[PATCH v2 1/3] vstatus: Allow the n_tty line dicipline to write to a user tty

[Walt Drummond]

Refactor the implementation of n_tty_write() into do_n_tty_write(),
and change n_tty_write() to call do_n_tty_write() after acquiring
tty.termios_rwsem.

This allows the n_tty line dicipline to write to a user tty via
do_n_tty_write() when already holding tty.termios_rwsem.

Signed-off-by: Walt Drummond <walt@xxxxxxxxxxx>
---
 drivers/tty/n_tty.c | 69 +++++++++++++++++++++++++++------------------
 include/linux/tty.h |  2 +-
 2 files changed, 42 insertions(+), 29 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 0ec93f1a61f5..64a058a4c63b 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2231,45 +2231,24 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 	return retval;
 }
 
-/**
- *	n_tty_write		-	write function for tty
- *	@tty: tty device
- *	@file: file object
- *	@buf: userspace buffer pointer
- *	@nr: size of I/O
- *
- *	Write function of the terminal device.  This is serialized with
- *	respect to other write callers but not to termios changes, reads
- *	and other such events.  Since the receive code will echo characters,
- *	thus calling driver write methods, the output_lock is used in
- *	the output processing functions called here as well as in the
- *	echo processing function to protect the column state and space
- *	left in the buffer.
- *
- *	This code must be sure never to sleep through a hangup.
- *
- *	Locking: output_lock to protect column state and space left
- *		 (note that the process_output*() functions take this
- *		  lock themselves)
- */
-
-static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
-			   const unsigned char *buf, size_t nr)
+static ssize_t do_n_tty_write(struct tty_struct *tty, struct file *file,
+			      const unsigned char *buf, size_t nr)
 {
 	const unsigned char *b = buf;
 	DEFINE_WAIT_FUNC(wait, woken_wake_function);
 	int c;
 	ssize_t retval = 0;
 
+	lockdep_assert_held_read(&tty->termios_rwsem);
+
 	/* Job control check -- must be done at start (POSIX.1 7.1.1.4). */
-	if (L_TOSTOP(tty) && file->f_op->write_iter != redirected_tty_write) {
+	if (L_TOSTOP(tty) &&
+	    !(file && file->f_op->write_iter != redirected_tty_write)) {
 		retval = tty_check_change(tty);
 		if (retval)
 			return retval;
 	}
 
-	down_read(&tty->termios_rwsem);
-
 	/* Write out any echoed characters that are still pending */
 	process_echoes(tty);
 
@@ -2336,10 +2315,44 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
 	remove_wait_queue(&tty->write_wait, &wait);
 	if (nr && tty->fasync)
 		set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
-	up_read(&tty->termios_rwsem);
+
 	return (b - buf) ? b - buf : retval;
 }
 
+/**
+ *	n_tty_write		-	write function for tty
+ *	@tty: tty device
+ *	@file: file object
+ *	@buf: userspace buffer pointer
+ *	@nr: size of I/O
+ *
+ *	Write function of the terminal device.  This is serialized with
+ *	respect to other write callers but not to termios changes, reads
+ *	and other such events.  Since the receive code will echo characters,
+ *	thus calling driver write methods, the output_lock is used in
+ *	the output processing functions called here as well as in the
+ *	echo processing function to protect the column state and space
+ *	left in the buffer.
+ *
+ *	This code must be sure never to sleep through a hangup.
+ *
+ *	Locking: output_lock to protect column state and space left
+ *		 (note that the process_output*() functions take this
+ *		  lock themselves)
+ */
+
+static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
+			   const unsigned char *buf, size_t nr)
+{
+	ssize_t retval = 0;
+
+	down_read(&tty->termios_rwsem);
+	retval = do_n_tty_write(tty, file, buf, nr);
+	up_read(&tty->termios_rwsem);
+
+	return retval;
+}
+
 /**
  *	n_tty_poll		-	poll method for N_TTY
  *	@tty: terminal device
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 168e57e40bbb..cbe5d535a69d 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -237,7 +237,7 @@ struct tty_file_private {
 
 static inline bool tty_io_nonblock(struct tty_struct *tty, struct file *file)
 {
-	return file->f_flags & O_NONBLOCK ||
+	return (file && file->f_flags & O_NONBLOCK) ||
 		test_bit(TTY_LDISC_CHANGING, &tty->flags);
 }
 
-- 
2.30.2