Fix: Update writability of the last page after loading as a workaround to calculating the offset, w/ G

src/userprog/exception.c

@@ -179,18 +179,23 @@ try_fetch_page (void *upage, bool write)
   if (page == NULL)
     return false;
 
+  /* An attempt to write to a non-writeable should fail. */
   if (write && !page->writable)
-    {
-      pagedir_set_writable(thread_current()->pagedir, upage, write);
-      page->writable = true;
-      return true;
-    }
+    return false;
 
   /* Load the page into memory based on the type of data it is expecting. */
+  bool success = false;
   switch (page->type) {
     case PAGE_EXECUTABLE:
-      return page_load (page, write);
+      success = page_load (page, page->writable);
+      break;
     default:
       return false;
   }
+
+  if (success && page->writable &&
+      !pagedir_is_writable(thread_current()->pagedir, upage))
+    pagedir_set_writable(thread_current()->pagedir, upage, true);
+
+  return success;
 }