答复: [PATCH][v4] tty: fix race between flush_to_ldisc and tty_open

["Li,Rongqing" <lirongqing@baidu.com>]

> -----邮件原件-----
> 发件人: linux-kernel-owner@vger.kernel.org
> [mailto:linux-kernel-owner@vger.kernel.org] 代表 Greg KH
> 发送时间: 2019年1月30日 18:19
> 收件人: Li,Rongqing <lirongqing@baidu.com>
> 抄送: jslaby@suse.com; linux-kernel@vger.kernel.org; gkohli@codeaurora.org
> 主题: Re: [PATCH][v4] tty: fix race between flush_to_ldisc and tty_open
> 
> On Fri, Jan 18, 2019 at 05:27:17PM +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 n_tty_open and
> > tty->driver_data may be NULL.
> >
> > CPU0                                    CPU1
> > ----                                    ----
> >                                  n_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 put 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
> >
> > Signed-off-by: Wang Li <wangli39@baidu.com>
> > Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
> > Signed-off-by: Li RongQing <lirongqing@baidu.com>
> > ---
> > 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/tty_port.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c index
> > 044c3cbdcfa4..86d0bec38322 100644
> > --- a/drivers/tty/tty_port.c
> > +++ b/drivers/tty/tty_port.c
> > @@ -31,6 +31,9 @@ static int tty_port_default_receive_buf(struct tty_port
> *port,
> >  	if (!tty)
> >  		return 0;
> >
> > +	if (!tty->driver_data)
> > +		return 0;
> > +
> 
> How is this working?  What is setting driver_data to NULL to "stop" this race?
> 


if tty->driver_data is NULL and return,  tty_port_default_receive_buf will not step to
uart_start which access tty->driver_data and trigger panic before tty_open, so it can
fix the system panic

> There's no requirement that a tty driver set this field to NULL when it is "done"
> with the tty device, so I think you are just getting lucky in that your specific
> driver happens to be doing this.
> 

when tty_open is running, tty is allocated by kzalloc in tty_init_dev which called
by tty_open_by_driver, tty is inited to 0

> What driver are you testing this against?
> 

8250

Thanks

-RongQing

> thanks,
> 
> greg k-h