[PATCH resend v2] 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 is barely lost after the restore. So we
would like to have an ability to dump and restore it.

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 pairs 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 see it in read buffer.

I tried several approaches (including simply reuse of canon_copy_from_read_buf
and copy_from_read_buf) but it makes code more complicated, so I end up
in plain copying of buffer data.

Strictly speaking, we could try handle it inside CRIU itself, as
Peter Hurley proposed (switch terminal to raw mode, read data,
and return original termios back) but in this case we may hit
the scenario when we read data and failed to restore it back
(due to any error) leaving the task we're dumping without
read buffer, thus peeking data looks like be the only
safe way.

v2:
 - Use @char in ioctl definition.

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
@@ -2420,6 +2420,26 @@ 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)
 {
@@ -2437,6 +2457,8 @@ 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, char)
 
 #define FIONCLEX	0x5450
 #define FIOCLEX		0x5451