[Bug fortran/67588] New: module.c heap use after free

["zeccav at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67588

            Bug ID: 67588
           Summary: module.c heap use after free
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zeccav at gmail dot com
  Target Milestone: ---

Let us look at module.c:800 and next:

  use_list = module_list;
  for (; module_list->next; use_list = use_list->next)
    {
      module_list = use_list->next;
      free (use_list);
    }

The Asan sanitizer detects that after the first iteration use_list is freed,
you can see that by inspection. But in the for statement it is dereferenced.

So this loop is wrong.

Maybe it should be

for (; module_list->next; use_list = module_list)
    {
      module_list = use_list->next;
      free (use_list);
    }