linux-kernel - Re: [PATCH 1/4] tty: Drop tty->count on tty

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Date:   Wed, 29 Aug 2018 17:13:27 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     Jiri Slaby <jslaby@...e.cz>, linux-kernel@...r.kernel.org
Cc:     Daniel Axtens <dja@...ens.net>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Peter Hurley <peter@...leysoftware.com>,
        Pasi Kärkkäinen <pasik@....fi>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michael Neuling <mikey@...ling.org>,
        Mikulas Patocka <mpatocka@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH 1/4] tty: Drop tty->count on tty_reopen() failure

On Wed, 2018-08-29 at 16:38 +0200, Jiri Slaby wrote:
> On 08/29/2018, 04:23 AM, Dmitry Safonov wrote:
> > In case of tty_ldisc_reinit() failure, tty->count should be
> > decremented
> > back, otherwise we will never release_tty().
> > Never seen it in the real life, but it seems not really hard to
> > hit.
> 
> I did see it. And this fixes it.

Thanks, I'll add your tested-by, if I'll have to resend.

> 
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: Jiri Slaby <jslaby@...e.com>
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Dmitry Safonov <dima@...sta.com>
> > ---
> >  drivers/tty/tty_io.c | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> > index 32bc3e3fe4d3..5e5da9acaf0a 100644
> > --- a/drivers/tty/tty_io.c
> > +++ b/drivers/tty/tty_io.c
> > @@ -1255,6 +1255,7 @@ static void tty_driver_remove_tty(struct
> > tty_driver *driver, struct tty_struct *
> >  static int tty_reopen(struct tty_struct *tty)
> >  {
> >  	struct tty_driver *driver = tty->driver;
> > +	int retval;
> >  
> >  	if (driver->type == TTY_DRIVER_TYPE_PTY &&
> >  	    driver->subtype == PTY_TYPE_MASTER)
> > @@ -1268,10 +1269,14 @@ static int tty_reopen(struct tty_struct
> > *tty)
> >  
> >  	tty->count++;
> >  
> > -	if (!tty->ldisc)
> > -		return tty_ldisc_reinit(tty, tty->termios.c_line);
> > +	if (tty->ldisc)
> > +		return 0;
> >  
> > -	return 0;
> > +	retval = tty_ldisc_reinit(tty, tty->termios.c_line);
> > +	if (retval)
> > +		tty->count--;
> 
> I would just do:
>   if (!retval)
>     tty->count++;
> here. Nobody from ldiscs should rely on tty->count.

I thought about that and probably should have described in commit
message why I haven't done that: I prefer to keep it as was as I did Cc
stable tree - to keep the chance of regression to minimum.

I agree that your way is cleaner, but probably it may be done as
cleanup on top for linux-next..

-- 
Thanks,
             Dmitry