linux-kernel - Re: [PATCH printk v3 33/40] printk, xen: fbfront: create/use safe function for forcing preferred

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Date:   Fri, 11 Nov 2022 11:48:03 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Helge Deller <deller@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Tom Rix <trix@...hat.com>, linux-fbdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH printk v3 33/40] printk, xen: fbfront: create/use safe
 function for forcing preferred

On Mon 2022-11-07 15:22:31, John Ogness wrote:
> With commit 9e124fe16ff2("xen: Enable console tty by default in domU
> if it's not a dummy") a hack was implemented to make sure that the
> tty console remains the console behind the /dev/console device. The
> main problem with the hack is that, after getting the console pointer
> to the tty console, it is assumed the pointer is still valid after
> releasing the console_sem. This assumption is incorrect and unsafe.
> 
> Make the hack safe by introducing a new function
> console_force_preferred_locked() and perform the full operation
> under the console_list_lock.
> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3457,6 +3458,43 @@ int unregister_console(struct console *console)
>  }
>  EXPORT_SYMBOL(unregister_console);
>  
> +/**
> + * console_force_preferred_locked - force a registered console preferred
> + * @con: The registered console to force preferred.
> + *
> + * Must be called under console_list_lock().
> + */
> +void console_force_preferred_locked(struct console *con)
> +{
> +	struct console *cur_pref_con;

One more thing. It would make sense to check that it has
really been called under console_list_lock():

	lockdep_assert_console_list_lock_held();

> +
> +	if (!console_is_registered_locked(con))
> +		return;
> +
> +	cur_pref_con = console_first();
> +
> +	/* Already preferred? */
> +	if (cur_pref_con == con)
> +		return;
> +

Best Regards,
Petr