futex2: Add sysfs entry for syscall numbers

In the course of futex2 development, it will be rebased on top of
different kernel releases, and the syscall number can change in this
process. Expose futex2 syscall number via sysfs so tools that are
experimenting with futex2 (like Proton/Wine) can test it and set the
syscall number at runtime, rather than setting it at compilation time.

Signed-off-by: André Almeida <andrealmeid@collabora.com>

kernel/futex2.c

@@ -115,3 +115,27 @@ SYSCALL_DEFINE4(futex_waitv, struct futex_waitv __user *, waiters,
 	kfree(futexv);
 	return ret;
 }
+
+static ssize_t waitv_show(struct kobject *kobj, struct kobj_attribute *attr,
+			     char *buf)
+{
+	return sprintf(buf, "%u\n", __NR_futex_waitv);
+
+}
+static struct kobj_attribute futex2_waitv_attr = __ATTR_RO(waitv);
+
+static struct attribute *futex2_sysfs_attrs[] = {
+	&futex2_waitv_attr.attr,
+	NULL,
+};
+
+static const struct attribute_group futex2_sysfs_attr_group = {
+	.attrs = futex2_sysfs_attrs,
+	.name = "futex2",
+};
+
+static int __init futex2_sysfs_init(void)
+{
+	return sysfs_create_group(kernel_kobj, &futex2_sysfs_attr_group);
+}
+subsys_initcall(futex2_sysfs_init);