linux-kernel - Re: Change of behaviour when console=null and ttynull driver is used

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Date:   Fri, 17 Mar 2023 13:51:18 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Guillermo Rodriguez Garcia <guille.rodriguez@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Alejandro Vazquez <avazquez.dev@...il.com>,
        Shreyas Joshi <shreyas.joshi@...mp.com>,
        sergey.senozhatsky@...il.com, rostedt@...dmis.org,
        shreyasjoshi15@...il.com, Guenter Roeck <linux@...ck-us.net>
Subject: Re: Change of behaviour when console=null and ttynull driver is used

On Thu 2023-03-16 11:29:26, Guillermo Rodriguez Garcia wrote:
> Hi all,
> 
> We have several embedded systems where pass console= or console=null
> in production to disable the console.
> 
> Later we check for this in user space: in our inittab we check if fd0
> is "associated with a terminal" (test -t 0); if so, we are in
> development mode and we open a debug shell; otherwise (console
> disabled) we just start the application.
> 
> Recently [1] this behaviour has changed and now if we pass console= or
> console=null, the new ttynull driver is used. This breaks the check we
> were doing (test -t 0 always true now).
>
> [1]: https://lore.kernel.org/lkml/X%2FcDG%2FxCCzSWW2cd@alley/t/

This is actually exactly the problem that the change tried to solve.
Some systems failed to boot when there was no console and they tried
to write something at stdout.

> Is there a way to get the previous behaviour? If not, is there an easy
> way for userspace to detect whether the console device is a "real" tty
> or ttynull (other than trying to parse the kernel boot args, which
> would be a bit fragile).

A solution would be to check that /proc/consoles has ttynull as the
only registred console, for example:

grep -q ttynull /proc/consoles && test `cat /proc/consoles | wc -l` -eq 1

Would this work for you, please?

Best Regards,
Petr