Date:	Sun, 28 Feb 2016 19:56:25 -0800
From:	Brian Bloniarz <brian.bloniarz@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	peter@...leysoftware.com, tsi@...oix.net
Subject: Re: n_tty: Check the other end of pty pair before returning EAGAIN
 on a read()

(Take 3, fix compile error in n_hdlc.c)

Hi Peter, I saw Marc Aurele La France's proposed patch to n_tty to fix
OpenSSH, and your feedback. Patch below is an attempt to address that
feedback. Please let me know if this is the change you envisioned;
(see Marc's excellent original writeup for details on the issue).

[PATCH] n_tty: wait for buffer work in read() and poll().

Undoes the following four changes:

1) f95499c3030fe1bfad57745f2db1959c5b43dca8
    n_tty: Don't wait for buffer work in read() loop

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

These changes caused a regression in OpenSSH, as it assumes that the
first read() to return EAGAIN after a SIGCHLD means that all the child's
output has been returned.

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          | 34 +++++++++++++++-------------------
 drivers/tty/pty.c            |  4 +---
 drivers/tty/tty_buffer.c     | 29 +----------------------------
 include/linux/tty.h          |  1 -
 6 files changed, 19 insertions(+), 56 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..fc04011 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1952,10 +1952,20 @@ err:
 	return -ENOMEM;
 }
 
+/**
+ *	Synchronously pushes the terminal flip buffers to the line discipline
+ *	and checks for available data.
+ *
+ *	Must not be called from IRQ context.
+ */
 static inline int input_available_p(struct tty_struct *tty, int poll)
 {
 	struct n_tty_data *ldata = tty->disc_data;
-	int amt = poll && !TIME_CHAR(tty) && MIN_CHAR(tty) ? MIN_CHAR(tty) : 1;
+	int amt;
+
+	flush_work(&tty->port->buf.work);
+
+	amt = poll && !TIME_CHAR(tty) && MIN_CHAR(tty) ? MIN_CHAR(tty) : 1;
 
 	if (ldata->icanon && !L_EXTPROC(tty))
 		return ldata->canon_head != ldata->read_tail;
@@ -1963,18 +1973,6 @@ 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 +2168,7 @@ 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;
@@ -2238,10 +2236,8 @@ 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) {
+			if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
 				retval = -EIO;
 				break;
 			}
@@ -2445,12 +2441,12 @@ 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;
 	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..3c0e914 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;
diff --git a/include/linux/tty.h b/include/linux/tty.h
index d9fb4b0..af18365 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 */

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