linux-kernel - Re: [patch 3/3] audit: Use rcu for task lookup protection

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Date:	Tue, 7 Sep 2010 20:25:36 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Paris <eparis@...hat.com>
Subject: Re: [patch 3/3] audit: Use rcu for task lookup protection

On 09/07, Thomas Gleixner wrote:
>
> Protect the task lookups in audit_receive_msg() with rcu_read_lock()
> instead of tasklist_lock and use lock/unlock_sighand to protect
> against the exit race.

I do not understand audit, but I belive both 1/3 and 3/3 patches are
fine (I didn't get 2/3).



But, sorry, can't resists ;) off-topic nit.

> @@ -873,17 +873,16 @@ static int audit_receive_msg(struct sk_b
>  	case AUDIT_TTY_GET: {
>  		struct audit_tty_status s;
>  		struct task_struct *tsk;
> +		unsigned long flags;
>
> -		read_lock(&tasklist_lock);
> +		rcu_read_lock();
>  		tsk = find_task_by_vpid(pid);
> -		if (!tsk)
> -			err = -ESRCH;
> -		else {
> -			spin_lock_irq(&tsk->sighand->siglock);
> +		if (tsk && lock_task_sighand(tsk, &flags)) {
>  			s.enabled = tsk->signal->audit_tty != 0;

Yes, this is what original code does, it takes ->siglock every time
around read/write of ->audit_tty. And this looks absolutely bogus.
Say, tty_audit_fork(). Why does it take ->siglock ?

As for ->tty_audit_buf, I am not sure ->siglock is the best choice,
perhaps task_lock() would be better.


Once again, I think the patch is fine. Just it seems to me this code
needs more cleanups.

Oleg.

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