tty: Add NULL TTY driver

If no console driver is enabled (or if a non-present driver is selected
with something like console=null in an attempt to disable the console),
opening /dev/console errors out, and init scripts and other userspace
code that relies on the existence of a console will fail.  Symlinking
/dev/null to /dev/console does not solve the problem since /dev/null
does not behave like a real TTY.

To just provide a dummy console to userspace when no console driver is
available or desired, add a ttynull driver which simply discards all
writes.  It can be chosen on the command line in the standard way, i.e.
with console=ttynull.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 109 insertions, 0 deletions

diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
new file mode 100644
index 000000000000..17f05b7eb6d3
--- /dev/null
+++ b/drivers/tty/ttynull.c
@@ -0,0 +1,109 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2019 Axis Communications AB
+ *
+ * Based on ttyprintk.c:
+ *  Copyright (C) 2010 Samo Pogacnik
+ */
+
+#include <linux/console.h>
+#include <linux/module.h>
+#include <linux/tty.h>
+
+static const struct tty_port_operations ttynull_port_ops;
+static struct tty_driver *ttynull_driver;
+static struct tty_port ttynull_port;
+
+static int ttynull_open(struct tty_struct *tty, struct file *filp)
+{
+	return tty_port_open(&ttynull_port, tty, filp);
+}
+
+static void ttynull_close(struct tty_struct *tty, struct file *filp)
+{
+	tty_port_close(&ttynull_port, tty, filp);
+}
+
+static void ttynull_hangup(struct tty_struct *tty)
+{
+	tty_port_hangup(&ttynull_port);
+}
+
+static int ttynull_write(struct tty_struct *tty, const unsigned char *buf,
+			 int count)
+{
+	return count;
+}
+
+static int ttynull_write_room(struct tty_struct *tty)
+{
+	return 65536;
+}
+
+static const struct tty_operations ttynull_ops = {
+	.open = ttynull_open,
+	.close = ttynull_close,
+	.hangup = ttynull_hangup,
+	.write = ttynull_write,
+	.write_room = ttynull_write_room,
+};
+
+static struct tty_driver *ttynull_device(struct console *c, int *index)
+{
+	*index = 0;
+	return ttynull_driver;
+}
+
+static struct console ttynull_console = {
+	.name = "ttynull",
+	.device = ttynull_device,
+};
+
+static int __init ttynull_init(void)
+{
+	struct tty_driver *driver;
+	int ret;
+
+	driver = tty_alloc_driver(1,
+		TTY_DRIVER_RESET_TERMIOS |
+		TTY_DRIVER_REAL_RAW |
+		TTY_DRIVER_UNNUMBERED_NODE);
+	if (IS_ERR(driver))
+		return PTR_ERR(driver);
+
+	tty_port_init(&ttynull_port);
+	ttynull_port.ops = &ttynull_port_ops;
+
+	driver->driver_name = "ttynull";
+	driver->name = "ttynull";
+	driver->type = TTY_DRIVER_TYPE_CONSOLE;
+	driver->init_termios = tty_std_termios;
+	driver->init_termios.c_oflag = OPOST | OCRNL | ONOCR | ONLRET;
+	tty_set_operations(driver, &ttynull_ops);
+	tty_port_link_device(&ttynull_port, driver, 0);
+
+	ret = tty_register_driver(driver);
+	if (ret < 0) {
+		put_tty_driver(driver);
+		tty_port_destroy(&ttynull_port);
+		return ret;
+	}
+
+	ttynull_driver = driver;
+	register_console(&ttynull_console);
+
+	return 0;
+}
+
+static void __exit ttynull_exit(void)
+{
+	unregister_console(&ttynull_console);
+	tty_unregister_driver(ttynull_driver);
+	put_tty_driver(ttynull_driver);
+	tty_port_destroy(&ttynull_port);
+}
+
+module_init(ttynull_init);
+module_exit(ttynull_exit);
+
+MODULE_LICENSE("GPL v2");