[PATCH 4.9 10/77] tty: fix __tty_insert_flip_char regression

Greg Kroah-Hartman
2017-09-24T21:19:41Z

From: Arnd Bergmann <arnd@arndb.de>

commit 8a5a90a2a477b86a3dc2eaa5a706db9bfdd647ca upstream.

Sergey noticed a small but fatal mistake in __tty_insert_flip_char,
leading to an oops in an interrupt handler when using any serial
port.

The problem is that I accidentally took the tty_buffer pointer
before calling __tty_buffer_request_room(), which replaces the
buffer. This moves the pointer lookup to the right place after
allocating the new buffer space.

Fixes: 979990c62848 ("tty: improve tty_insert_flip_char() fast path")
Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Tested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/tty/tty_buffer.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -372,12 +372,13 @@ EXPORT_SYMBOL(tty_insert_flip_string_fla
*/
int __tty_insert_flip_char(struct tty_port *port, unsigned char ch, char flag)
{
- struct tty_buffer *tb = port->buf.tail;
+ struct tty_buffer *tb;
int flags = (flag == TTY_NORMAL) ? TTYB_NORMAL : 0;

if (!__tty_buffer_request_room(port, 1, flags))
return 0;

+ tb = port->buf.tail;
if (~tb->flags & TTYB_NORMAL)
*flag_buf_ptr(tb, tb->used) = flag;
*char_buf_ptr(tb, tb->used++) = ch;