Container-init signal semantics

Container-init must behave like global-init to processes within the
container and hence it must be immune to unhandled fatal signals from
within the container (i.e SIG_DFL signals that terminate the process).

But the same container-init must behave like a normal process to 
processes in ancestor namespaces and so if it receives the same fatal
signal from a process in ancestor namespace, the signal must be
processed.

Implementing these semantics requires that send_signal() determine pid
namespace of the sender but since signals can originate from workqueues/
interrupt-handlers, determining pid namespace of sender may not always
be possible or safe.

This patchset implements the design/simplified semantics suggested by
Oleg Nesterov.  The simplified semantics for container-init are:

	- container-init must never be terminated by a signal from a
	  descendant process.

	- container-init must never be immune to SIGKILL from an ancestor
	  namespace (so a process in parent namespace must always be able
	  to terminate a descendant container).

	- container-init may be immune to unhandled fatal signals (like
	  SIGUSR1) even if they are from ancestor namespace (SIGKILL is
	  the only reliable signal from ancestor namespace).

Patches in this set:

	[PATCH 1/7] Remove 'handler' parameter to tracehook functions
	[PATCH 2/7] Protect init from unwanted signals more
	[PATCH 3/7] Define siginfo_from_ancestor_ns()
	[PATCH 4/7] Protect cinit from unblocked SIG_DFL signals
	[PATCH 5/7] Protect cinit from blocked fatal signals
	[PATCH 6/7] SI_USER: Masquerade si_pid when crossing pid ns boundary
	[PATCH 7/7] SI_TKILL: Masquerade si_pid when crossing pid ns boundary

Changelog[v4]:
	- Remove SIGNAL_UNKILLABLE_FROM_NS flag and simplify logic as
	  suggested by Oleg Nesterov.
	- Check ns == NULL in siginfo_from_ancestor_ns() (Patch 3/7). 
	  Although http://lkml.org/lkml/2008/12/16/502 makes it less likely
	  that ns == NULL, looks like an explicit check won't hurt ?
	- Dropped patch that set SIGNAL_UNKILLABLE_FROM_NS and set
	  SIGNAL_UNKILLABLE in patch 5/7 to be bisect-safe.
	- Add a warning in rt_sigqueueinfo() if SI_ASYNCIO is used
	  (patch 3/7)
	- Added two patches (6/7 and 7/7) to masquerade si_pid for
	  SI_USER and SI_TKILL


Changelog[v3]:
	Changes based on discussions of previous version:
		http://lkml.org/lkml/2008/11/25/458

	Major changes:

	- Define SIGNAL_UNKILLABLE_FROM_NS and use in container-inits to
	  skip fatal signals from same namespace but process SIGKILL/SIGSTOP
	  from ancestor namespace.
	- Use SI_FROMUSER() and si_code != SI_ASYNCIO to determine if
	  it is safe to dereference pid-namespace of caller. Highly
	  experimental :-)
	- Masquerading si_pid when crossing namespace boundary: relevant
	  patches merged in -mm and dropped from this set.

	Minor changes:

	- Remove 'handler' parameter to tracehook functions
	- Update sig_ignored() to drop SIG_DFL signals to global init early
	  (tried to address Roland's  and Oleg's comments)
	- Use 'same_ns' flag to drop SIGKILL/SIGSTOP to cinit from same
	  namespace


TODO:
	- Use sig_task_unkillable() in fs/proc/array.c:task_sig()
	  to correctly report ignored signals for container/global
	  init.


Limitations/side-effects of current design

	- Container-init is immune to suicide - kill(getpid(), SIGKILL) is
	  ignored. Use exit() :-)