inaccessible page under the user stack page, to help exec deal w/ too-large args

vm.c

@@ -363,3 +363,16 @@ copyout(pde_t *pgdir, uint va, void *p, uint len)
   }
   return 0;
 }
+
+// Clear PTE_U on a page. Used to create an inaccessible
+// page beneath the user stack.
+void
+clear_pte_u(pde_t *pgdir, char *uva)
+{
+  pte_t *pte;
+
+  pte = walkpgdir(pgdir, uva, 0);
+  if(pte == 0)
+    panic("clear_pte_u");
+  *pte &= ~PTE_U;
+}