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, 4 insertions, 6 deletions

diff --git a/drivers/tty/n_r3964.c b/drivers/tty/n_r3964.c
index 934dd2fb2ec8..3161f0a535e3 100644
--- a/drivers/tty/n_r3964.c
+++ b/drivers/tty/n_r3964.c
@@ -129,7 +129,7 @@ static void remove_client_block(struct r3964_info *pInfo,
 static int r3964_open(struct tty_struct *tty);
 static void r3964_close(struct tty_struct *tty);
 static ssize_t r3964_read(struct tty_struct *tty, struct file *file,
-		unsigned char __user * buf, size_t nr);
+		void *cookie, unsigned char *buf, size_t nr);
 static ssize_t r3964_write(struct tty_struct *tty, struct file *file,
 		const unsigned char *buf, size_t nr);
 static int r3964_ioctl(struct tty_struct *tty, struct file *file,
@@ -1058,7 +1058,8 @@ static void r3964_close(struct tty_struct *tty)
 }
 
 static ssize_t r3964_read(struct tty_struct *tty, struct file *file,
-			  unsigned char __user * buf, size_t nr)
+			  unsigned char *kbuf, size_t nr,
+			  void **cookie, unsigned long offset)
 {
 	struct r3964_info *pInfo = tty->disc_data;
 	struct r3964_client_info *pClient;
@@ -1109,10 +1110,7 @@ static ssize_t r3964_read(struct tty_struct *tty, struct file *file,
 		kfree(pMsg);
 		TRACE_M("r3964_read - msg kfree %p", pMsg);
 
-		if (copy_to_user(buf, &theMsg, ret)) {
-			ret = -EFAULT;
-			goto unlock;
-		}
+		memcpy(kbuf, &theMsg, ret);
 
 		TRACE_PS("read - return %d", ret);
 		goto unlock;