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SubjectRe: [PATCH-next 3/3] serial/sysrq: Add MAGIC_SYSRQ_SERIAL_SEQUENCE
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@arista.com>
> Signed-off-by: Dmitry Safonov <dima@arista.com>
> ---
> 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

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