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Commit f63ee72e authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
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[PATCH] introduce lock_task_sighand() helper 



Add lock_task_sighand() helper and converts group_send_sig_info() to use
it.  Hopefully we will have more users soon.

This patch also removes '!sighand->count' and '!p->usage' checks, I think
they both are bogus, racy and unneeded (but probably it makes sense to
restore them as BUG_ON()s).

->sighand is cleared and it's ->count is decremented in release_task() with
sighand->siglock held, so it is a bug to have '!p->usage || !->count' after
we already locked and verified it is the same.  On the other hand, an
already dead task without ->sighand can have a non-zero ->usage due to
ptrace, for example.

If we read the stale value of ->sighand we must see the change after
spin_lock(), because that change was done while holding that same old
->sighand.siglock.

Signed-off-by: default avatarOleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent aa1757f9

include/linux/sched.h

+9 −0
Original line number Diff line number Diff line
@@ -1225,6 +1225,15 @@ static inline void task_unlock(struct task_struct *p) 
	spin_unlock(&p->alloc_lock);

}



extern struct sighand_struct *lock_task_sighand(struct task_struct *tsk,

							unsigned long *flags);



static inline void unlock_task_sighand(struct task_struct *tsk,

						unsigned long *flags)

{

	spin_unlock_irqrestore(&tsk->sighand->siglock, *flags);

}



#ifndef __HAVE_THREAD_FUNCTIONS



#define task_thread_info(task) (task)->thread_info

kernel/signal.c

+24 −14
Original line number Diff line number Diff line
@@ -1120,27 +1120,37 @@ void zap_other_threads(struct task_struct *p) 
/*

 * Must be called under rcu_read_lock() or with tasklist_lock read-held.

 */

struct sighand_struct *lock_task_sighand(struct task_struct *tsk, unsigned long *flags)

{

	struct sighand_struct *sighand;



	for (;;) {

		sighand = rcu_dereference(tsk->sighand);

		if (unlikely(sighand == NULL))

			break;



		spin_lock_irqsave(&sighand->siglock, *flags);

		if (likely(sighand == tsk->sighand))

			break;

		spin_unlock_irqrestore(&sighand->siglock, *flags);

	}



	return sighand;

}



int group_send_sig_info(int sig, struct siginfo *info, struct task_struct *p)

{

	unsigned long flags;

	struct sighand_struct *sp;

	int ret;



retry:

	ret = check_kill_permission(sig, info, p);

	if (!ret && sig && (sp = rcu_dereference(p->sighand))) {

		spin_lock_irqsave(&sp->siglock, flags);

		if (p->sighand != sp) {

			spin_unlock_irqrestore(&sp->siglock, flags);

			goto retry;

		}

		if ((atomic_read(&sp->count) == 0) ||

				(atomic_read(&p->usage) == 0)) {

			spin_unlock_irqrestore(&sp->siglock, flags);

			return -ESRCH;

		}



	if (!ret && sig) {

		ret = -ESRCH;

		if (lock_task_sighand(p, &flags)) {

			ret = __group_send_sig_info(sig, info, p);

		spin_unlock_irqrestore(&sp->siglock, flags);

			unlock_task_sighand(p, &flags);

		}

	}



	return ret;