Commit 9bb48c82 ("tty: implement write_iter") converted the tty

layer to use write_iter. Fix the redirected_tty_write declaration
also in n_tty and change the comparisons to use write_iter instead of
write.

[ Also moved the declaration of redirected_tty_write() to the proper
  location in a header file. The reason for the bug was the bogus extern
  declaration in n_tty.c silently not matching the changed definition in
  tty_io.c, and because it wasn't in a shared header file, there was no
  cross-checking of the declaration.

  Sami noticed because Clang's Control Flow Integrity checking ended up
  incidentally noticing the inconsistent declaration.    - Linus ]

Fixes: 9bb48c82 ("tty: implement write_iter")
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/tty/n_tty.c

@@ -2081,9 +2081,6 @@ static int canon_copy_from_read_buf(struct tty_struct *tty,
 	return 0;
 }
 
-extern ssize_t redirected_tty_write(struct file *, const char __user *,
-							size_t, loff_t *);
-
 /**
  *	job_control		-	check job control
  *	@tty: tty
@@ -2105,7 +2102,7 @@ static int job_control(struct tty_struct *tty, struct file *file)
 	/* NOTE: not yet done after every sleep pending a thorough
 	   check of the logic of this change. -- jlc */
 	/* don't stop on /dev/console */
-	if (file->f_op->write == redirected_tty_write)
+	if (file->f_op->write_iter == redirected_tty_write)
 		return 0;
 
 	return __tty_check_change(tty, SIGTTIN);
@@ -2309,7 +2306,7 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
 	ssize_t retval = 0;
 
 	/* Job control check -- must be done at start (POSIX.1 7.1.1.4). */
-	if (L_TOSTOP(tty) && file->f_op->write != redirected_tty_write) {
+	if (L_TOSTOP(tty) && file->f_op->write_iter != redirected_tty_write) {
 		retval = tty_check_change(tty);
 		if (retval)
 			return retval;

drivers/tty/tty_io.c

@@ -144,10 +144,8 @@ DEFINE_MUTEX(tty_mutex);
 
 static ssize_t tty_read(struct file *, char __user *, size_t, loff_t *);
 static ssize_t tty_write(struct kiocb *, struct iov_iter *);
-ssize_t redirected_tty_write(struct kiocb *, struct iov_iter *);
 static __poll_t tty_poll(struct file *, poll_table *);
 static int tty_open(struct inode *, struct file *);
-long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
 #ifdef CONFIG_COMPAT
 static long tty_compat_ioctl(struct file *file, unsigned int cmd,
 				unsigned long arg);

include/linux/tty.h

@@ -421,6 +421,7 @@ extern void tty_kclose(struct tty_struct *tty);
 extern int tty_dev_name_to_number(const char *name, dev_t *number);
 extern int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout);
 extern void tty_ldisc_unlock(struct tty_struct *tty);
+extern ssize_t redirected_tty_write(struct kiocb *, struct iov_iter *);
 #else
 static inline void tty_kref_put(struct tty_struct *tty)
 { }