Re: [PATCH v12 1/1] printk: Add an option to allow ttynull to be a default console device

[Petr Mladek <pmladek@suse.com>]

On Fri 2025-03-14 12:07:49, adamsimonelli@gmail.com wrote:
> From: Adam Simonelli <adamsimonelli@gmail.com>
> 
> 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>

JFYI, the patch have been comitted into printk/linux.git,
branch for-6.15.

Best Regards,
Petr