tty/sysrq: alpha: export and use __sysrq_get_key_op()

Export a pointer to the sysrq_get_key_op(). This way we can cleanly
unregister it, instead of the current solutions of modifuing it inplace.

Since __sysrq_get_key_op() is no longer used externally, let's make it
a static function.

This patch will allow us to limit access to each and every sysrq op and
constify the sysrq handling.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-kernel@vger.kernel.org
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: linux-alpha@vger.kernel.org
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://lore.kernel.org/r/20200513214351.2138580-1-emil.l.velikov@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/alpha/kernel/setup.c

@@ -430,6 +430,15 @@ register_cpus(void)
 
 arch_initcall(register_cpus);
 
+#ifdef CONFIG_MAGIC_SYSRQ
+static struct sysrq_key_op srm_sysrq_reboot_op = {
+	.handler	= machine_halt,
+	.help_msg       = "reboot(b)",
+	.action_msg     = "Resetting",
+	.enable_mask    = SYSRQ_ENABLE_BOOT,
+};
+#endif
+
 void __init
 setup_arch(char **cmdline_p)
 {
@@ -550,8 +559,8 @@ setup_arch(char **cmdline_p)
 	/* If we're using SRM, make sysrq-b halt back to the prom,
 	   not auto-reboot.  */
 	if (alpha_using_srm) {
-		struct sysrq_key_op *op = __sysrq_get_key_op('b');
-		op->handler = (void *) machine_halt;
+		unregister_sysrq_key('b', __sysrq_reboot_op);
+		register_sysrq_key('b', &srm_sysrq_reboot_op);
 	}
 #endif
 

drivers/tty/sysrq.c

@@ -172,6 +172,8 @@ static struct sysrq_key_op sysrq_reboot_op = {
 	.enable_mask	= SYSRQ_ENABLE_BOOT,
 };
 
+struct sysrq_key_op *__sysrq_reboot_op = &sysrq_reboot_op;
+
 static void sysrq_handle_sync(int key)
 {
 	emergency_sync();
@@ -516,7 +518,7 @@ static int sysrq_key_table_key2index(int key)
 /*
  * get and put functions for the table, exposed to modules.
  */
-struct sysrq_key_op *__sysrq_get_key_op(int key)
+static struct sysrq_key_op *__sysrq_get_key_op(int key)
 {
         struct sysrq_key_op *op_p = NULL;
         int i;

include/linux/sysrq.h

@@ -47,7 +47,7 @@ void handle_sysrq(int key);
 void __handle_sysrq(int key, bool 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);
+extern struct sysrq_key_op *__sysrq_reboot_op;
 
 int sysrq_toggle_support(int enable_mask);
 int sysrq_mask(void);