Re: [PATCH v4 2/2] tty: Change order of ttynull to be linked sooner if enabled as a console.

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Sun, Feb 23, 2025 at 03:44:56PM -0500, adamsimonelli@gmail.com wrote:
> From: Adam Simonelli <adamsimonelli@gmail.com>
> 
> If CONFIG_NULL_TTY_CONSOLE is enabled, and CONFIG_VT is disabled, ttynull
> will become the default primary console device, based on the link order.
> 
> 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_CONSOLE option will give users and distribution
> maintainers an option to avoid this. Disabling CONFIG_VT and enabling
> CONFIG_NULL_TTY_CONSOLE will ensure the default kernel console behavior
> is not dependant 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@gmail.com>
> ---
>  drivers/tty/Makefile | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile
> index 07aca5184a55..1a1051ecb1af 100644
> --- a/drivers/tty/Makefile
> +++ b/drivers/tty/Makefile
> @@ -11,6 +11,10 @@ obj-$(CONFIG_N_HDLC)		+= n_hdlc.o
>  obj-$(CONFIG_N_GSM)		+= n_gsm.o
>  
>  obj-y				+= vt/
> +#If ttynull is configured to be a console by default, ensure that it is linked
> +#earlier before a real one is selected.
> +obj-$(CONFIG_NULL_TTY_CONSOLE)	+= ttynull.o
> +
>  obj-$(CONFIG_HVC_DRIVER)	+= hvc/
>  obj-y				+= serial/
>  obj-$(CONFIG_SERIAL_DEV_BUS)	+= serdev/
> @@ -20,7 +24,13 @@ obj-$(CONFIG_AMIGA_BUILTIN_SERIAL) += amiserial.o
>  obj-$(CONFIG_MOXA_INTELLIO)	+= moxa.o
>  obj-$(CONFIG_MOXA_SMARTIO)	+= mxser.o
>  obj-$(CONFIG_NOZOMI)		+= nozomi.o
> +
> +#If ttynull is enabled, but not as a boot console, it is linked and used later
> +#after the real ones.
> +ifneq ($(CONFIG_NULL_TTY_CONSOLE),y)
>  obj-$(CONFIG_NULL_TTY)	        += ttynull.o
> +endif

Nit, a " " needs to be after the "#" character, right?

And ick, this is going to be tricky, changing the link order depending
on the configuration option setting?  This feels wrong, and messy, and
very fragile.

thanks,

greg k-h