tty: convert tty_ldisc_ops 'read()' function to take a kernel pointer

The tty line discipline .read() function was passed the final user
pointer destination as an argument, which doesn't match the 'write()'
function, and makes it very inconvenient to do a splice method for
ttys.

This is a conversion to use a kernel buffer instead.

NOTE! It does this by passing the tty line discipline ->read() function
an additional "cookie" to fill in, and an offset into the cookie data.

The line discipline can fill in the cookie data with its own private
information, and then the reader will repeat the read until either the
cookie is cleared or it runs out of data.

The only real user of this is N_HDLC, which can use this to handle big
packets, even if the kernel buffer is smaller than the whole packet.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/drivers/tty/n_null.c b/drivers/tty/n_null.c
index 96feabae4740..ce03ae78f5c6 100644
--- a/drivers/tty/n_null.c
+++ b/drivers/tty/n_null.c
@@ -20,7 +20,8 @@ static void n_null_close(struct tty_struct *tty)
 }
 
 static ssize_t n_null_read(struct tty_struct *tty, struct file *file,
-			   unsigned char __user * buf, size_t nr)
+			   unsigned char *buf, size_t nr,
+			   void **cookie, unsigned long offset)
 {
 	return -EOPNOTSUPP;
 }