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    Subject[RFC][PATCH 0/6][v3] 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.

    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

    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 1/6] Remove 'handler' parameter to tracehook functions
    [PATCH 2/6] Protect init from unwanted signals more
    [PATCH 3/6] Define/set SIGNAL_UNKILLABLE_FROM_NS
    [PATCH 4/6] Define siginfo_from_ancestor_ns()
    [PATCH 5/6] Protect cinit from unblocked SIG_DFL signals
    [PATCH 6/6] Protect cinit from blocked fatal signals

    TODO:
    - Use sig_task_unkillable() in fs/proc/array.c:task_sig() to
    correctly report ignored signals for container/global init.
    - Make SI_ASYNCIO a kernel signal ?
    - Compile/touch tested. Need so real testing ;-)

    Limitations/side-effects of current design

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


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