serial: do not accept sysrq characters via serial port

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

[john@phrozen.org: sent upstream 22.12.2016]

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/linux/serial_core.h

@@ -455,7 +455,7 @@ 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)
 {

lib/Kconfig.debug

@@ -396,6 +396,11 @@ 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
+
 config DEBUG_KERNEL
 	bool "Kernel debugging"
 	help