From: Adam Simonelli <adamsimonelli@xxxxxxxxx>
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.
Signed-off-by: Adam Simonelli <adamsimonelli@xxxxxxxxx>
---
Documentation/admin-guide/serial-console.rst | 4 +++-
drivers/tty/Kconfig | 19 ++++++++++++++++++-
kernel/printk/printk.c | 5 +++++
3 files changed, 26 insertions(+), 2 deletions(-)
diff --git a/Documentation/admin-guide/serial-console.rst b/Documentation/admin-guide/serial-console.rst
index a3dfc2c66e01..1609e7479249 100644
--- 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 63a494d36a1f..7fb81bbaee60 100644
--- 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 07668433644b..2590498d8353 100644
--- 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();
--
2.45.2
