reboot: hide from sysfs not applicable settings

Not all the reboot settings from both the kernel command line or sysfs
interface are available to all platforms.

Filter out reboot_type and reboot_force which are x86 only, and also
remove reboot_cpu on kernels without SMP support.

This saves some space, and avoid confusing the user with settings which
will have no effect.

Link: https://lkml.kernel.org/r/20201130173717.198952-3-mcroce@linux.microsoft.com
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/reboot.c

@@ -668,6 +668,29 @@ static ssize_t mode_store(struct kobject *kobj, struct kobj_attribute *attr,
 }
 static struct kobj_attribute reboot_mode_attr = __ATTR_RW(mode);
 
+#ifdef CONFIG_X86
+static ssize_t force_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%d\n", reboot_force);
+}
+static ssize_t force_store(struct kobject *kobj, struct kobj_attribute *attr,
+			  const char *buf, size_t count)
+{
+	bool res;
+
+	if (!capable(CAP_SYS_BOOT))
+		return -EPERM;
+
+	if (kstrtobool(buf, &res))
+		return -EINVAL;
+
+	reboot_default = 0;
+	reboot_force = res;
+
+	return count;
+}
+static struct kobj_attribute reboot_force_attr = __ATTR_RW(force);
+
 static ssize_t type_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
 {
 	const char *val;
@@ -723,7 +746,9 @@ static ssize_t type_store(struct kobject *kobj, struct kobj_attribute *attr,
 	return count;
 }
 static struct kobj_attribute reboot_type_attr = __ATTR_RW(type);
+#endif
 
+#ifdef CONFIG_SMP
 static ssize_t cpu_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
 {
 	return sprintf(buf, "%d\n", reboot_cpu);
@@ -751,34 +776,17 @@ static ssize_t cpu_store(struct kobject *kobj, struct kobj_attribute *attr,
 	return count;
 }
 static struct kobj_attribute reboot_cpu_attr = __ATTR_RW(cpu);
-
-static ssize_t force_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
-{
-	return sprintf(buf, "%d\n", reboot_force);
-}
-static ssize_t force_store(struct kobject *kobj, struct kobj_attribute *attr,
-			  const char *buf, size_t count)
-{
-	bool res;
-
-	if (!capable(CAP_SYS_BOOT))
-		return -EPERM;
-
-	if (kstrtobool(buf, &res))
-		return -EINVAL;
-
-	reboot_default = 0;
-	reboot_force = res;
-
-	return count;
-}
-static struct kobj_attribute reboot_force_attr = __ATTR_RW(force);
+#endif
 
 static struct attribute *reboot_attrs[] = {
 	&reboot_mode_attr.attr,
+#ifdef CONFIG_X86
+	&reboot_force_attr.attr,
 	&reboot_type_attr.attr,
+#endif
+#ifdef CONFIG_SMP
 	&reboot_cpu_attr.attr,
-	&reboot_force_attr.attr,
+#endif
 	NULL,
 };