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    Subject[PATCH 2/3] make kernel threads invisible to /sbin/init
    1. rename reparent_to_init() to reparent_kthread() and export it

    2. use init_pid_ns.child_reaper instead of child_reaper(current)

    3. set ->exit_signal = -1, so init can't see us and we don't use
    it to reap the task.

    4. add reparent_kthread() to kthread() and stopmachine()

    See also

    http://marc.info/?t=117580282200003&r=1
    http://marc.info/?t=95299284800003&r=1

    Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

    include/linux/sched.h | 1 +
    kernel/exit.c | 16 ++++++++--------
    kernel/kthread.c | 1 +
    kernel/stop_machine.c | 1 +
    4 files changed, 11 insertions(+), 8 deletions(-)

    --- 2.6.21-rc5/include/linux/sched.h~2_DETACH 2007-04-05 12:18:28.000000000 +0400
    +++ 2.6.21-rc5/include/linux/sched.h 2007-04-10 21:52:27.000000000 +0400
    @@ -1401,6 +1401,7 @@ extern void exit_itimers(struct signal_s

    extern NORET_TYPE void do_group_exit(int);

    +extern void reparent_kthread(void);
    extern void daemonize(const char *, ...);
    extern int allow_signal(int);
    extern int disallow_signal(int);
    --- 2.6.21-rc5/kernel/exit.c~2_DETACH 2007-04-10 21:32:44.000000000 +0400
    +++ 2.6.21-rc5/kernel/exit.c 2007-04-10 21:59:41.000000000 +0400
    @@ -255,7 +255,7 @@ static int has_stopped_jobs(struct pid *
    }

    /**
    - * reparent_to_init - Reparent the calling kernel thread to the init task of the pid space that the thread belongs to.
    + * reparent_kthread - Reparent the calling kernel thread to the init task of the pid space that the thread belongs to.
    *
    * If a kernel thread is launched as a result of a system call, or if
    * it ever exits, it should generally reparent itself to init so that
    @@ -264,20 +264,20 @@ static int has_stopped_jobs(struct pid *
    * The various task state such as scheduling policy and priority may have
    * been inherited from a user process, so we reset them to sane values here.
    *
    - * NOTE that reparent_to_init() gives the caller full capabilities.
    + * NOTE that reparent_kthread() gives the caller full capabilities.
    */
    -static void reparent_to_init(void)
    +void reparent_kthread(void)
    {
    write_lock_irq(&tasklist_lock);

    ptrace_unlink(current);
    remove_parent(current);
    - current->parent = child_reaper(current);
    - current->real_parent = child_reaper(current);
    + current->parent = init_pid_ns.child_reaper;
    + current->real_parent = current->parent;
    add_parent(current);

    - /* Set the exit signal to SIGCHLD so we signal init on exit */
    - current->exit_signal = SIGCHLD;
    + /* make the task auto-reap */
    + current->exit_signal = -1;

    security_task_reparent_to_init(current);
    write_unlock_irq(&tasklist_lock);
    @@ -391,7 +391,7 @@ void daemonize(const char *name, ...)
    current->files = init_task.files;
    atomic_inc(&current->files->count);

    - reparent_to_init();
    + reparent_kthread();

    if (!has_rt_policy(current) && (task_nice(current) < 0))
    set_user_nice(current, 0);
    --- 2.6.21-rc5/kernel/kthread.c~2_DETACH 2007-04-05 12:18:28.000000000 +0400
    +++ 2.6.21-rc5/kernel/kthread.c 2007-04-10 22:02:31.000000000 +0400
    @@ -82,6 +82,7 @@ static int kthread(void *_create)
    sigset_t blocked;
    int ret = -EINTR;

    + reparent_kthread();
    kthread_exit_files();

    /* Copy data: it's on keventd's stack */
    --- 2.6.21-rc5/kernel/stop_machine.c~2_DETACH 2006-10-22 18:24:03.000000000 +0400
    +++ 2.6.21-rc5/kernel/stop_machine.c 2007-04-10 22:04:23.000000000 +0400
    @@ -33,6 +33,7 @@ static int stopmachine(void *cpu)
    int irqs_disabled = 0;
    int prepared = 0;

    + reparent_kthread();
    set_cpus_allowed(current, cpumask_of_cpu((int)(long)cpu));

    /* Ack: we are alive */
    -
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