Feat: pointer validation checks string across multiple pages and handle kernel page faults

src/userprog/exception.c

@@ -146,19 +146,19 @@ page_fault (struct intr_frame *f)
   write = (f->error_code & PF_W) != 0;
   user = (f->error_code & PF_U) != 0;
 
-  /* Kernel page fault is further handled by the kernel itself. */
-  if (kernel)
+  if (user && not_present)
     {
-      f->eip = (void *)f->eax;
+  	  if (try_alloc_new_page (fault_addr, f->esp))
+        return;
+    }
+  else
+    {
+      if (try_alloc_new_page (fault_addr, thread_current ()->curr_esp))
+        return;
+	  f->eip = (void *)f->eax;
       f->eax = 0xffffffff;
       return;
-    }
-
-   if (user)
-     {
-       if (try_alloc_new_page (fault_addr, f->esp))
-        return;
-     }
+	}
 
   /* To implement virtual memory, delete the rest of the function
      body, and replace it with code that brings in the page to