linux-kernel - Re: [PATCH] ttyprintk: Add TTY hangup callback.

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Date:   Sun, 18 Apr 2021 13:16:05 +0200
From:   Samo Pogačnik <samo_pogacnik@....net>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        linux-kernel@...r.kernel.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [PATCH] ttyprintk: Add TTY hangup callback.

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@...kaller.appspotmail.com>
> Reported-by: syzbot <syzbot+3ed715090790806d8b18@...kaller.appspotmail.com>
> Tested-by: syzbot <syzbot+43e93968b964e369db0b@...kaller.appspotmail.com>
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> 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.

best regards, Samo