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

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Message-ID: <755a8541-adf5-4454-9bd5-9ea439a5a4ea@roeck-us.net>
Date:   Wed, 22 Mar 2023 08:05:27 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Guillermo Rodriguez Garcia <guille.rodriguez@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.org,
        Alejandro Vazquez <avazquez.dev@...il.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 09:33:32AM +0100, Guillermo Rodriguez Garcia wrote:
> Hi Petr,
> 
> El vie, 17 mar 2023 a las 13:51, Petr Mladek (<pmladek@...e.com>) escribió:
> >
> > 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.
> 
> Well, I did not have any problem before this change.
> 
> I understand that some systems had a problem (many others didn't, and
> setting console= or console=null has been standard practice for a long
> time). Since this change in behaviour could (and did) break things in
> user space, perhaps it should have been made opt-in...
> 
> >
> > > 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?
> 
> I was trying to avoid something like this as it feels like userspace
> now needs to have too much knowledge of what the kernel is doing
> internally, however I guess this is the best option.
> 
> The suggested check seems to work but now I am seeing a different
> (related?) issue: when I try to reboot from the console (using
> busybox's reboot command), the system reboots normally if I am using a
> "normal" console (e.g. console=ttyXXX), however when using
> console=ttynull, the command takes 20-25 seconds to complete. I am not
> sure why this would happen; if I understand correctly, for userspace
> ttynull is just like a regular tty driver, so why would there be a
> difference?
> 

I think we may have seen something similar, causing us to revert the
upstream patches in ChromeOS. We did not try to find out what exactly
caused the problems.

Guenter