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    Subject[PATCH 5/6][v5] Protect cinit from blocked fatal signals

    From: Sukadev Bhattiprolu <>
    Date: Wed, 24 Dec 2008 14:04:24 -0800
    Subject: [PATCH 5/6][v5] Protect cinit from blocked fatal signals

    Normally SIG_DFL signals to global and container-init are dropped early.
    But if a signal is blocked when it is posted, we cannot drop the signal
    since the receiver may install a handler before unblocking the signal.
    Once this signal is queued however, the receiver container-init has
    no way of knowing if the signal was sent from an ancestor or descendant
    namespace. This patch ensures that contianer-init drops all SIG_DFL
    signals in get_signal_to_deliver() except SIGKILL/SIGSTOP.

    If SIGSTOP/SIGKILL originate from a descendant of container-init they
    are never queued (i.e dropped in sig_ignored() in an earler patch).

    If SIGSTOP/SIGKILL originate from parent namespace, the signal is queued
    and container-init processes the signal.

    And now that we have all pieces in place, set SIGNAL_UNKILLABLE for

    - (Oleg Nesterov) Drop signal_unkillable(), simplify check in
    get_signal_to_deliver() and drop check for signal_group_exit()
    since it is covered by sig_kernel_only().

    - Rename sig_unkillable() to unkillable_by_sig()
    - Remove SIGNAL_UNKILLABLE_FROM_NS flag and simplify (Oleg Nesterov)
    - Set SIGNAL_UNKILLABLE for container-init in this patch.

    Signed-off-by: Sukadev Bhattiprolu <>
    kernel/fork.c | 2 ++
    kernel/signal.c | 6 ++++--
    2 files changed, 6 insertions(+), 2 deletions(-)

    diff --git a/kernel/fork.c b/kernel/fork.c
    index dba2d3f..d3e93ef 100644
    --- a/kernel/fork.c
    +++ b/kernel/fork.c
    @@ -812,6 +812,8 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
    atomic_set(&sig->live, 1);
    sig->flags = 0;
    + if (clone_flags & CLONE_NEWPID)
    + sig->flags |= SIGNAL_UNKILLABLE;
    sig->group_exit_code = 0;
    sig->group_exit_task = NULL;
    sig->group_stop_count = 0;
    diff --git a/kernel/signal.c b/kernel/signal.c
    index b51781e..d90b033 100644
    --- a/kernel/signal.c
    +++ b/kernel/signal.c
    @@ -1914,9 +1914,11 @@ relock:

    * Global init gets no signals it doesn't want.
    + * Container-init gets no signals it doesn't want from same
    + * container.
    - if (unlikely(signal->flags & SIGNAL_UNKILLABLE) &&
    - !signal_group_exit(signal))
    + if ((signal->flags & SIGNAL_UNKILLABLE) &&
    + !sig_kernel_only(signr))

    if (sig_kernel_stop(signr)) {

     \ /
      Last update: 2008-12-27 21:59    [W:0.023 / U:2.256 seconds]
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