Created attachment 370763 [details]
simple test case to reproduce the bug

Function g_markup_parse_context_end_parse() dosen't call current_element() function carefully. and can cause null pointer dereference error while parsing an malformed input.

The definition of function current_element() is as below:

static inline const gchar*
current_element (GMarkupParseContext *context)
{
  return context->tag_stack->data; // context->tag_stack may be NULL;
}

This bug resides in gmarkup.c ,near line 1837, in  case "STATE_AFTER_CLOSE_TAG_NAME":

1834    case STATE_AFTER_CLOSE_TAG_NAME:
1835      set_error (context, error, G_MARKUP_ERROR_PARSE,
1836                 _("Document ended unexpectedly inside the close tag for "
1837                   "element '%s'"), current_element (context));
1838      break;

here set_error is called and use the return value of "current_element (context)" as it's parameter, without checking whether "context->tag_stack" is NULL or not. Parsing invalid data may lead to null pointer dereference.

I think this bug may exits in other place where current_element() is not carefully called, a good way to fix it is to add an assertion in current_element().

The attachment is a simple test case to reproduce it.

credit: ADLab of Venustech