sysrq: Allow magic SysRq key functions to be disabled through Kconfig

Turn the initial value of sysctl kernel.sysrq (SYSRQ_DEFAULT_ENABLE)
into a Kconfig variable.

Original version by Bastian Blank <[e-mail]>.

Signed-off-by: Ben Hutchings <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/sysrq.txt

···
 You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
 configuring the kernel. When running a kernel with SysRq compiled in,
 /proc/sys/kernel/sysrq controls the functions allowed to be invoked via
-the SysRq key. By default the file contains 1 which means that every
-possible SysRq request is allowed (in older versions SysRq was disabled
-by default, and you were required to specifically enable it at run-time
-but this is not the case any more). Here is the list of possible values
-in /proc/sys/kernel/sysrq:
+the SysRq key. The default value in this file is set by the
+CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults
+to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:
    0 - disable sysrq completely
    1 - enable all functions of sysrq
   >1 - bitmask of allowed sysrq functions (see below for detailed function
···
 You can set the value in the file by the following command:
     echo "number" >/proc/sys/kernel/sysrq
 
-The number may be written either as decimal or as hexadecimal with the
-0x prefix.
+The number may be written here either as decimal or as hexadecimal
+with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be
+written in hexadecimal.
 
 Note that the value of /proc/sys/kernel/sysrq influences only the invocation
 via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always

drivers/tty/sysrq.c

···
 #include <asm/irq_regs.h>
 
 /* Whether we react on sysrq keys or just ignore them */
-static int __read_mostly sysrq_enabled = SYSRQ_DEFAULT_ENABLE;
+static int __read_mostly sysrq_enabled = CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE;
 static bool __read_mostly sysrq_always_enabled;
 
 unsigned short platform_sysrq_reset_seq[] __weak = { KEY_RESERVED };

include/linux/sysrq.h

···
 #include <linux/errno.h>
 #include <linux/types.h>
 
-/* Enable/disable SYSRQ support by default (0==no, 1==yes). */
-#define SYSRQ_DEFAULT_ENABLE	1
-
 /* Possible values of bitmask for enabling sysrq functions */
 /* 0x0001 is reserved for enable everything */
 #define SYSRQ_ENABLE_LOG	0x0002

kernel/sysctl.c

···
 
 #ifdef CONFIG_MAGIC_SYSRQ
 /* Note: sysrq code uses it's own private copy */
-static int __sysrq_enabled = SYSRQ_DEFAULT_ENABLE;
+static int __sysrq_enabled = CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE;
 
 static int sysrq_sysctl_handler(ctl_table *table, int write,
 				void __user *buffer, size_t *lenp,

lib/Kconfig.debug

···
 	  keys are documented in <file:Documentation/sysrq.txt>. Don't say Y
 	  unless you really know what this hack does.
 
+config MAGIC_SYSRQ_DEFAULT_ENABLE
+	hex "Enable magic SysRq key functions by default"
+	depends on MAGIC_SYSRQ
+	default 0x1
+	help
+	  Specifies which SysRq key functions are enabled by default.
+	  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 DEBUG_KERNEL
 	bool "Kernel debugging"
 	help