linux-kernel - Re: [PATCH v2 1/2] printk: Check valid console index for preferred console

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Date:   Wed, 11 Oct 2023 17:26:07 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Tony Lindgren <tony@...mide.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] printk: Check valid console index for preferred
 console

On Wed 2023-10-11 12:18:03, Tony Lindgren wrote:
> * Greg Kroah-Hartman <gregkh@...uxfoundation.org> [231011 07:53]:
> > On Wed, Oct 11, 2023 at 10:43:25AM +0300, Tony Lindgren wrote:
> > > Let's check for valid console index values to avoid bogus console index
> > > numbers from kernel command line. While struct console uses short for
> > > index, and negative index values are used by some device drivers, we do
> > > not want to allow negative values for preferred console.
> > 
> > What drivers use a negative index for the console?
> 
> This is based on grepping with $ git grep "co->index =" drivers/tty/
> 
> Not sure what all might be stopping making struct console index unsigned.

The value -1 is used for initializing struct console, see:

$> git grep -A10 "struct console.*=" | \
   grep -e "struct console" -e index | \
   grep -B1 index
[...]
drivers/tty/serial/8250/8250_core.c:static struct console univ8250_console = {
drivers/tty/serial/8250/8250_core.c-    .index          = -1,
[...]
drivers/tty/serial/imx.c:static struct console imx_uart_console = {
drivers/tty/serial/imx.c-       .index          = -1,
drivers/tty/serial/ip22zilog.c:static struct console ip22zilog_console = {
drivers/tty/serial/ip22zilog.c- .index  =       -1,
drivers/tty/serial/kgdb_nmi.c:static struct console kgdb_nmi_console = {
drivers/tty/serial/kgdb_nmi.c-  .index  = -1,
drivers/tty/serial/lantiq.c:static struct console lqasc_console = {
drivers/tty/serial/lantiq.c-    .index =        -1,
drivers/tty/serial/liteuart.c:static struct console liteuart_console = {
drivers/tty/serial/liteuart.c-  .index = -1,
drivers/tty/serial/lpc32xx_hs.c:static struct console lpc32xx_hsuart_console = {
drivers/tty/serial/lpc32xx_hs.c-        .index          = -1,
drivers/tty/serial/ma35d1_serial.c:static struct console ma35d1serial_console = {
drivers/tty/serial/ma35d1_serial.c-     .index   = -1,
drivers/tty/serial/mcf.c:static struct console mcf_console = {
drivers/tty/serial/mcf.c-       .index          = -1,
[...]

It means that the index still has be get assigned. For example, it is
used here:

static int try_enable_preferred_console(struct console *newcon,
					bool user_specified)
{
[...]
			if (newcon->index < 0)
				newcon->index = c->index;
[...]


Resume:

1. We must either keep signed short in struct console or
   use another check for non-yet assigned index.

2. We should fix the commit message and the comment. We should
   explain that negative value is used in struct console
   to distinguish a non-yet-registered/assigned index/port.

Best Regards,
Petr