Re: [PATCH-next 3/3] serial/sysrq: Add MAGIC_SYSRQ_SERIAL_SEQUENCE

[Greg Kroah-Hartman]

On Thu, Jan 09, 2020 at 09:54:44PM +0000, Dmitry Safonov wrote:
> Many embedded boards have a disconnected TTL level serial which can
> generate some garbage that can lead to spurious false sysrq detects.
> 
> Currently, sysrq can be either completely disabled for serial console
> or always disabled (with CONFIG_MAGIC_SYSRQ_SERIAL), since
> commit 732dbf3a6104 ("serial: do not accept sysrq characters via serial port")
> 
> At Arista, we have such boards that can generate BREAK and random
> garbage. While disabling sysrq for serial console would solve
> the problem with spurious false sysrq triggers, it's also desirable
> to have a way to enable sysrq back.
> 
> As a measure of balance between on and off options, add
> MAGIC_SYSRQ_SERIAL_SEQUENCE which is a string sequence that can enable
> sysrq if it follows BREAK on a serial line. The longer the string - the
> less likely it may be in the garbage.
> 
> Having the way to enable sysrq was beneficial to debug lockups with
> a manual investigation in field and on the other side preventing false
> sysrq detections.
> 
> Based-on-patch-by: Vasiliy Khoruzhick <vasilykh@xxxxxxxxxx>
> Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
> ---
>  drivers/tty/serial/serial_core.c | 52 ++++++++++++++++++++++++++++----
>  include/linux/serial_core.h      |  2 +-
>  lib/Kconfig.debug                |  8 +++++
>  3 files changed, 55 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 6ac9dfed3423..f70eba032d0b 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -3081,6 +3081,38 @@ void uart_insert_char(struct uart_port *port, unsigned int status,
>  }
>  EXPORT_SYMBOL_GPL(uart_insert_char);
>  
> +const char sysrq_toggle_seq[] = CONFIG_MAGIC_SYSRQ_SERIAL_SEQUENCE;
> +
> +static void uart_sysrq_on(struct work_struct *w)
> +{
> +	sysrq_toggle_support(1);
> +	pr_info("SysRq is enabled by magic sequience on serial\n");

Do we want to say what serial port it is enabled on?

And why is this done in a workqueue?

> +}
> +static DECLARE_WORK(sysrq_enable_work, uart_sysrq_on);
> +
> +static int uart_try_toggle_sysrq(struct uart_port *port, unsigned int ch)
> +{
> +	if (sysrq_toggle_seq[0] == '\0')
> +		return 0;

Is constantly checking the data stream like this going to slow things
down overall?  Ah, we are just checking this after BREAK, right?  So
that hopefully will not be that bad...

> +
> +	BUILD_BUG_ON(ARRAY_SIZE(sysrq_toggle_seq) >= sizeof(port->sysrq_seq)*U8_MAX);
> +	if (sysrq_toggle_seq[port->sysrq_seq] != ch) {
> +		port->sysrq_seq = 0;
> +		return 0;
> +	}
> +
> +	/* Without the last \0 */
> +	if (++port->sysrq_seq < (ARRAY_SIZE(sysrq_toggle_seq) - 1)) {
> +		port->sysrq = jiffies + HZ*5;

5 second delay?  You should document what this value is for somewhere
here...

> +		return 1;
> +	}
> +
> +	schedule_work(&sysrq_enable_work);
> +
> +	port->sysrq = 0;
> +	return 1;
> +}
> +
>  int uart_handle_sysrq_char(struct uart_port *port, unsigned int ch)
>  {
>  	if (!IS_ENABLED(CONFIG_MAGIC_SYSRQ_SERIAL))
> @@ -3090,9 +3122,13 @@ int uart_handle_sysrq_char(struct uart_port *port, unsigned int ch)
>  		return 0;
>  
>  	if (ch && time_before(jiffies, port->sysrq)) {
> -		handle_sysrq(ch);
> -		port->sysrq = 0;
> -		return 1;
> +		if (sysrq_get_mask()) {
> +			handle_sysrq(ch);
> +			port->sysrq = 0;
> +			return 1;
> +		}

Isn't this change to test for sysrq_get_mask() a different change than
checking for the "magic" data stream?

> +		if (uart_try_toggle_sysrq(port, ch))
> +			return 1;
>  	}
>  	port->sysrq = 0;
>  
> @@ -3109,9 +3145,13 @@ int uart_prepare_sysrq_char(struct uart_port *port, unsigned int ch)
>  		return 0;
>  
>  	if (ch && time_before(jiffies, port->sysrq)) {
> -		port->sysrq_ch = ch;
> -		port->sysrq = 0;
> -		return 1;
> +		if (sysrq_get_mask()) {
> +			port->sysrq_ch = ch;
> +			port->sysrq = 0;
> +			return 1;
> +		}
> +		if (uart_try_toggle_sysrq(port, ch))
> +			return 1;
>  	}
>  	port->sysrq = 0;
>  
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index 255e86a474e9..1f4443db5474 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -243,10 +243,10 @@ struct uart_port {
>  	unsigned long		sysrq;			/* sysrq timeout */
>  	unsigned int		sysrq_ch;		/* char for sysrq */
>  	unsigned char		has_sysrq;
> +	unsigned char		sysrq_seq;		/* index in sysrq_toggle_seq */
>  
>  	unsigned char		hub6;			/* this should be in the 8250 driver */
>  	unsigned char		suspended;
> -	unsigned char		unused;

This is an unrelated change, let's leave it for a different patch that
cleans up the layout of this structure, ok?

thanks,

greg k-h