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

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Message-ID: <20201008122042.GB127397@jagdpanzerIV.localdomain>
Date:   Thu, 8 Oct 2020 21:20:42 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        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 (20/10/08 10:50), Petr Mladek wrote:
> On Wed 2020-10-07 21:30:44, Sergey Senozhatsky wrote:
> > On (20/10/07 09:28), Petr Mladek wrote:
> > > 
> > > 		/*
> > > 		 * Dirty hack to prevent using any console with tty
> > > 		 * binding as a fallback and adding the empty
> > > 		 * name into console_cmdline array.
> > > 		 */
> > > 		preferred_console = MAX_CMDLINECONSOLES;
> > 
> > Let me dump my findings so far. I still don't understand what exactly
> > crashes the laptop (blank screen is not very helpful).
> > 
> > So, things start with the "preferred_console = -1". In console_setup()
> > we call __add_preferred_console(). Since we have no consoles, the
> > name matching loop is not executed, and console selection counter remains
> > at 0. After the loop, despite the fact that we don't have the console
> > (`name' is empty), we still set `preferred_console', to 0.
> 
> Heh, we actually add the console.

To the console drovers list? I don't think so. At least on my laptop
what I have is as follows:

	/* See if this console matches one we selected on the command line */
	err = try_enable_new_console(newcon, true);

	/* If not, try to match against the platform default(s) */
	if (err == -ENOENT)
		err = try_enable_new_console(newcon, false);

	/* printk() messages are not printed to the Braille console. */
	if (err || newcon->flags & CON_BRL)
		return;

We hit this error return. Because both try_enable_new_console() return
-ENOENT. So this is never executed

	...
	console_lock();
	if ((newcon->flags & CON_CONSDEV) || console_drivers == NULL) {
		newcon->next = console_drivers;
		console_drivers = newcon;
		if (newcon->next)
			newcon->next->flags &= ~CON_CONSDEV;
		/* Ensure this flag is always set for the head of the list */
		newcon->flags |= CON_CONSDEV;
	} else {
		newcon->next = console_drivers->next;
		console_drivers->next = newcon;
	}
	...

The console driver list is 0x00.

> I wonder if you see the problem solved by the commit 2d3145f8d2809592ef8
> ("early init: fix error handling when opening /dev/console").

/dev/console does exist. What does not exist is console driver, because
console drivers list is NULL. So the failure here is not filp_open()
per se, but tty_lookup_driver()->console_device(), which returns NULL.
As far as I'm concerned.

> I am also curious about the commit 74f1a299107b9e1a56 "Revert "fs:
> remove ksys_dup()"". I wonder why it was safe to call ksys_dup(0);
> even though the previous ksys_open() failed.

I'm quite sure ksys_dup(0) fails, in fact. I guess the issue here boils
down to user-space that does modprobe/fsck/mount and what kind of things
it attempts to do with standard file descriptors 0/1/2.

> PS: I am quite busy with something else this week.

Sure, no prob. Thanks.

	-ss