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Subject[PATCH] change kernel threads to ignore signals instead of blocking them
On top of Eric's

kthread-dont-depend-on-work-queues-take-2.patch

Currently kernel threads use sigprocmask(SIG_BLOCK) to protect against signals.
This doesn't prevent the signal delivery, this only blocks signal_wake_up().
Every "killall -33 kthreadd" means a "struct siginfo" leak.

Change kthreadd_setup() to set all handlers to SIG_IGN instead of blocking them
(make a new helper ignore_signals() for that). If the kernel thread needs some
signal, it should use allow_signal() anyway, and in that case it should not use
CLONE_SIGHAND.

Note that we can't change daemonize() (should die!) in the same way, because
it can be used along with CLONE_SIGHAND. This means that allow_signal() still
should unblock the signal to work correctly with daemonize()ed threads.

However, disallow_signal() doesn't block the signal any longer but ignores it.

NOTE: with or without this patch the kernel threads are not protected from
handle_stop_signal(), this seems harmless, but not good.

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

include/linux/sched.h | 1 +
kernel/exit.c | 2 +-
kernel/kthread.c | 17 +++--------------
kernel/signal.c | 10 ++++++++++
4 files changed, 15 insertions(+), 15 deletions(-)
--- 2.6.21-rc5/include/linux/sched.h~1_SIGIGN 2007-04-05 12:18:28.000000000 +0400
+++ 2.6.21-rc5/include/linux/sched.h 2007-04-13 00:09:56.000000000 +0400
@@ -1299,6 +1299,7 @@ extern int in_egroup_p(gid_t);

extern void proc_caches_init(void);
extern void flush_signals(struct task_struct *);
+extern void ignore_signals(struct task_struct *);
extern void flush_signal_handlers(struct task_struct *, int force_default);
extern int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info);

--- 2.6.21-rc5/kernel/signal.c~1_SIGIGN 2007-04-05 12:18:28.000000000 +0400
+++ 2.6.21-rc5/kernel/signal.c 2007-04-13 02:14:06.000000000 +0400
@@ -329,6 +329,16 @@ void flush_signals(struct task_struct *t
spin_unlock_irqrestore(&t->sighand->siglock, flags);
}

+void ignore_signals(struct task_struct *t)
+{
+ int i;
+
+ for (i = 0; i < _NSIG; ++i)
+ t->sighand->action[i].sa.sa_handler = SIG_IGN;
+
+ flush_signals(t);
+}
+
/*
* Flush all handlers for a task.
*/
--- 2.6.21-rc5/kernel/kthread.c~1_SIGIGN 2007-04-12 23:18:09.000000000 +0400
+++ 2.6.21-rc5/kernel/kthread.c 2007-04-13 02:27:39.000000000 +0400
@@ -215,24 +215,13 @@ EXPORT_SYMBOL(kthread_stop);
static __init void kthreadd_setup(void)
{
struct task_struct *tsk = current;
- struct k_sigaction sa;
- sigset_t blocked;

set_task_comm(tsk, "kthreadd");

- /* Block and flush all signals */
- sigfillset(&blocked);
- sigprocmask(SIG_BLOCK, &blocked, NULL);
- flush_signals(tsk);
-
- /* SIG_IGN makes children autoreap: see do_notify_parent(). */
- sa.sa.sa_handler = SIG_IGN;
- sa.sa.sa_flags = 0;
- siginitset(&sa.sa.sa_mask, sigmask(SIGCHLD));
- do_sigaction(SIGCHLD, &sa, (struct k_sigaction *)0);
+ ignore_signals(tsk);

- set_user_nice(current, -5);
- set_cpus_allowed(current, CPU_MASK_ALL);
+ set_user_nice(tsk, -5);
+ set_cpus_allowed(tsk, CPU_MASK_ALL);
}

int kthreadd(void *unused)
--- 2.6.21-rc5/kernel/exit.c~1_SIGIGN 2007-04-12 23:23:50.000000000 +0400
+++ 2.6.21-rc5/kernel/exit.c 2007-04-13 10:17:06.000000000 +0400
@@ -348,7 +348,7 @@ int disallow_signal(int sig)
return -EINVAL;

spin_lock_irq(&current->sighand->siglock);
- sigaddset(&current->blocked, sig);
+ current->sighand->action[(sig)-1].sa.sa_handler = SIG_IGN;
recalc_sigpending();
spin_unlock_irq(&current->sighand->siglock);
return 0;
-
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