MIME-Version: 1.0
From:   Guillermo Rodriguez Garcia <guille.rodriguez@gmail.com>
Date:   Thu, 16 Mar 2023 11:29:26 +0100
Message-ID: <CABDcavZWz=YOvZnW8pkQmuTVjTDxPPoa0zOiC7A_0HAEg_Vi3w@mail.gmail.com>
Subject: Change of behaviour when console=null and ttynull driver is used
To:     linux-kernel@vger.kernel.org
Cc:     Petr Mladek <pmladek@suse.com>,
        Alejandro Vazquez <avazquez.dev@gmail.com>,
        Shreyas Joshi <shreyas.joshi@biamp.com>,
        sergey.senozhatsky@gmail.com, rostedt@goodmis.org,
        shreyasjoshi15@gmail.com, Guenter Roeck <linux@roeck-us.net>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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/

-- 
Guillermo Rodriguez Garcia
guille.rodriguez@gmail.com