linux-kernel - Re: [PATCH] n_tty_read: check for hanging tty while waiting for input

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Message-ID: <20150218145805.GD13666@redhat.com>
Date:	Wed, 18 Feb 2015 09:58:06 -0500
From:	Aristeu Rozanski <aris@...hat.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH] n_tty_read: check for hanging tty while waiting for input

Hi Peter,
On Tue, Feb 17, 2015 at 05:35:10PM -0500, Peter Hurley wrote:
> I realize that. But hanging up the tty that is /dev/console only affects
> open descriptors that are not /dev/console.
> 
> So readers using the /dev/ttyS0 file descriptor will see a hungup fops,
> but readers using /dev/console will not, and /dev/ttyS0 will _not_
> be closed or released because of the still-open descriptor on /dev/console.

I see.

> Ok, so the process sleeping on /dev/console read() should have received
> SIGHUP, which would wake the process and cause it to exit the
> n_tty_read() loop, thus dropping the ldisc reference it holds.
> Did it ignore the signal or perhaps the signal is masked?

Not masked on the test case (attached). Sent sighup manually and it did
receive it.

-- 
Aristeu

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <termios.h>
#include <errno.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <sys/ioctl.h>

static char *default_console = "/dev/console";
static char *default_tty = "/dev/ttyS0";
struct data {
	char *console;
	char *tty;
};

static void *reader(void *d)
{
	struct data *data = (struct data *)d;
	struct termios old_termio;
	char buff[512];
	int fd = -1, rc;

	while (1) {
		if (fd == -1) {
			fd = open(data->console, O_RDWR);
			if (fd < 0)
				exit(1);
			if (tcgetattr(fd, &old_termio) == -1)
				exit(1);
			old_termio.c_lflag = ICANON;
			if (tcsetattr(fd, 0, &old_termio) == -1)
				exit(1);
		}
		rc = read(fd, buff, sizeof(buff));
		if (rc < 0 && errno == EAGAIN)
			continue;
		close(fd);
		fd = -1;
	}
}

void launch(void *(*fn)(void *), struct data *data)
{
	if (fork() == 0)
		fn(data);
}

int main(int argc, char *argv[])
{
	struct data data;
	int fd;

	fd = open("/dev/ttyS0", O_RDWR);
	close(0);
	close(1);
	close(2);
	ioctl(fd, TIOCSCTTY, 1);

	data.console = default_console;
	data.tty = default_tty;

	launch(reader, &data);

	waitpid(-1, NULL, 0);

	return 0;
}

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