tty: implement read_iter

Now that the ldisc read() function takes kernel pointers, it's fairly
straightforward to make the tty file operations use .read_iter() instead
of .read().

That automatically gives us vread() and friends, and also makes it
possible to do .splice_read() on ttys again.

Fixes: 36e2c742 ("fs: don't allow splice read/write without explicit ops")
Reported-by: NOliver Giles <ohw.giles@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/tty/tty_io.c

@@ -142,7 +142,7 @@ LIST_HEAD(tty_drivers);			/* linked list of tty drivers */
 /* Mutex to protect creating and releasing a tty */
 DEFINE_MUTEX(tty_mutex);
 
-static ssize_t tty_read(struct file *, char __user *, size_t, loff_t *);
+static ssize_t tty_read(struct kiocb *, struct iov_iter *);
 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 *);
@@ -476,8 +476,9 @@ static void tty_show_fdinfo(struct seq_file *m, struct file *file)
 
 static const struct file_operations tty_fops = {
 	.llseek		= no_llseek,
-	.read		= tty_read,
+	.read_iter	= tty_read,
 	.write_iter	= tty_write,
+	.splice_read	= generic_file_splice_read,
 	.splice_write	= iter_file_splice_write,
 	.poll		= tty_poll,
 	.unlocked_ioctl	= tty_ioctl,
@@ -490,8 +491,9 @@ static const struct file_operations tty_fops = {
 
 static const struct file_operations console_fops = {
 	.llseek		= no_llseek,
-	.read		= tty_read,
+	.read_iter	= tty_read,
 	.write_iter	= redirected_tty_write,
+	.splice_read	= generic_file_splice_read,
 	.splice_write	= iter_file_splice_write,
 	.poll		= tty_poll,
 	.unlocked_ioctl	= tty_ioctl,
@@ -844,16 +846,17 @@ static void tty_update_time(struct timespec64 *time)
  * data or clears the cookie. The cookie may be something that the
  * ldisc maintains state for and needs to free.
  */
-static int iterate_tty_read(struct tty_ldisc *ld, struct tty_struct *tty, struct file *file,
-		char __user *buf, size_t count)
+static int iterate_tty_read(struct tty_ldisc *ld, struct tty_struct *tty,
+		struct file *file, struct iov_iter *to)
 {
 	int retval = 0;
 	void *cookie = NULL;
 	unsigned long offset = 0;
 	char kernel_buf[64];
+	size_t count = iov_iter_count(to);
 
 	do {
-		int size, uncopied;
+		int size, copied;
 
 		size = count > sizeof(kernel_buf) ? sizeof(kernel_buf) : count;
 		size = ld->ops->read(tty, file, kernel_buf, size, &cookie, offset);
@@ -869,10 +872,9 @@ static int iterate_tty_read(struct tty_ldisc *ld, struct tty_struct *tty, struct
 			return size;
 		}
 
-		uncopied = copy_to_user(buf+offset, kernel_buf, size);
-		size -= uncopied;
-		offset += size;
-		count -= size;
+		copied = copy_to_iter(kernel_buf, size, to);
+		offset += copied;
+		count -= copied;
 
 		/*
 		 * If the user copy failed, we still need to do another ->read()
@@ -880,7 +882,7 @@ static int iterate_tty_read(struct tty_ldisc *ld, struct tty_struct *tty, struct
 		 *
 		 * But make sure size is zeroed.
 		 */
-		if (unlikely(uncopied)) {
+		if (unlikely(copied != size)) {
 			count = 0;
 			retval = -EFAULT;
 		}
@@ -907,10 +909,10 @@ static int iterate_tty_read(struct tty_ldisc *ld, struct tty_struct *tty, struct
  *	read calls may be outstanding in parallel.
  */
 
-static ssize_t tty_read(struct file *file, char __user *buf, size_t count,
-			loff_t *ppos)
+static ssize_t tty_read(struct kiocb *iocb, struct iov_iter *to)
 {
 	int i;
+	struct file *file = iocb->ki_filp;
 	struct inode *inode = file_inode(file);
 	struct tty_struct *tty = file_tty(file);
 	struct tty_ldisc *ld;
@@ -923,11 +925,9 @@ static ssize_t tty_read(struct file *file, char __user *buf, size_t count,
 	/* We want to wait for the line discipline to sort out in this
 	   situation */
 	ld = tty_ldisc_ref_wait(tty);
-	if (!ld)
-		return hung_up_tty_read(file, buf, count, ppos);
 	i = -EIO;
-	if (ld->ops->read)
-		i = iterate_tty_read(ld, tty, file, buf, count);
+	if (ld && ld->ops->read)
+		i = iterate_tty_read(ld, tty, file, to);
 	tty_ldisc_deref(ld);
 
 	if (i > 0)
@@ -2927,7 +2927,7 @@ static long tty_compat_ioctl(struct file *file, unsigned int cmd,
 
 static int this_tty(const void *t, struct file *file, unsigned fd)
 {
-	if (likely(file->f_op->read != tty_read))
+	if (likely(file->f_op->read_iter != tty_read))
 		return 0;
 	return file_tty(file) != t ? 0 : fd + 1;
 }