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>

diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index 0ebb8d6ab341005c342dd7b5272bad55c25b0394..80f65767aacfef8682e831b13ed165f6d22a320f 100644 (file)
--- a/drivers/tty/Kconfig
+++ b/drivers/tty/Kconfig
@@ -374,6 +374,20 @@ config PPC_EARLY_DEBUG_EHV_BC_HANDLE
          there simply will be no early console output.  This is true also
          if you don't boot under a hypervisor at all.
 
+config NULL_TTY
+       tristate "NULL TTY driver"
+       help
+         Say Y here if you want a NULL TTY which simply discards messages.
+
+         This is useful to allow userspace applications which expect a console
+         device to work without modifications even when no console is
+         available or desired.
+
+         In order to use this driver, you should redirect the console to this
+         TTY, or boot the kernel with console=ttynull.
+
+         If unsure, say N.
+
 config GOLDFISH_TTY
        tristate "Goldfish TTY Driver"
        depends on GOLDFISH

diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile
index c72cafdf32b40a10f50e2b1147dc543d87c7ec10..020b1cd9294fdc3342cb145dda766cf4e00127a7 100644 (file)
--- a/drivers/tty/Makefile
+++ b/drivers/tty/Makefile
@@ -25,6 +25,7 @@ obj-$(CONFIG_ISI)             += isicom.o
 obj-$(CONFIG_MOXA_INTELLIO)    += moxa.o
 obj-$(CONFIG_MOXA_SMARTIO)     += mxser.o
 obj-$(CONFIG_NOZOMI)           += nozomi.o
+obj-$(CONFIG_NULL_TTY)         += ttynull.o
 obj-$(CONFIG_ROCKETPORT)       += rocket.o
 obj-$(CONFIG_SYNCLINK_GT)      += synclink_gt.o
 obj-$(CONFIG_SYNCLINKMP)       += synclinkmp.o

diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
new file mode 100644 (file)
index 0000000..17f05b7
--- /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");