[PATCH] serial: do not accept sysrq characters via serial port

[John Crispin]

From: Felix Fietkau <nbd@xxxxxxxx>

many embedded boards have a disconnected TTL level serial which can
generate some garbage that can lead to spurious false sysrq detects.

Signed-off-by: John Crispin <john@xxxxxxxxxxx>
Signed-off-by: Felix Fietkau <nbd@xxxxxxxx>
---
 include/linux/serial_core.h |    2 +-
 lib/Kconfig.debug           |   10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 3442014..7e6165b 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -448,7 +448,7 @@ extern void uart_handle_cts_change(struct uart_port *uport,
 extern void uart_insert_char(struct uart_port *port, unsigned int status,
 		 unsigned int overrun, unsigned int ch, unsigned int flag);
 
-#ifdef SUPPORT_SYSRQ
+#if defined(SUPPORT_SYSRQ) && defined(CONFIG_MAGIC_SYSRQ_SERIAL)
 static inline int
 uart_handle_sysrq_char(struct uart_port *port, unsigned int ch)
 {
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index a6c8db1..e370d0a 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -397,6 +397,16 @@ config MAGIC_SYSRQ_DEFAULT_ENABLE
 	  This may be set to 1 or 0 to enable or disable them all, or
 	  to a bitmask as described in Documentation/sysrq.txt.
 
+config MAGIC_SYSRQ_SERIAL
+	bool "Enable magic SysRq key over serial"
+	depends on MAGIC_SYSRQ
+	default y
+	help
+	  Many embedded boards have a disconnected TTL level serial which can
+	  generate some garbage that can lead to spurious false sysrq detects.
+	  This option allows you to decide whether you want to enable the
+	  magic SysRq key.
+
 config DEBUG_KERNEL
 	bool "Kernel debugging"
 	help
-- 
1.7.10.4