Notify special task kill using wait* functions

[Andi Kleen]

> I didn't even finish the line that starts "From now on.." before I started
> wondering "How can I abuse this to hang or crash a system?"  And it only took
> me a few seconds to come up with an attack. All you need to do is find a way to
> sigsegv /bin/bash... and that's easy to do by forking, excecve /bin/bash, and
> then use ptrace() to screw the child process's stack and cause a sigsegv.
> 
> Say goodnight Gracie...

Yes there is certainly DoS potential, but that's kind of inevitable
for the proposal. It's a trade between allowing attacks and allowing DoS,
with the idea that a DoS is more benign.

I'm more worried that it doesn't actually prevent the attacks 
unless we make sure systemd and other supervisor daemons understand it,
so that they don't restart.

Any caching of state is inherently insecure because any caches of limited
size can be always thrashed by a purposeful attacker. I suppose the
only thing that would work is to actually write something to the 
executable itself on disk, but of course that doesn't always work either.

-Andi