METH_VARARGS, stat_float_times__doc__},
-#ifdef HAVE_SYMLINK
+#if defined(HAVE_SYMLINK) && !defined(MS_WINDOWS)
     {"symlink",         posix_symlink, METH_VARARGS, posix_symlink__doc__},
 #endif /* HAVE_SYMLINK */
-#if !defined(HAVE_SYMLINK) && defined(MS_WINDOWS)
-    {"symlink", (PyCFunction)win_symlink, METH_VARARGS | METH_KEYWORDS,
-                win_symlink__doc__},
-#endif /* !defined(HAVE_SYMLINK) && defined(MS_WINDOWS) */
+#if defined(HAVE_SYMLINK) && defined(MS_WINDOWS)
+    {"_symlink", (PyCFunction)win_symlink, METH_VARARGS | METH_KEYWORDS,
+                 win_symlink__doc__},
+#endif /* defined(HAVE_SYMLINK) && defined(MS_WINDOWS) */
 #ifdef HAVE_SYSTEM
     {"system",          posix_system, METH_VARARGS, posix_system__doc__},
 #endif
@@ -8099,6 +8102,46 @@ static int insertvalues(PyObject *module)
 }
 #endif
 
+#if defined(HAVE_SYMLINK) && defined(MS_WINDOWS)
+void
+enable_symlink()
+{
+    HANDLE tok;
+    TOKEN_PRIVILEGES tok_priv;
+    LUID luid;
+    int meth_idx = 0;
+
+    if (!OpenProcessToken(GetCurrentProcess(), TOKEN_ALL_ACCESS, &tok))
+        return;
+
+    if (!LookupPrivilegeValue(NULL, SE_CREATE_SYMBOLIC_LINK_NAME, &luid))
+        return;
+
+    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;
+
+    if(GetLastError() == ERROR_NOT_ALL_ASSIGNED) {
+        /* We couldn't acquire the necessary privilege, so leave the
+           method hidden for this user. */
+        return;
+    } else {
+        /* We've successfully acquired the symlink privilege so rename
+           the method to it's proper "os.symlink" name. */
+        while(posix_methods[meth_idx].ml_meth != (PyCFunction)win_symlink)
+            meth_idx++;
+        posix_methods[meth_idx].ml_name = "symlink";
+
+        return;
+    }
+}
+#endif /* defined(HAVE_SYMLINK) && defined(MS_WINDOWS) */
+
 static int
 all_ins(PyObject *d)
 {
@@ -8360,6 +8403,10 @@ INITFUNC(void)
 {
     PyObject *m, *v;
 
+#if defined(HAVE_SYMLINK) && defined(MS_WINDOWS)
+    enable_symlink();
+#endif
+
     m = PyModule_Create(&posixmodule);
     if (m == NULL)
         return NULL;