Message-ID: <56D5BEA9.5040200@hurleysoftware.com>
Date:	Tue, 1 Mar 2016 08:09:13 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	Brian Bloniarz <brian.bloniarz@...il.com>
Cc:	linux-kernel@...r.kernel.org, tsi@...oix.net
Subject: Re: [PATCH] Fix OpenSSH pty regression on close

Hi Brian,

Couple of procedural notes:

I'm not a maintainer so can't take this patch directly.
This patch should be addressed to the tty maintainers and cc me.

What tree was this diff'd from? You should be using GregKH's
tty-next branch on this tree
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git


On 02/29/2016 10:40 PM, Brian Bloniarz wrote:
> OpenSSH expects the (non-blocking) read() of pty master to return
> EAGAIN only if it has received all of the slave-side output after
> it has received SIGCHLD. This used to work on pre-3.12 kernels.
> 
> This fix effectively forces non-blocking read() and poll() to
> block for parallel i/o to complete for all ttys. It unwinds these
> changes:
> 
> 1) f95499c3030fe1bfad57745f2db1959c5b43dca8
>    n_tty: Don't wait for buffer work in read() loop

I would drop the above reference as you won't be waiting for
buffer work unless the initial test for input fails.


> 
> 2) f8747d4a466ab2cafe56112c51b3379f9fdb7a12
>    tty: Fix pty master read() after slave closes
> 
> 3) 52bce7f8d4fc633c9a9d0646eef58ba6ae9a3b73
>    pty, n_tty: Simplify input processing on final close
> 
> 4) 1a48632ffed61352a7810ce089dc5a8bcd505a60
>    pty: Fix input race when closing
> 
> Inspired by analysis and patch from Marc Aurele La France <tsi@...oix.net>
> 
> Reported-by: Volth <openssh@...th.com>
> Reported-by: Marc Aurele La France <tsi@...oix.net>
> BugLink: https://bugzilla.mindrot.org/show_bug.cgi?id=52
> BugLink: https://bugzilla.mindrot.org/show_bug.cgi?id=2492
> Signed-off-by: Brian Bloniarz <brian.bloniarz@...il.com>
> ---
>  Documentation/serial/tty.txt |  3 --
>  drivers/tty/n_hdlc.c         |  4 +--
>  drivers/tty/n_tty.c          | 70 +++++++++++++++++++++-----------------------
>  drivers/tty/pty.c            |  4 +--
>  drivers/tty/tty_buffer.c     | 34 ++++-----------------
>  include/linux/tty.h          |  2 +-
>  6 files changed, 43 insertions(+), 74 deletions(-)
> 
> diff --git a/Documentation/serial/tty.txt b/Documentation/serial/tty.txt
> index bc3842d..e2dea3d 100644
> --- a/Documentation/serial/tty.txt
> +++ b/Documentation/serial/tty.txt
> @@ -213,9 +213,6 @@ TTY_IO_ERROR		If set, causes all subsequent userspace read/write
>  
>  TTY_OTHER_CLOSED	Device is a pty and the other side has closed.
>  
> -TTY_OTHER_DONE		Device is a pty and the other side has closed and
> -			all pending input processing has been completed.
> -
>  TTY_NO_WRITE_SPLIT	Prevent driver from splitting up writes into
>  			smaller chunks.
>  
> diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
> index bbc4ce6..644ddb8 100644
> --- a/drivers/tty/n_hdlc.c
> +++ b/drivers/tty/n_hdlc.c
> @@ -600,7 +600,7 @@ static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
>  	add_wait_queue(&tty->read_wait, &wait);
>  
>  	for (;;) {
> -		if (test_bit(TTY_OTHER_DONE, &tty->flags)) {
> +		if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
>  			ret = -EIO;
>  			break;
>  		}
> @@ -828,7 +828,7 @@ static unsigned int n_hdlc_tty_poll(struct tty_struct *tty, struct file *filp,
>  		/* set bits for operations that won't block */
>  		if (n_hdlc->rx_buf_list.head)
>  			mask |= POLLIN | POLLRDNORM;	/* readable */
> -		if (test_bit(TTY_OTHER_DONE, &tty->flags))
> +		if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
>  			mask |= POLLHUP;
>  		if (tty_hung_up_p(filp))
>  			mask |= POLLHUP;
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index b280abaa..c12def7 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -1963,18 +1963,6 @@ static inline int input_available_p(struct tty_struct *tty, int poll)
>  		return ldata->commit_head - ldata->read_tail >= amt;
>  }
>  
> -static inline int check_other_done(struct tty_struct *tty)
> -{
> -	int done = test_bit(TTY_OTHER_DONE, &tty->flags);
> -	if (done) {
> -		/* paired with cmpxchg() in check_other_closed(); ensures
> -		 * read buffer head index is not stale
> -		 */
> -		smp_mb__after_atomic();
> -	}
> -	return done;
> -}
> -
>  /**
>   *	copy_from_read_buf	-	copy read data directly
>   *	@tty: terminal device
> @@ -2170,7 +2158,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
>  	struct n_tty_data *ldata = tty->disc_data;
>  	unsigned char __user *b = buf;
>  	DEFINE_WAIT_FUNC(wait, woken_wake_function);
> -	int c, done;
> +	int c;
>  	int minimum, time;
>  	ssize_t retval = 0;
>  	long timeout;
> @@ -2238,32 +2226,35 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
>  		    ((minimum - (b - buf)) >= 1))
>  			ldata->minimum_to_wake = (minimum - (b - buf));
>  
> -		done = check_other_done(tty);
> -
>  		if (!input_available_p(tty, 0)) {
> -			if (done) {
> -				retval = -EIO;
> -				break;
> -			}
> -			if (tty_hung_up_p(file))
> -				break;
> -			if (!timeout)
> -				break;
> -			if (file->f_flags & O_NONBLOCK) {
> -				retval = -EAGAIN;
> -				break;
> -			}
> -			if (signal_pending(current)) {
> -				retval = -ERESTARTSYS;
> -				break;
> -			}
>  			up_read(&tty->termios_rwsem);
> +			tty_buffer_flush_work(tty->port);
> +			down_read(&tty->termios_rwsem);
> +			if (!input_available_p(tty, 0)) {
> +				if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
> +					retval = -EIO;
> +					break;
> +				}
> +				if (tty_hung_up_p(file))
> +					break;
> +				if (!timeout)
> +					break;
> +				if (file->f_flags & O_NONBLOCK) {
> +					retval = -EAGAIN;
> +					break;
> +				}
> +				if (signal_pending(current)) {
> +					retval = -ERESTARTSYS;
> +					break;
> +				}
> +				up_read(&tty->termios_rwsem);
>  
> -			timeout = wait_woken(&wait, TASK_INTERRUPTIBLE,
> -					     timeout);
> +				timeout = wait_woken(&wait, TASK_INTERRUPTIBLE,
> +						timeout);
>  
> -			down_read(&tty->termios_rwsem);
> -			continue;
> +				down_read(&tty->termios_rwsem);
> +				continue;
> +			}
>  		}
>  
>  		if (ldata->icanon && !L_EXTPROC(tty)) {
> @@ -2445,12 +2436,17 @@ static unsigned int n_tty_poll(struct tty_struct *tty, struct file *file,
>  
>  	poll_wait(file, &tty->read_wait, wait);
>  	poll_wait(file, &tty->write_wait, wait);
> -	if (check_other_done(tty))
> -		mask |= POLLHUP;
>  	if (input_available_p(tty, 1))
>  		mask |= POLLIN | POLLRDNORM;
> +	else {
> +		tty_buffer_flush_work(tty->port);
> +		if (input_available_p(tty, 1))
> +			mask |= POLLIN | POLLRDNORM;
> +	}
>  	if (tty->packet && tty->link->ctrl_status)
>  		mask |= POLLPRI | POLLIN | POLLRDNORM;
> +	if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
> +		mask |= POLLHUP;
>  	if (tty_hung_up_p(file))
>  		mask |= POLLHUP;
>  	if (!(mask & (POLLHUP | POLLIN | POLLRDNORM))) {
> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
> index 2348fa6..6427a39 100644
> --- a/drivers/tty/pty.c
> +++ b/drivers/tty/pty.c
> @@ -59,7 +59,7 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
>  	if (!tty->link)
>  		return;
>  	set_bit(TTY_OTHER_CLOSED, &tty->link->flags);
> -	tty_flip_buffer_push(tty->link->port);
> +	wake_up_interruptible(&tty->link->read_wait);
>  	wake_up_interruptible(&tty->link->write_wait);
>  	if (tty->driver->subtype == PTY_TYPE_MASTER) {
>  		set_bit(TTY_OTHER_CLOSED, &tty->flags);
> @@ -247,9 +247,7 @@ static int pty_open(struct tty_struct *tty, struct file *filp)
>  		goto out;
>  
>  	clear_bit(TTY_IO_ERROR, &tty->flags);
> -	/* TTY_OTHER_CLOSED must be cleared before TTY_OTHER_DONE */
>  	clear_bit(TTY_OTHER_CLOSED, &tty->link->flags);
> -	clear_bit(TTY_OTHER_DONE, &tty->link->flags);
>  	set_bit(TTY_THROTTLED, &tty->flags);
>  	return 0;
>  
> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
> index 3cd31e0..fb31eec 100644
> --- a/drivers/tty/tty_buffer.c
> +++ b/drivers/tty/tty_buffer.c
> @@ -37,29 +37,6 @@
>  
>  #define TTY_BUFFER_PAGE	(((PAGE_SIZE - sizeof(struct tty_buffer)) / 2) & ~0xFF)
>  
> -/*
> - * If all tty flip buffers have been processed by flush_to_ldisc() or
> - * dropped by tty_buffer_flush(), check if the linked pty has been closed.
> - * If so, wake the reader/poll to process
> - */
> -static inline void check_other_closed(struct tty_struct *tty)
> -{
> -	unsigned long flags, old;
> -
> -	/* transition from TTY_OTHER_CLOSED => TTY_OTHER_DONE must be atomic */
> -	for (flags = ACCESS_ONCE(tty->flags);
> -	     test_bit(TTY_OTHER_CLOSED, &flags);
> -	     ) {
> -		old = flags;
> -		__set_bit(TTY_OTHER_DONE, &flags);
> -		flags = cmpxchg(&tty->flags, old, flags);
> -		if (old == flags) {
> -			wake_up_interruptible(&tty->read_wait);
> -			break;
> -		}
> -	}
> -}
> -
>  /**
>   *	tty_buffer_lock_exclusive	-	gain exclusive access to buffer
>   *	tty_buffer_unlock_exclusive	-	release exclusive access
> @@ -254,8 +231,6 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
>  	if (ld && ld->ops->flush_buffer)
>  		ld->ops->flush_buffer(tty);
>  
> -	check_other_closed(tty);
> -
>  	atomic_dec(&buf->priority);
>  	mutex_unlock(&buf->lock);
>  }
> @@ -505,10 +480,8 @@ static void flush_to_ldisc(struct work_struct *work)
>  		 */
>  		count = smp_load_acquire(&head->commit) - head->read;
>  		if (!count) {
> -			if (next == NULL) {
> -				check_other_closed(tty);
> +			if (next == NULL)
>  				break;
> -			}
>  			buf->head = next;
>  			tty_buffer_free(port, head);
>  			continue;
> @@ -597,3 +570,8 @@ bool tty_buffer_cancel_work(struct tty_port *port)
>  {
>  	return cancel_work_sync(&port->buf.work);
>  }
> +
> +void tty_buffer_flush_work(struct tty_port *port)
> +{
> +	flush_work(&port->buf.work);
> +}
> diff --git a/include/linux/tty.h b/include/linux/tty.h
> index d9fb4b0..663155b 100644
> --- a/include/linux/tty.h
> +++ b/include/linux/tty.h
> @@ -338,7 +338,6 @@ struct tty_file_private {
>  #define TTY_EXCLUSIVE 		3	/* Exclusive open mode */
>  #define TTY_DEBUG 		4	/* Debugging */
>  #define TTY_DO_WRITE_WAKEUP 	5	/* Call write_wakeup after queuing new */
> -#define TTY_OTHER_DONE		6	/* Closed pty has completed input processing */
>  #define TTY_LDISC_OPEN	 	11	/* Line discipline is open */
>  #define TTY_PTY_LOCK 		16	/* pty private */
>  #define TTY_NO_WRITE_SPLIT 	17	/* Preserve write boundaries to driver */
> @@ -464,6 +463,7 @@ extern void tty_buffer_init(struct tty_port *port);
>  extern void tty_buffer_set_lock_subclass(struct tty_port *port);
>  extern bool tty_buffer_restart_work(struct tty_port *port);
>  extern bool tty_buffer_cancel_work(struct tty_port *port);
> +extern void tty_buffer_flush_work(struct tty_port *port);
>  extern speed_t tty_termios_baud_rate(struct ktermios *termios);
>  extern speed_t tty_termios_input_baud_rate(struct ktermios *termios);
>  extern void tty_termios_encode_baud_rate(struct ktermios *termios,
> 

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