Re: [PATCH] ttyprintk: Add TTY hangup callback.

[Jiri Slaby]

On 18. 04. 21, 13:16, Samo Pogačnik wrote:
> Dne 15.04.2021 (čet) ob 09:22 +0900 je Tetsuo Handa napisal(a):
> > syzbot is reporting hung task due to flood of
> >
> >    tty_warn(tty, "%s: tty->count = 1 port count = %d\n", __func__,
> >             port->count);
> >
> > message [1], for ioctl(TIOCVHANGUP) prevents tty_port_close() from
> > decrementing port->count due to tty_hung_up_p() == true.
> >
> > ----------
> > #include <sys/types.h>
> > #include <sys/stat.h>
> > #include <fcntl.h>
> > #include <sys/ioctl.h>
> > #include <unistd.h>
> >
> > int main(int argc, char *argv[])
> > {
> > 	int i;
> > 	int fd[10];
> >
> > 	for (i = 0; i < 10; i++)
> > 		fd[i] = open("/dev/ttyprintk", O_WRONLY);
> > 	ioctl(fd[0], TIOCVHANGUP);
> > 	for (i = 0; i < 10; i++)
> > 		close(fd[i]);
> > 	close(open("/dev/ttyprintk", O_WRONLY));
> > 	return 0;
> > }
> > ----------
> >
> > When TTY hangup happens, port->count needs to be reset via
> > "struct tty_operations"->hangup callback.
> >
> > [1]
> > https://syzkaller.appspot.com/bug?id=39ea6caa479af471183997376dc7e90bc7d64a6a
> >
> > Reported-by: syzbot <syzbot+43e93968b964e369db0b@xxxxxxxxxxxxxxxxxxxxxxxxx>
> > Reported-by: syzbot <syzbot+3ed715090790806d8b18@xxxxxxxxxxxxxxxxxxxxxxxxx>
> > Tested-by: syzbot <syzbot+43e93968b964e369db0b@xxxxxxxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> > Fixes: 24b4b67d17c308aa ("add ttyprintk driver")
> > ---
> >   drivers/char/ttyprintk.c | 11 +++++++++++
> >   1 file changed, 11 insertions(+)
> >
> > diff --git a/drivers/char/ttyprintk.c b/drivers/char/ttyprintk.c
> > index 6a0059e508e3..93f5d11c830b 100644
> > --- a/drivers/char/ttyprintk.c
> > +++ b/drivers/char/ttyprintk.c
> > @@ -158,12 +158,23 @@ static int tpk_ioctl(struct tty_struct *tty,
> >   	return 0;
> >   }
> >   
> > +/*
> > + * TTY operations hangup function.
> > + */
> > +static void tpk_hangup(struct tty_struct *tty)
> > +{
> > +	struct ttyprintk_port *tpkp = tty->driver_data;
> > +
> > +	tty_port_hangup(&tpkp->port);
> > +}
> > +
> >   static const struct tty_operations ttyprintk_ops = {
> >   	.open = tpk_open,
> >   	.close = tpk_close,
> >   	.write = tpk_write,
> >   	.write_room = tpk_write_room,
> >   	.ioctl = tpk_ioctl,
> > +	.hangup = tpk_hangup,
> >   };
> >   
> >   static const struct tty_port_operations null_ops = { };
>
> Using the supplied test code, i've tested the patch on my desktop running the
> 5.4 kernel. After applying the patch, the kernel warnings like "ttyprintk:
> tty_port_close_start: tty->count = 1 port count = 11" do not appear any more,
> when the test code is run.
> I think the patch is ok.

I wonder if the buffer shouldn't be flushed in hangup too? Or better, 
the flush moved from tty_ops->close to tty_port->ops->shutdown?

thanks,
--
js
suse labs