4.5 do_allocate functional

kernel/vm.c

@@ -377,6 +377,14 @@ int load_from_file(char* file,
   }
 
 int do_allocate(pagetable_t pagetable, struct proc* p, uint64 addr){
+  
+  pte_t* pte = walk(pagetable, addr, 0);
+  if(pte == 0 || !(*pte & PTE_V)){
+    return ENOMEM;
+  }
+  if(!(*pte & PTE_U)){
+    return EBADPERM;
+  }
   return 0;
 }