Re: n_tty: Check the other end of pty pair before returning EAGAIN on a read()

[Brian Bloniarz <brian.bloniarz@gmail.com>]

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@tuyoix.net>

Reported-by: Volth <openssh@volth.com>
Reported-by: Marc Aurele La France <tsi@tuyoix.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@gmail.com>
---
 Documentation/serial/tty.txt |  3 ---
 drivers/tty/n_hdlc.c         |  2 +-
 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, 18 insertions(+), 55 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..c35abef 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;
                }
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 */