linux-kernel - Re: printk: Add kernel parameter to disable writes to /dev/kmsg

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Message-ID: <20160425185015.GJ3448@twins.programming.kicks-ass.net>
Date:	Mon, 25 Apr 2016 20:50:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: printk: Add kernel parameter to disable writes to /dev/kmsg

On Mon, Apr 25, 2016 at 01:06:29PM -0400, Steven Rostedt wrote:
> Over the weekend my server was acting funny. The display wasn't working
> well, and I assumed that a driver was going bad. I went to look at the
> kernel dmesg, but the buffer only had the following over and over:
> 
> [226062.401405] systemd-logind[3511]: Removed session 4168.
> [226063.381051] systemd-logind[3511]: Removed session 4169.

> The kernel buffer was completely overridden by useless spewing from

Yes it does this. I usually fix this with: apt-get install sysvinit

> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index bfbf284e4218..b704b48415a0 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -85,6 +85,15 @@ static struct lockdep_map console_lock_dep_map = {
>  };
>  #endif
>  
> +static bool devkmsg_disabled;
> +static int __init disable_devkmsg(char *str)
> +{
> +	devkmsg_disabled = true;
> +	return 0;
> +}
> +__setup("disable_devkmsg_write", disable_devkmsg);
> +

Can't we default enable this? Or at the very least make this easier to
type? I'll never remember how exactly this thing will be called.