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Subject: Re: [PATCH] ttyprintk: Add TTY hangup callback.
To:     =?UTF-8?Q?Samo_Poga=c4=8dnik?= <samo_pogacnik@t-2.net>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        John Ogness <john.ogness@linutronix.de>,
        linux-kernel@vger.kernel.org,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
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From:   Jiri Slaby <jirislaby@kernel.org>
Message-ID: <699d0312-ee68-8f05-db2d-07511eaad576@kernel.org>
Date:   Fri, 23 Apr 2021 06:22:54 +0200
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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@syzkaller.appspotmail.com>
>> Reported-by: syzbot <syzbot+3ed715090790806d8b18@syzkaller.appspotmail.com>
>> Tested-by: syzbot <syzbot+43e93968b964e369db0b@syzkaller.appspotmail.com>
>> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.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.

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