tty/sysrq: constify the sysrq API

The user is not supposed to thinker with the underlying sysrq_key_op.
Make that explicit by adding a handful of const notations.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://lore.kernel.org/r/20200513214351.2138580-2-emil.l.velikov@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/admin-guide/sysrq.rst

@@ -231,13 +231,13 @@ prints help, and C) an action_msg string, that will print right before your
 handler is called. Your handler must conform to the prototype in 'sysrq.h'.
 
 After the ``sysrq_key_op`` is created, you can call the kernel function
-``register_sysrq_key(int key, struct sysrq_key_op *op_p);`` this will
+``register_sysrq_key(int key, const struct sysrq_key_op *op_p);`` this will
 register the operation pointed to by ``op_p`` at table key 'key',
 if that slot in the table is blank. At module unload time, you must call
-the function ``unregister_sysrq_key(int key, struct sysrq_key_op *op_p)``, which
-will remove the key op pointed to by 'op_p' from the key 'key', if and only if
-it is currently registered in that slot. This is in case the slot has been
-overwritten since you registered it.
+the function ``unregister_sysrq_key(int key, const struct sysrq_key_op *op_p)``,
+which will remove the key op pointed to by 'op_p' from the key 'key', if and
+only if it is currently registered in that slot. This is in case the slot has
+been overwritten since you registered it.
 
 The Magic SysRQ system works by registering key operations against a key op
 lookup table, which is defined in 'drivers/tty/sysrq.c'. This key table has

drivers/tty/sysrq.c

@@ -518,9 +518,9 @@ static int sysrq_key_table_key2index(int key)
 /*
  * get and put functions for the table, exposed to modules.
  */
-static struct sysrq_key_op *__sysrq_get_key_op(int key)
+static const struct sysrq_key_op *__sysrq_get_key_op(int key)
 {
-        struct sysrq_key_op *op_p = NULL;
+        const struct sysrq_key_op *op_p = NULL;
         int i;
 
 	i = sysrq_key_table_key2index(key);
@@ -530,7 +530,7 @@ static struct sysrq_key_op *__sysrq_get_key_op(int key)
         return op_p;
 }
 
-static void __sysrq_put_key_op(int key, struct sysrq_key_op *op_p)
+static void __sysrq_put_key_op(int key, const struct sysrq_key_op *op_p)
 {
         int i = sysrq_key_table_key2index(key);
 
@@ -540,7 +540,7 @@ static void __sysrq_put_key_op(int key, struct sysrq_key_op *op_p)
 
 void __handle_sysrq(int key, bool check_mask)
 {
-	struct sysrq_key_op *op_p;
+	const struct sysrq_key_op *op_p;
 	int orig_log_level;
 	int orig_suppress_printk;
 	int i;
@@ -1063,8 +1063,8 @@ int sysrq_toggle_support(int enable_mask)
 }
 EXPORT_SYMBOL_GPL(sysrq_toggle_support);
 
-static int __sysrq_swap_key_ops(int key, struct sysrq_key_op *insert_op_p,
-                                struct sysrq_key_op *remove_op_p)
+static int __sysrq_swap_key_ops(int key, const struct sysrq_key_op *insert_op_p,
+                                const struct sysrq_key_op *remove_op_p)
 {
 	int retval;
 
@@ -1087,13 +1087,13 @@ static int __sysrq_swap_key_ops(int key, struct sysrq_key_op *insert_op_p,
 	return retval;
 }
 
-int register_sysrq_key(int key, struct sysrq_key_op *op_p)
+int register_sysrq_key(int key, const struct sysrq_key_op *op_p)
 {
 	return __sysrq_swap_key_ops(key, op_p, NULL);
 }
 EXPORT_SYMBOL(register_sysrq_key);
 
-int unregister_sysrq_key(int key, struct sysrq_key_op *op_p)
+int unregister_sysrq_key(int key, const struct sysrq_key_op *op_p)
 {
 	return __sysrq_swap_key_ops(key, NULL, op_p);
 }

include/linux/sysrq.h

@@ -30,10 +30,10 @@
 #define SYSRQ_ENABLE_RTNICE	0x0100
 
 struct sysrq_key_op {
-	void (*handler)(int);
-	char *help_msg;
-	char *action_msg;
-	int enable_mask;
+	void (* const handler)(int);
+	const char * const help_msg;
+	const char * const action_msg;
+	const int enable_mask;
 };
 
 #ifdef CONFIG_MAGIC_SYSRQ
@@ -45,8 +45,8 @@ struct sysrq_key_op {
 
 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);
+int register_sysrq_key(int key, const struct sysrq_key_op *op);
+int unregister_sysrq_key(int key, const struct sysrq_key_op *op);
 extern struct sysrq_key_op *__sysrq_reboot_op;
 
 int sysrq_toggle_support(int enable_mask);
@@ -62,12 +62,12 @@ static inline void __handle_sysrq(int key, bool check_mask)
 {
 }
 
-static inline int register_sysrq_key(int key, struct sysrq_key_op *op)
+static inline int register_sysrq_key(int key, const struct sysrq_key_op *op)
 {
 	return -EINVAL;
 }
 
-static inline int unregister_sysrq_key(int key, struct sysrq_key_op *op)
+static inline int unregister_sysrq_key(int key, const struct sysrq_key_op *op)
 {
 	return -EINVAL;
 }