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

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Message-ID: <20201008055238.GA554@jagdpanzerIV.localdomain>
Date:   Thu, 8 Oct 2020 14:52:38 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Guenter Roeck <linux@...ck-us.net>, Petr Mladek <pmladek@...e.com>
Cc:     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>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] printk: handle blank console arguments passed in.

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.
@@ -2189,6 +2193,11 @@ static int __init console_setup(char *str)
 	char *s, *options, *brl_options = NULL;
 	int idx;
 
+	if (str[0] == 0) {
+		mute_consoles = true;
+		return 0;
+	}
+
 	if (_braille_console_setup(&str, &brl_options))
 		return 1;
 
@@ -2630,6 +2639,9 @@ EXPORT_SYMBOL(console_stop);
 
 void console_start(struct console *console)
 {
+	if (mute_consoles)
+		return;
+
 	console_lock();
 	console->flags |= CON_ENABLED;
 	console_unlock();
@@ -2811,6 +2823,9 @@ void register_console(struct console *newcon)
 		console_drivers->next = newcon;
 	}
 
+	if (mute_consoles)
+		newcon->flags &= ~CON_ENABLED;
+
 	if (newcon->flags & CON_EXTENDED)
 		nr_ext_console_drivers++;