[PATCH v2 0/8] exec: Use sane stack rlimit under secureexec

[Kees Cook]

As discussed with Linus and Andy, we need to reset the stack rlimit
before we do memory layouts when execing a privilege-gaining (e.g.
setuid) program. This moves security_bprm_secureexec() earlier (with
required changes), and then lowers the stack limit when appropriate.

As a side-effect, dumpability and pdeath_signal clearing is expanded
to cover LSM definitions of secureexec (and Smack can drop its special
handler for pdeath_signal clearing).

I'd appreciate some extra eyes on this to make sure this isn't
broken in some special way. I couldn't find anything that _depended_
on security_bprm_secureexec() being called late.

Thanks!

-Kees

v2:
- fix missed current_security() uses in LSMs.
- research/consolidate dumpability setting logic
- research/consolidate pdeath_signal clearing logic
- split up logical steps a little more for easier review (and bisection)
- fix some old broken comments