linux-kernel - Re: [PATCH] printk: Add loglevel for "do not print to consoles".

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Date:   Sat, 25 Apr 2020 01:34:15 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org, Dmitry Safonov <dima@...sta.com>,
        Michal Hocko <mhocko@...e.com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH] printk: Add loglevel for "do not print to consoles".

On 2020/04/25 1:21, Steven Rostedt wrote:
> On Sat, 25 Apr 2020 01:10:15 +0900
> Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp> wrote:
> 
>> KERN_NO_CONSOLES is a mechanism for implementing user decided policy. As long as
>> userspace can control whether to use KERN_NO_CONSOLES (e.g. sysctl), there should
>> be no problem with adding KERN_NO_CONSOLES (i.e. this patch) to the kernel side.
> 
> How would you define what gets "KERN_NO_CONSOLES"? Is it going to be a
> sysctl switch?

Yes. See
https://lkml.kernel.org/r/c95dfafb-fe9c-19d7-8d42-bcd7d0946867@i-love.sakura.ne.jp .

> 
> Also, how does one control the log level of prints with KERN_NO_CONSOLES?

I couldn't understand the question.

KERN_NO_CONSOLES is just a flag for not to call call_console_drivers().
Messages printed with printk(KERN_$LOGLEVEL KERN_NO_CONSOLES ...) will be
read by userspace syslog daemon and be filtered based on KERN_$LOGLEVEL.