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

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Date:	Tue, 7 Sep 2010 22:25:32 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Oleg Nesterov <oleg@...hat.com>
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 Tue, 7 Sep 2010, Oleg Nesterov wrote:

> 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.

Yeah, thought about that, but that's not in the scope of what I was
working on. I leave that to the audit folks. :)
 
Thanks,

	tglx
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