Re: nss_ldap broken

On Thu, Apr 01, 2004 at 04:04:42PM +0200, Oliver Eikemeier wrote:
> Daniel Eischen wrote:
> 
> >On Thu, 1 Apr 2004, Oliver Eikemeier wrote:
> [...]
> >>- it should be documented somewhere (bsd.port.mk gives you only 
> >>PTHREAD_LIBS)
> 
> As far as I understand the problem, every application that doesn't link to
> pthreads, but uses a library that does crashes on -CURRENT. Am I right
> there?

No.  Problems only occur if run-time loading/unloading via
dlopen/dlclose is used, and then only if a lock or other thread
library data structure might be held ``across'' the load or unload.
So I think it is a rare, but troublesome, condition.

When you changed the way OpenLDAP was linked, this condition occurred
within libc due to dynamic loading of nss_ldap.  So, lots of people's
working systems suddenly starting crashing after upgrading.

I was able to work around the problem in libc, because there, we
have the freedom to putz with internals like __isthreaded.  Real
applications don't have that luxury.

[...]
> >I think it is dependent on the library.  If the library truly is
> >creating threads behind the scene (suppose there were a libaio)
> >then it needs the threads library.
> >
> >On the other hand, for applications that want to use libaio, you
> >could force them to link to a threads library instead of having
> >it automatically brought in by libaio.
> 
> I guess the latter approach will be preferrable, especially since the
> former does seem to trigger the problem we have...

So far no library has been found that does ``the former'' (create
threads behind the scene).


It seems to me we need one of a few things to happen to our threads
implementation*s*:

  (a) pthread.h provides all the magic needed to make pthread_*
      symbols weak, i.e. transparently providing the functionality of
      the `libgcc hack' which Dan says would avoid the problem.

  (b) ``somehow'' arrange for the unloading of a thread library to
      fixup the pthread stubs `back to normal'.  er, that sounds like
      a load of work and dangerous to boot.

  (c) teach rtld to treat thread libraries specially: ignore them during
      dlopen even if they are specified in DT_NEEDED.  perhaps we could
      add some info to the ELF headers of our thread libraries that rtld
      could use to implement this.  hacky.

Cheers,
-- 
Jacques Vidrine / nectar_at_celabo.org / jvidrine_at_verio.net / nectar_at_freebsd.org