linux-kernel - Re: [PATCH] printk: handle blank console arguments passed in.

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Date:   Thu, 8 Oct 2020 11:01:55 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Shreyas Joshi <shreyas.joshi@...mp.com>, rostedt@...dmis.org,
        shreyasjoshi15@...il.com, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] printk: handle blank console arguments passed in.

On Thu 2020-10-08 14:52:38, Sergey Senozhatsky wrote:
> On (20/10/08 01:29), Sergey Senozhatsky wrote:
> > On (20/10/07 08:57), Guenter Roeck wrote:
> > > On 10/7/20 5:30 AM, Sergey Senozhatsky wrote:
> > 
> > [..]
> > 
> > > I can see to options: Link /dev/console to /dev/null if there is no console,
> > > or do something like
> > > 
> > > 	if (IS_ERR(file)) {
> > >                 pr_warn("Warning: unable to open an initial console.\n");
> > >                 file = filp_open("/dev/null", O_RDWR, 0);
> > > 		if (IS_ERR(file))
> > >                 	return;
> > >         }
> > 
> > As far as I can tell, /dev/null does not exist yet on this stage
> > (at least not in my system). But generally the idea looks interesting.
> 
> Hmm. How about this. console= is undocumented and unspecified - it
> may work sometimes or it may kill the system (and theoretically even
> corrupt some files, depending on what fd 1 and fd 2 point to). So
> maybe we can document console= and handle it in printk, rather than
> somewhere deep in init/main.c
> 
> IOW add one more flag (yeah, I know) and set it when console_setup()
> sees console= boot param. The idea is allow console registration,
> but all consoles should be disabled (cleared CON_ENABLED bit). This
> would be easier to document, at least.
> 
> Schematically:
> 
> ---
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 929e86a01148..b71ff9d87693 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -281,6 +281,7 @@ static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES];
>  
>  static int preferred_console = -1;
>  static bool has_preferred_console;
> +static bool mute_consoles = false;
>  int console_set_on_cmdline;
>  EXPORT_SYMBOL(console_set_on_cmdline);
>  
> @@ -2141,6 +2142,9 @@ static int __add_preferred_console(char *name, int idx, char *options,
>  	struct console_cmdline *c;
>  	int i;
>  
> +	if (mute_consoles)
> +		return;
> +
>  	/*
>  	 *	See if this tty is not yet registered, and
>  	 *	if we have a slot free.

Interesting idea. Well, it looks like yet another mess:

  + it would show the consoles in /proc/consoles
    even thought they will be basically unusable

  + it is yet another way to affect the amount of messages
    on console. We already have console_loglevel, ignore_loglevel.

  + this effect is far from obvious when using console=""


IMHO, we should try to understand why it actually crashes first.
It might help to solve the problem some cleaner way.

Thanks a lot for digging into it.

Best Regards,
Petr