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>

diff --git a/Documentation/admin-guide/serial-console.rst b/Documentation/admin-guide/serial-console.rst
index a3dfc2c66e019df5e15c6d3ca3674454bc71a162..1609e7479249f0721fd0b4fbbd70eda13ffc09e4 100644 (file)
--- a/Documentation/admin-guide/serial-console.rst
+++ b/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.

diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index 63a494d36a1fdceba5a7b39f4516060e48af0cc6..7fb81bbaee60b359afed9e1a297b7a06b3348816 100644 (file)
--- a/drivers/tty/Kconfig
+++ b/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.
 

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 07668433644b8a746229d170fde6ea416a09422b..2590498d8353a08e7887bf706b3c519409edeecf 100644 (file)
--- a/kernel/printk/printk.c
+++ b/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();