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

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Date:   Wed, 22 Mar 2023 08:03:24 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Guillermo Rodriguez Garcia <guille.rodriguez@...il.com>
Cc:     linux-kernel@...r.kernel.org, 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 Wed, Mar 22, 2023 at 09:23:28AM +0100, Guillermo Rodriguez Garcia wrote:
> El jue, 16 mar 2023 a las 14:40, Guenter Roeck (<linux@...ck-us.net>) escribió:
> >
> > 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).
> 
> The kernel seems to specifically check for "" (empty string) or "null"
> and in this case uses the ttynull driver. But apparently we can use
> any other invalid string (e.g "invalid") to get the previous behaviour
> (no console).
> 
> I am still checking if this has other side effects.
> 

Interesting. I'll check if that works for ChromeOS; that would be better
than having to carry the reverts.

Thanks,
Guenter

> Best regards,
> 
> Guillermo Rodriguez Garcia
> guille.rodriguez@...il.com