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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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