linux-kernel - Re: [PATCHv3 4/6] tty/lockdep: Add ldisc

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Message-ID: <1536670424.2710.22.camel@arista.com>
Date:   Tue, 11 Sep 2018 13:53:44 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Daniel Axtens <dja@...ens.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Neuling <mikey@...ling.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Nathan March <nathan@...net>,
        Pasi Kärkkäinen <pasik@....fi>,
        Peter Hurley <peter@...leysoftware.com>,
        "Rong, Chen" <rong.a.chen@...el.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>
Subject: Re: [PATCHv3 4/6] tty/lockdep: Add ldisc_sem asserts

On Tue, 2018-09-11 at 14:01 +0200, Peter Zijlstra wrote:
> On Tue, Sep 11, 2018 at 02:48:19AM +0100, Dmitry Safonov wrote:
> > Make sure under CONFIG_LOCKDEP that each change to line discipline
> > is done with held write semaphor.
> 
> But you don't do that. You just assert it is held, not for writing.
> 
> > Otherwise potential reader will have a good time dereferencing
> > incomplete/uninitialized ldisc.
> > 
> > Exception here is tty_ldisc_open(), as it's called without
> > ldisc_sem
> > locked by tty_init_dev() for the tty->link.
> 
> You fail to explain how that is not broken...

I'll add the explanation to the next version.

> 
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: Jiri Slaby <jslaby@...e.com>
> > Signed-off-by: Dmitry Safonov <dima@...sta.com>
> > ---
> >  drivers/tty/tty_ldisc.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
> > index fc4c97cae01e..202cb645582f 100644
> > --- a/drivers/tty/tty_ldisc.c
> > +++ b/drivers/tty/tty_ldisc.c
> > @@ -471,6 +471,7 @@ static int tty_ldisc_open(struct tty_struct
> > *tty, struct tty_ldisc *ld)
> >  
> >  static void tty_ldisc_close(struct tty_struct *tty, struct
> > tty_ldisc *ld)
> >  {
> > +	lockdep_assert_held(&tty->ldisc_sem);
> 
> Did you want:
> 
> 	lockdep_assert_held_exclusive(&tty->ldisc_sem);
> 
> ?

Oh, yes, thanks.

> 
> >  	WARN_ON(!test_bit(TTY_LDISC_OPEN, &tty->flags));
> >  	clear_bit(TTY_LDISC_OPEN, &tty->flags);
> >  	if (ld->ops->close)

-- 
Thanks,
             Dmitry