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

include/uapi/asm-generic/ioctls.h line 80

Quoted 7 lines · Cyrill Gorcunov

include/uapi/asm-generic/ioctls.h +1 -0

@@ -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 *)

Hi, this is not 32/64bit compatible and would need a compat ioctl number and a wrapper. Since it is char *, could you make it just 'char'?

Quoted 11 lines · Jiri Slaby

include/uapi/asm-generic/ioctls.h line 80

Quoted 7 lines · Cyrill Gorcunov

include/uapi/asm-generic/ioctls.h +1 -0

@@ -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 *)

Hi, this is not 32/64bit compatible and would need a compat ioctl number and a wrapper. Since it is char *, could you make it just 'char'?

Hi Jiri, sure, thanks! Lets wait for more feedback, maybe there some other more elegant solution possible, which I failed to invent.

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 <[email protected]>

CC Jiri Slaby <[email protected]>

CC Peter Hurley <[email protected]>

CC Greg Kroah-Hartman <[email protected]>

CC Andrey Vagin <[email protected]>

CC Pavel Emelianov <[email protected]>

CC Vladimir Davydov <[email protected]>

CC Konstantin Khorenko <[email protected]>

---
 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
===================================================================

drivers/tty/n_tty.c +22 -0

@@ -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
===================================================================

include/uapi/asm-generic/ioctls.h +1 -0

@@ -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

Quoted 3 lines

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).

Maybe I'm overlooking something obvious, but why not do just that; ie., save the termios, reset termios to raw mode and read the entire ldisc read buffer?

Note that saving and resetting termios to raw mode is only necessary for the slave side, as the master side is always in raw mode.

Then, two options for restore are:
1) set termios to raw mode, write to peer, restore the saved termios; or
2) restore the saved termios and write to peer.

option 1 will preserve the contents as read but not preserve the line termination state; ie., it will be possible to read multiple lines with a single canonical read.

option 2 will preserve the line termination state (for the most part) but not necessarily the contents which might be re-interpreted.

These two options are not necessarily exclusive; it may be possible to construct a mixed mode for restore based on the original saved termios that reconstitutes both line termination state and read buffer contents.

One thing not accounted for is the column position.

Quoted 10 lines · Peter Hurley

Quoted 3 lines

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).

Maybe I'm overlooking something obvious, but why not do just that; ie., save the termios, reset termios to raw mode and read the entire ldisc read buffer?

Note that saving and resetting termios to raw mode is only necessary for the slave side, as the master side is always in raw mode.

Hi Peter! Yes we can do that, but there is one significant problem:
when we read the buffer (draining it) and then during later stages of checkpoint something fails -- we need to bring the dumping program back to the former state so it won't notice that someone has been dumping it. Which means code flow like

- read ldisk buffer
- ... some other work for checkpoint sake ...
- ... obtained some error ...
- write ldisk buffer back to restore original content

and if here write fails (for any reason), the program being dupmed will loose ldisk buffer content. we simply can't allow this to happen because otherwise checkpoint will work in destructive way.

Moreover there is an option in criu where we allow to dump program and leave it in running state, say someone needs a snapshot of a state, thus we always must work in non- destructive manner.

Sure, not modifying the kernel would be a preferred way for me too.

Quoted 13 lines

Then, two options for restore are:
1) set termios to raw mode, write to peer, restore the saved termios; or
2) restore the saved termios and write to peer.

option 1 will preserve the contents as read but not preserve the line termination state; ie., it will be possible to read multiple lines with a single canonical read.

option 2 will preserve the line termination state (for the most part) but not necessarily the contents which might be re-interpreted.

Quoted 6 lines

These two options are not necessarily exclusive; it may be possible to construct a mixed mode for restore based on the original saved termios that reconstitutes both line termination state and read buffer contents.

One thing not accounted for is the column position.