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.

Signed-off-by: John Crispin <[e-mail]>
Signed-off-by: Felix Fietkau <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

include/linux/serial_core.h

···
 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

···
 	  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