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

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Date:	Wed, 8 Sep 2010 14:44:06 +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:
>
> On Tue, 7 Sep 2010, Oleg Nesterov wrote:
>
> > 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 ?

Yes, I still think ->audit_tty doesn't need the locking.

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

OOPS, I misread the code. ->tty_audit_buf is per-process (of course!).
Well, unless I missed something again, tty_audit_push() and
tty_audit_tiocsti() can access ->tty_audit_buf lockless.

> > 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. :)

Yes, yes, sure.

Oleg.

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