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

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Message-ID: <83c9306a-dbfe-819a-16d8-6242437b2810@roeck-us.net>
Date:   Thu, 16 Mar 2023 06:40:37 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Guillermo Rodriguez Garcia <guille.rodriguez@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     Petr Mladek <pmladek@...e.com>,
        Alejandro Vazquez <avazquez.dev@...il.com>,
        Shreyas Joshi <shreyas.joshi@...mp.com>,
        sergey.senozhatsky@...il.com, rostedt@...dmis.org,
        shreyasjoshi15@...il.com
Subject: Re: Change of behaviour when console=null and ttynull driver is used

On 3/16/23 03:29, 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).
> 
> 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).
> 
> Thank you,
> 
> (If possible, please CC me in any replies)
> 
>   [1]: https://lore.kernel.org/lkml/X%2FcDG%2FxCCzSWW2cd@alley/t/
> 

Let me know if/when you find a solution. In ChromeOS we have to carry
reverts of commit 48021f981308 ("printk: handle blank console arguments
passed in.") and commit 3cffa06aeef7 ("printk/console: Allow to disable
console output by using console="" or console=null") to handle the
same problem (the above mentioned commit didn't work and had odd side
effects).

Guenter