[PATCH] debug: add sysrq_always_enabled boot option

Most distributions enable sysrq support but set it to 0 by default. Add a
sysrq_always_enabled boot option to always-enable sysrq keys. Useful for
debugging - without having to modify the disribution's config files (which
might not be possible if the kernel is on a live CD, etc.).

Also, while at it, clean up the sysrq interfaces.

[[e-mail]: make sysrq_always_enabled_setup() static]
Signed-off-by: Ingo Molnar <[e-mail]>
Signed-off-by: Adrian Bunk <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/kernel-parameters.txt

···
 	sym53c416=	[HW,SCSI]
 			See header of drivers/scsi/sym53c416.c.
 
+	sysrq_always_enabled
+			[KNL]
+			Ignore sysrq setting - this boot parameter will
+			neutralize any effect of /proc/sys/kernel/sysrq.
+			Useful for debugging.
+
 	t128=		[HW,SCSI]
 			See header of drivers/scsi/t128.c.
 

drivers/char/sysrq.c

···
 #include <asm/irq_regs.h>
 
 /* Whether we react on sysrq keys or just ignore them */
-int sysrq_enabled = 1;
+int __read_mostly __sysrq_enabled = 1;
+
+static int __read_mostly sysrq_always_enabled;
+
+int sysrq_on(void)
+{
+	return __sysrq_enabled || sysrq_always_enabled;
+}
+
+/*
+ * A value of 1 means 'all', other nonzero values are an op mask:
+ */
+static inline int sysrq_on_mask(int mask)
+{
+	return sysrq_always_enabled || __sysrq_enabled == 1 ||
+						(__sysrq_enabled & mask);
+}
+
+static int __init sysrq_always_enabled_setup(char *str)
+{
+	sysrq_always_enabled = 1;
+	printk(KERN_INFO "debug: sysrq always enabled.\n");
+
+	return 1;
+}
+
+__setup("sysrq_always_enabled", sysrq_always_enabled_setup);
+
 
 static void sysrq_handle_loglevel(int key, struct tty_struct *tty)
 {
···
 		 * Should we check for enabled operations (/proc/sysrq-trigger
 		 * should not) and is the invoked operation enabled?
 		 */
-		if (!check_mask || sysrq_enabled == 1 ||
-		    (sysrq_enabled & op_p->enable_mask)) {
+		if (!check_mask || sysrq_on_mask(op_p->enable_mask)) {
 			printk("%s\n", op_p->action_msg);
 			console_loglevel = orig_log_level;
 			op_p->handler(key, tty);
···
  */
 void handle_sysrq(int key, struct tty_struct *tty)
 {
-	if (!sysrq_enabled)
-		return;
-	__handle_sysrq(key, tty, 1);
+	if (sysrq_on())
+		__handle_sysrq(key, tty, 1);
 }
 EXPORT_SYMBOL(handle_sysrq);
 

drivers/char/viocons.c

···
 static DEFINE_SPINLOCK(consolelock);
 static DEFINE_SPINLOCK(consoleloglock);
 
-#ifdef CONFIG_MAGIC_SYSRQ
 static int vio_sysrq_pressed;
-extern int sysrq_enabled;
-#endif
 
 #define VIOCHAR_NUM_BUF		16
 
···
 	 */
 	num_pushed = 0;
 	for (index = 0; index < cevent->len; index++) {
-#ifdef CONFIG_MAGIC_SYSRQ
-		if (sysrq_enabled) {
+		/*
+		 * Will be optimized away if !CONFIG_MAGIC_SYSRQ:
+		 */
+		if (sysrq_on()) {
 			/* 0x0f is the ascii character for ^O */
 			if (cevent->data[index] == '\x0f') {
 				vio_sysrq_pressed = 1;
···
 				continue;
 			}
 		}
-#endif
 		/*
 		 * The sysrq sequence isn't included in this check if
 		 * sysrq is enabled and compiled into the kernel because

include/linux/sysrq.h

···
 
 #ifdef CONFIG_MAGIC_SYSRQ
 
+extern int sysrq_on(void);
+
+/*
+ * Do not use this one directly:
+ */
+extern int __sysrq_enabled;
+
 /* Generic SysRq interface -- you may call it from any device driver, supplying
  * ASCII code of the key, pointer to registers and kbd/tty structs (if they
  * are available -- else NULL's).
  */
 
-void handle_sysrq(int, struct tty_struct *);
-void __handle_sysrq(int, struct tty_struct *, int check_mask);
-int register_sysrq_key(int, struct sysrq_key_op *);
-int unregister_sysrq_key(int, struct sysrq_key_op *);
+void handle_sysrq(int key, struct tty_struct *tty);
+void __handle_sysrq(int key, struct tty_struct *tty, int check_mask);
+int register_sysrq_key(int key, struct sysrq_key_op *op);
+int unregister_sysrq_key(int key, struct sysrq_key_op *op);
 struct sysrq_key_op *__sysrq_get_key_op(int key);
 
 #else
 
+static inline int sysrq_on(void)
+{
+	return 0;
+}
 static inline int __reterr(void)
 {
 	return -EINVAL;
+}
+static inline void handle_sysrq(int key, struct tty_struct *tty)
+{
 }
 
 #define register_sysrq_key(ig,nore) __reterr()

kernel/sysctl.c

···
 extern int sysctl_overcommit_ratio;
 extern int sysctl_panic_on_oom;
 extern int max_threads;
-extern int sysrq_enabled;
 extern int core_uses_pid;
 extern int suid_dumpable;
 extern char core_pattern[];
···
 	{
 		.ctl_name	= KERN_SYSRQ,
 		.procname	= "sysrq",
-		.data		= &sysrq_enabled,
+		.data		= &__sysrq_enabled,
 		.maxlen		= sizeof (int),
 		.mode		= 0644,
 		.proc_handler	= &proc_dointvec,