Question

I’m inside a shared object (code) loaded with dlopen. I want to know the flags given to the loading call. I do not have access to the loader (code) – e.g. it may be a script interpreter – but I have to create subsequent dlopen calls with the same flags.

How can I do this?

Answer 1

I think this is NOT possible without the aid of debuggers.

From the latest glibc code , here is the source code of dlopen

void *
dlopen (const char *file, int mode)
{
  return __dlopen (file, mode, RETURN_ADDRESS (0));
}

And __dlopen is in turn defined as

void *
__dlopen (const char *file, int mode DL_CALLER_DECL)
{
# ifdef SHARED
  if (__builtin_expect (_dlfcn_hook != NULL, 0))
    return _dlfcn_hook->dlopen (file, mode, DL_CALLER);
# endif

  struct dlopen_args args;
  args.file = file;
  args.mode = mode;
  args.caller = DL_CALLER;

# ifdef SHARED
  return _dlerror_run (dlopen_doit, &args) ? NULL : args.new;
# else
  if (_dlerror_run (dlopen_doit, &args))
    return NULL;

  __libc_register_dl_open_hook ((struct link_map *) args.new);
  __libc_register_dlfcn_hook ((struct link_map *) args.new);

  return args.new;
# endif
}

The flags you are looking for, RTLD_LAZY, RTLD_NOW, RTLD_GLOBAL and RTLD_LOCAL are ORed and stored in mode variable. As you can see there is no route through which it is passed back or anything like that.

EDIT: It seems there indeed is a way of achieving what you want as shown by the other answer. If you can un-accept my answer, I can delete it so as to help future visitors