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SubjectRe: [RFC] tty: Add get- ioctls to fetch tty status

I don't know where this conversation comes from but putting ptys in
sysfs in combination with the newinstance mount option is completely
broken unless the device name and device number duplication is handled,
which I don't see here.

Cyrill Gorcunov <gorcunov@openvz.org> writes:
>
> Guys, you mean something like below? Look, I must admit I'm not really
> sure if I've done all locking right, and there is no need for additional
> kref counting on tty_struct. Could you please check if it looks more-less
> sane (I've tested it but still...)
>
> ---
> From: Cyrill Gorcunov <gorcunov@openvz.org>
> Subject: tty, pty: Add pty_state attribute to fetch tty flags
>
> For checkpoint/restore we need to know if tty has
> exclusive or packet mode set, as well as if pty
> is currently locked (just to be able to restore
> this characteristics).
>
> To serve this the pty_state attribute is introduced
> for pty devices. A typical output looks like
>
> | [root@neptune ~]# cat /sys/devices/virtual/tty/ptmp0/pty_state
> | locked: 0 exclusive: 0 packet: 0
>
> Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
> CC: Alan Cox <alan@lxorguk.ukuu.org.uk>
> CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> CC: "H. Peter Anvin" <hpa@zytor.com>
> CC: Jiri Slaby <jslaby@suse.cz>
> CC: Pavel Emelyanov <xemul@parallels.com>
> ---
> drivers/tty/pty.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 44 insertions(+), 1 deletion(-)
>
> Index: tty.git/drivers/tty/pty.c
> ===================================================================
> --- tty.git.orig/drivers/tty/pty.c
> +++ tty.git/drivers/tty/pty.c
> @@ -283,6 +283,46 @@ done:
> return 0;
> }
>
> +static ssize_t pty_show_state(struct device *dev,
> + struct device_attribute *attr,
> + char *buf)
> +{
> + struct tty_struct *tty = dev_get_drvdata(dev);
> + int locked, exclusive, packet;
> +
> + tty_lock(tty);
> + locked = test_bit(TTY_PTY_LOCK, &tty->flags);
> + exclusive = test_bit(TTY_EXCLUSIVE, &tty->flags);
> + packet = tty->packet;
> + tty_unlock(tty);
> +
> + return snprintf(buf, PAGE_SIZE, "locked: %d exclusive: %d packet: %d\n",
> + locked, exclusive, packet);
> +}
> +
> +static DEVICE_ATTR(pty_state, S_IRUSR | S_IRGRP, pty_show_state, NULL);
> +
> +static struct attribute *pty_state_dev_attrs[] = {
> + &dev_attr_pty_state.attr,
> + NULL,
> +};
> +
> +static const struct attribute_group pty_dev_attr_group = {
> + .attrs = pty_state_dev_attrs,
> +};
> +
> +static const struct attribute_group *pty_dev_attr_groups[] = {
> + &pty_dev_attr_group,
> + NULL,
> +};
> +
> +static int pty_register_attr(struct tty_driver *driver, struct tty_struct *tty)
> +{
> + struct device *dev = tty_register_device_attr(driver, tty->index, tty->dev,
> + (void *)tty, pty_dev_attr_groups);
> + return PTR_RET(dev);
> +}
> +
> /**
> * pty_common_install - set up the pty pair
> * @driver: the pty driver
> @@ -351,7 +391,9 @@ static int pty_common_install(struct tty
>
> tty_driver_kref_get(driver);
> tty->count++;
> - return 0;
> +
> + return pty_register_attr(driver, tty);
> +
> err_free_termios:
> if (legacy)
> tty_free_termios(tty);
> @@ -368,6 +410,7 @@ err:
>
> static void pty_cleanup(struct tty_struct *tty)
> {
> + tty_unregister_device(tty->driver, tty->index);
> kfree(tty->port);
> }
>
> --
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