[RFC] tty: n_tty -- Add new TIOCPEEKRAW ioctl to peek unread data

[Cyrill Gorcunov <gorcunov@gmail.com>]

Currently when we checkpoint PTY connections we ignore data queued inside
read buffer of ldisk, such data us barely lost after the restore. So we
would like to improve the situation and being able to dump and restore
it as well.

Here is a new ioctl code which simply copies data from read buffer
into the userspace without any additional processing (just like
terminal is sitting in a raw mode).

The idea behind is when we checkpointing PTY pair we peek
this unread data into images on disk and on restore phase
we find the linked peer and write it back thus reading
peer will obtain it then.

I tried several approaches (including simply reuse of
canon_copy_from_read_buf and copy_from_read_buf) but
it not always do the trick: for example if data queued
doesn't have \n symbol the input_available_p helper
returns false and I shouldn't call for the helpers
above.

Another proposal was to try to implement something
like sys_tee but for tty layer (proposed by xemul@)
but I drop this idea because it will require a lot
of rework as far as I can say (including moving
read buffer outside of the n_tty_data structure
and such).

Still this ioctl looks somewhat ugly to me so
if someone has better idea how to fetch queued
data from ldisk this would be awesome, please
share. _Any_ comments are highly appreciated.

Signed-off-by: Cyrill Gorcunov <gorcunov@virtuozzo.com>
CC: Jiri Slaby <jslaby@suse.com>
CC: Peter Hurley <peter@hurleysoftware.com>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CC: Andrey Vagin <avagin@virtuozzo.com>
CC: Pavel Emelianov <xemul@virtuozzo.com>
CC: Vladimir Davydov <vdavydov@virtuozzo.com>
CC: Konstantin Khorenko <khorenko@virtuozzo.com>
---
 drivers/tty/n_tty.c               |   22 ++++++++++++++++++++++
 include/uapi/asm-generic/ioctls.h |    1 +
 2 files changed, 23 insertions(+)

Index: linux-ml.git/drivers/tty/n_tty.c
===================================================================
--- linux-ml.git.orig/drivers/tty/n_tty.c
+++ linux-ml.git/drivers/tty/n_tty.c
@@ -2485,6 +2485,26 @@ static unsigned long inq_canon(struct n_
 	return nr;
 }
 
+static ssize_t n_tty_peek_raw(struct tty_struct *tty, unsigned char __user *buf)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	ssize_t retval;
+
+	if (!mutex_trylock(&ldata->atomic_read_lock))
+		return -EAGAIN;
+
+	down_read(&tty->termios_rwsem);
+	retval = read_cnt(ldata);
+	if (retval) {
+		const unsigned char *from = read_buf_addr(ldata, ldata->read_tail);
+		if (copy_to_user(buf, from, retval))
+			retval = -EFAULT;
+	}
+	up_read(&tty->termios_rwsem);
+	mutex_unlock(&ldata->atomic_read_lock);
+	return retval;
+}
+
 static int n_tty_ioctl(struct tty_struct *tty, struct file *file,
 		       unsigned int cmd, unsigned long arg)
 {
@@ -2502,6 +2522,8 @@ static int n_tty_ioctl(struct tty_struct
 			retval = read_cnt(ldata);
 		up_write(&tty->termios_rwsem);
 		return put_user(retval, (unsigned int __user *) arg);
+	case TIOCPEEKRAW:
+		return n_tty_peek_raw(tty, (unsigned char __user *) arg);
 	default:
 		return n_tty_ioctl_helper(tty, file, cmd, arg);
 	}
Index: linux-ml.git/include/uapi/asm-generic/ioctls.h
===================================================================
--- linux-ml.git.orig/include/uapi/asm-generic/ioctls.h
+++ linux-ml.git/include/uapi/asm-generic/ioctls.h
@@ -77,6 +77,7 @@
 #define TIOCGPKT	_IOR('T', 0x38, int) /* Get packet mode state */
 #define TIOCGPTLCK	_IOR('T', 0x39, int) /* Get Pty lock state */
 #define TIOCGEXCL	_IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCPEEKRAW	_IOR('T', 0x41, void *)
 
 #define FIONCLEX	0x5450
 #define FIOCLEX		0x5451