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SubjectRe: n_tty: Check the other end of pty pair before returning EAGAIN on a read()
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Hi Brian,

On 02/28/2016 07:56 PM, Brian Bloniarz wrote:
> (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.


This is not descriptive enough of the requirements of OpenSSH.
For example, it fails to note that the attempted read() is non-blocking
for which EAGAIN is an expected result that doesn't not mean
the slave-side has been closed.

Something like:

Fix OpenSSH pty regression on close

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.

...


> 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 | 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.

No, see below.

> + *
> + * 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);

This is not necessary and can deadlock.

The wait for buffer work to finish is only necessary input_available_p()
returns false. Then in n_tty_read() the termios lock needs to be dropped
before waiting for buffer work and rechecking input_available_p().

Please test your patches with lockdep enabled.

Also, abstract the naked flush_work(), similar to tty_buffer_cancel_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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