Re: [PATCH 2/2] tty: add retry to tty_init_dev() to workaround a race condition

Sudip Mukherjee
2019-11-20T15:55:15Z

On Wed, Nov 20, 2019 at 04:44:50PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Nov 20, 2019 at 03:17:09PM +0000, Sudip Mukherjee wrote:
> > There seems to be a race condition in tty drivers and I could see on
> > many boot cycles a NULL pointer dereference as tty_init_dev() tries to
> > do 'tty->port->itty = tty' even though tty->port is NULL.
<snip>
> > +++ b/drivers/tty/pty.c
> > @@ -842,7 +842,7 @@ static int ptmx_open(struct inode *inode, struct file *filp)
> >
> >
> > mutex_lock(&tty_mutex);
> > - tty = tty_init_dev(ptm_driver, index);
> > + tty = tty_init_dev(ptm_driver, index, 0);
>
> Horrible naming scheme for this new "flag".
>
> Look at that call here, can you instantly tell what this call is doing
> with "0"? I sure can not :(

well, I also made the mistake of 1->0 in my initial patch. :(

>
> If you really want to do this, you make a different function,
> tty_init_dev_retry() and then have that pass in a retry flag in the tty
> core, so that any users always know what they are doing here.

will do.

>
> But, this really feels like a race in the code somewhere:
>
> > --- a/drivers/tty/tty_io.c
> > +++ b/drivers/tty/tty_io.c
> > @@ -1295,6 +1295,7 @@ static int tty_reopen(struct tty_struct *tty)
> > * tty_init_dev - initialise a tty device
> > * @driver: tty driver we are opening a device on
> > * @idx: device index
> > + * @retry: retry count if driver has not set tty->port yet
>
> Why would tty->port not be set up already? The caller has control over
> this, what is not happening correctly to cause this?

Will add more debugs to check what is happening now and then send you v2.

--
Regards
Sudip