[PATCH v2] tty: Fix lookahead_buf crash with serdev

[Vincent Whitchurch]

Do not follow a NULL pointer if the tty_port_client_operations does not
implement the ->lookahead_buf() callback, which is the case with
serdev's ttyport.

Fixes: 6bb6fa6908ebd3 ("tty: Implement lookahead to process XON/XOFF timely")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>
---

Notes:
    v2: Move more stuff into if block.

 drivers/tty/tty_buffer.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 9fdecc795b6b..5e287dedce01 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -470,7 +470,6 @@ static void lookahead_bufs(struct tty_port *port, struct tty_buffer *head)
 
 	while (head) {
 		struct tty_buffer *next;
-		unsigned char *p, *f = NULL;
 		unsigned int count;
 
 		/*
@@ -489,11 +488,16 @@ static void lookahead_bufs(struct tty_port *port, struct tty_buffer *head)
 			continue;
 		}
 
-		p = char_buf_ptr(head, head->lookahead);
-		if (~head->flags & TTYB_NORMAL)
-			f = flag_buf_ptr(head, head->lookahead);
+		if (port->client_ops->lookahead_buf) {
+			unsigned char *p, *f = NULL;
+
+			p = char_buf_ptr(head, head->lookahead);
+			if (~head->flags & TTYB_NORMAL)
+				f = flag_buf_ptr(head, head->lookahead);
+
+			port->client_ops->lookahead_buf(port, p, f, count);
+		}
 
-		port->client_ops->lookahead_buf(port, p, f, count);
 		head->lookahead += count;
 	}
 }
-- 
2.34.1