For pthread_attr_setstacksize(), POSIX.1-2001 says:

"The stacksize attribute shall define the minimum stack size (in bytes)
allocated for the created threads stack."

However, under NPTL, the allocated stack (as reported by pthread_getattr_np())
can be up to [STACK_MIN - 1] (15 on x86-32) bytes less than was requested.  

The problem line looks to be the following line in nptl/allocatestak.c:

      size &= ~__static_tls_align_m1;

which should probably be something like:

      size = (size + __static_tls_align_m1 + 1) & ~__static_tls_align_m1;