printk: Add an option to allow ttynull to be a default console device

The new option is CONFIG_NULL_TTY_DEFAULT_CONSOLE.

if enabled, and CONFIG_VT is disabled, ttynull will become the default
primary console device.

ttynull will be the only console device usually with this option enabled.
Some architectures do call add_preferred_console() which may add another
console though.

Motivation:

Many distributions ship with CONFIG_VT enabled. On tested desktop hardware
if CONFIG_VT is disabled, the default console device falls back to
/dev/ttyS0 instead of /dev/tty.

This could cause issues in user space, and hardware problems:

1. The user space issues include the case where  /dev/ttyS0 is
disconnected, and the TCGETS ioctl, which some user space libraries use
as a probe to determine if a file is a tty, is called on /dev/console and
fails. Programs that call isatty() on /dev/console and get an incorrect
false value may skip expected logging to /dev/console.

2. The hardware issues include the case if a user has a science instrument
or other device connected to the /dev/ttyS0 port, and they were to upgrade
to a kernel that is disabling the CONFIG_VT option, kernel logs will then
be sent to the device connected to /dev/ttyS0 unless they edit their
kernel command line manually.

The new CONFIG_NULL_TTY_DEFAULT_CONSOLE option will give users and
distribution maintainers an option to avoid this. Disabling CONFIG_VT and
enabling CONFIG_NULL_TTY_DEFAULT_CONSOLE will ensure the default kernel
console behavior is not dependent on hardware configuration by default, and
avoid unexpected new behavior on devices connected to the /dev/ttyS0 serial
port.

Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Adam Simonelli <adamsimonelli@gmail.com>
Link: https://lore.kernel.org/r/20250314160749.3286153-2-adamsimonelli@gmail.com
[pmladek@suse.com: Fixed indentation of the commit message.]
Signed-off-by: Petr Mladek <pmladek@suse.com>

Documentation/admin-guide/serial-console.rst

@@ -78,7 +78,9 @@ If no console device is specified, the first device found capable of
 acting as a system console will be used. At this time, the system
 first looks for a VGA card and then for a serial port. So if you don't
 have a VGA card in your system the first serial port will automatically
-become the console.
+become the console, unless the kernel is configured with the
+CONFIG_NULL_TTY_DEFAULT_CONSOLE option, then it will default to using the
+ttynull device.
 
 You will need to create a new device to use ``/dev/console``. The official
 ``/dev/console`` is now character device 5,1.

drivers/tty/Kconfig

@@ -383,7 +383,24 @@ config NULL_TTY
 	  available or desired.
 
 	  In order to use this driver, you should redirect the console to this
-	  TTY, or boot the kernel with console=ttynull.
+	  TTY, boot the kernel with console=ttynull, or enable
+	  NULL_TTY_DEFAULT_CONSOLE.
+
+	  If unsure, say N.
+
+config NULL_TTY_DEFAULT_CONSOLE
+	bool "Support for console on ttynull"
+	depends on NULL_TTY=y && !VT_CONSOLE
+	help
+	  Say Y here if you want the NULL TTY to be used as a /dev/console
+	  device by default.
+
+	  For example, it might be useful to prevent a VT-less kernel from
+	  writing the system log to a random device connected to the serial
+	  port.
+
+	  Another console driver still might get preferred via the command
+	  line, SPCR, or the device tree.
 
 	  If unsure, say N.
 

kernel/printk/printk.c

@@ -4277,6 +4277,11 @@ void __init console_init(void)
 	initcall_t call;
 	initcall_entry_t *ce;
 
+#ifdef CONFIG_NULL_TTY_DEFAULT_CONSOLE
+	if (!console_set_on_cmdline)
+		add_preferred_console("ttynull", 0, NULL);
+#endif
+
 	/* Setup the default TTY line discipline. */
 	n_tty_init();