Re: VT-less kernels, and /dev/console on x86

[Bagas Sanjaya <bagasdotme@gmail.com>]

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On Sat, Aug 17, 2024 at 08:09:20PM -0400, nerdopolis wrote:
> Hi
> 
> I originally brought this up on linux-serial, but I think it makes more sense
> that it's part of how printk console device selection works. Without VTs, while
> most software is able to handle the situation, some userspace programs expect
> /dev/console to still be responsive. Namely systemd. It calls isatty() against
> /dev/console, and since /dev/console on VT-less systems currently defaults to
> /dev/ttyS0, and when /dev/ttyS0 is disconnected, the ioctl's fail, and it
> refuses to write log messages to it.
> 
> There doesn't seem to be a mailing list for printk, so I had to use
> get_maintainer.pl. Hopefully this is correct
> 
> 
> After some grepping and guessing and testing, and playing around Something like
> diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
> index a45d423ad10f..f94a4632aab0 100644
> --- a/drivers/tty/Kconfig
> +++ b/drivers/tty/Kconfig
> @@ -384,9 +384,12 @@ config NULL_TTY
>  
>           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 NULL_TTY_CONSOLE
> +        bool "Supports /dev/ttynull as a console automatically"
> +        depends on NULL_TTY && !VT_CONSOLE
> +
>  config VCC
>         tristate "Sun Virtual Console Concentrator"
>         depends on SUN_LDOMS
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index dddb15f48d59..c1554a789de8 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3712,6 +3712,11 @@ void __init console_init(void)
>         initcall_t call;
>         initcall_entry_t *ce;
>  
> +#ifdef CONFIG_NULL_TTY_CONSOLE
> +       if (!strstr(boot_command_line, "console="))
> +               add_preferred_console("ttynull", 0, NULL);
> +#endif
> +
>         /* Setup the default TTY line discipline. */
>         n_tty_init();
>  
> 
> 
> 
> seems to work, it conflicts with CONFIG_VT_CONSOLE since it is effectively
> redundant, it is optional, so that it doesn't cause any changes to
> configurations, that historically had CONFIG_VT_CONSOLE turned off in the past,
> and for bootloader configs, it won't change any behavior if the kernel command
> line has a console device specified
> 
> With ttynull as the console device, isatty() no longer fails on /dev/console,
> systemd writes the log messages fine to /dev/console, and when Plymouth calls
> TIOCCONS on its PTY, it is able to get the log messages.

Then what does /dev/ttynull do other than just to satisfy systemd? I expect
it to be like /dev/null, though.

Confused...

-- 
An old man doll... just what I always wanted! - Clara

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