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Date:   Thu, 31 Jan 2019 19:44:09 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     "Li,Rongqing" <lirongqing@baidu.com>
Cc:     "linux-serial@vger.kernel.org" <linux-serial@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "jslaby@suse.com" <jslaby@suse.com>,
        "gkohli@codeaurora.org" <gkohli@codeaurora.org>
Subject: Re: =?utf-8?B?562U5aSNOiBbUEFUQ0hdW1Y1?= =?utf-8?Q?=5D?= tty: fix
 race between flush_to_ldisc and tty_open
Message-ID: <20190131184409.GA5238@kroah.com>
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 <20190131105527.GB8271@kroah.com>
 <22a5d8639b154119af7d7661e17025f2@baidu.com>
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On Thu, Jan 31, 2019 at 11:15:48AM +0000, Li,Rongqing wrote:
> 
> 
> > -----邮件原件-----
> > 发件人: linux-kernel-owner@vger.kernel.org
> > [mailto:linux-kernel-owner@vger.kernel.org] 代表 Greg Kroah-Hartman
> > 发送时间: 2019年1月31日 18:55
> > 收件人: Li,Rongqing <lirongqing@baidu.com>
> > 抄送: linux-serial@vger.kernel.org; linux-kernel@vger.kernel.org;
> > jslaby@suse.com; gkohli@codeaurora.org
> > 主题: Re: [PATCH][V5] tty: fix race between flush_to_ldisc and tty_open
> > 
> > On Thu, Jan 31, 2019 at 05:43:16PM +0800, Li RongQing wrote:
> > > There still is a race window after the commit b027e2298bd588
> > > ("tty: fix data race between tty_init_dev and flush of buf"), and we
> > > encountered this crash issue if receive_buf call comes before tty
> > > initialization completes in tty_open and
> > > tty->driver_data may be NULL.
> > >
> > > CPU0                                    CPU1
> > > ----                                    ----
> > >                                   tty_open
> > >                                    tty_init_dev
> > >                                      tty_ldisc_unlock
> > >                                        schedule flush_to_ldisc
> > > receive_buf
> > >   tty_port_default_receive_buf
> > >    tty_ldisc_receive_buf
> > >     n_tty_receive_buf_common
> > >       __receive_buf
> > >        uart_flush_chars
> > >         uart_start
> > >         /*tty->driver_data is NULL*/
> > >                                    tty->ops->open
> > >                                    /*init tty->driver_data*/
> > >
> > > it can be fixed by extending ldisc semaphore lock in tty_init_dev to
> > > driver_data initialized completely after tty->ops->open(), but this
> > > will lead to get lock on one function and unlock in some other
> > > function, and hard to maintain, so fix this race only by checking
> > > tty->driver_data when receiving, and return if tty->driver_data
> > > is NULL, and n_tty_receive_buf_common maybe calls uart_unthrottle, so
> > > add the same check
> > >
> > > Signed-off-by: Wang Li <wangli39@baidu.com>
> > > Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
> > > Signed-off-by: Li RongQing <lirongqing@baidu.com>
> > > ---
> > > V5: move check into uart_start from n_tty_receive_buf_common
> > > V4: add version information
> > > V3: not used ldisc semaphore lock, only checking tty->driver_data with
> > > NULL
> > > V2: fix building error by EXPORT_SYMBOL tty_ldisc_unlock
> > > V1: extend ldisc lock to protect that tty->driver_data is inited
> > >
> > >  drivers/tty/serial/serial_core.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > >
> > > diff --git a/drivers/tty/serial/serial_core.c
> > > b/drivers/tty/serial/serial_core.c
> > > index 5c01bb6d1c24..556f50aa1b58 100644
> > > --- a/drivers/tty/serial/serial_core.c
> > > +++ b/drivers/tty/serial/serial_core.c
> > > @@ -130,6 +130,9 @@ static void uart_start(struct tty_struct *tty)
> > >  	struct uart_port *port;
> > >  	unsigned long flags;
> > >
> > > +	if (!state)
> > > +		return;
> > > +
> > >  	port = uart_port_lock(state, flags);
> > >  	__uart_start(tty);
> > >  	uart_port_unlock(port, flags);
> > > @@ -727,6 +730,9 @@ static void uart_unthrottle(struct tty_struct *tty)
> > >  	upstat_t mask = UPSTAT_SYNC_FIFO;
> > >  	struct uart_port *port;
> > >
> > > +	if (!state)
> > > +		return;
> > > +
> > >  	port = uart_port_ref(state);
> > >  	if (!port)
> > >  		return;
> > > --
> > > 2.16.2
> > 
> > 
> > Hm, I wrote this patch, not you, right?  So shouldn't I get the
> > credit/blame for it?  :)
> > 
> 
> Welcome you to add your credit/blame/signature
> and I am not clear the rule, and be afraid to become fake

No problem, I've fixed this up when committing this, and added some
wording change to the changelog text.

Thanks so much for working through all of this, it's a bug that has
always been there for forever it seems, nice catch!

greg k-h