kernel/ksysfs.c: export kernel cpu byteorder

Certain files in procfs are formatted in byteorder-dependent formats.
For example the IP addresses in /proc/net/udp.

When using emulation like qemu-user, applications are not guaranteed to
be using the same byteorder as the kernel.
Therefore the kernel needs to provide a way for applications to discover
the byteorder used in API-filesystems.
Using systemcalls is not enough because these are intercepted and
translated by the emulation.

Also this makes it easier for non-compiled applications like
shellscripts to discover the byteorder.
Signed-off-by: NThomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20221103152407.3348-1-linux@weissschuh.netSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-kernel-cpu_byteorder

+What:		/sys/kernel/cpu_byteorder
+Date:		February 2023
+KernelVersion:	6.2
+Contact:	Thomas Weißschuh <linux@weissschuh.net>
+Description:
+		The endianness of the running kernel.
+
+		Access: Read
+
+		Valid values:
+			"little", "big"
+Users:		util-linux

kernel/ksysfs.c

@@ -6,6 +6,7 @@
  * Copyright (C) 2004 Kay Sievers <kay.sievers@vrfy.org>
  */
 
+#include <asm/byteorder.h>
 #include <linux/kobject.h>
 #include <linux/string.h>
 #include <linux/sysfs.h>
@@ -20,6 +21,14 @@
 
 #include <linux/rcupdate.h>	/* rcu_expedited and rcu_normal */
 
+#if defined(__LITTLE_ENDIAN)
+#define CPU_BYTEORDER_STRING	"little"
+#elif defined(__BIG_ENDIAN)
+#define CPU_BYTEORDER_STRING	"big"
+#else
+#error Unknown byteorder
+#endif
+
 #define KERNEL_ATTR_RO(_name) \
 static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
 
@@ -34,6 +43,14 @@ static ssize_t uevent_seqnum_show(struct kobject *kobj,
 }
 KERNEL_ATTR_RO(uevent_seqnum);
 
+/* cpu byteorder */
+static ssize_t cpu_byteorder_show(struct kobject *kobj,
+				  struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%s\n", CPU_BYTEORDER_STRING);
+}
+KERNEL_ATTR_RO(cpu_byteorder);
+
 #ifdef CONFIG_UEVENT_HELPER
 /* uevent helper program, used during early boot */
 static ssize_t uevent_helper_show(struct kobject *kobj,
@@ -215,6 +232,7 @@ EXPORT_SYMBOL_GPL(kernel_kobj);
 static struct attribute * kernel_attrs[] = {
 	&fscaps_attr.attr,
 	&uevent_seqnum_attr.attr,
+	&cpu_byteorder_attr.attr,
 #ifdef CONFIG_UEVENT_HELPER
 	&uevent_helper_attr.attr,
 #endif