[PATCH v2] proc: add byteorder file

["Thomas Weißschuh" <linux@weissschuh.net>]

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

Assuming the byteorder of the userspace program is not guaranteed to be
correct in the face of emulation as for example with qemu-user.

Also this makes it easier for non-compiled applications like
shellscripts to discover the byteorder.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

---

Development of userspace part: https://github.com/util-linux/util-linux/pull/1872

v1: https://lore.kernel.org/lkml/20221101005043.1791-1-linux@weissschuh.net/
v1->v2:
  * Move file to /sys/kernel/byteorder
---
 .../ABI/testing/sysfs-kernel-byteorder         | 12 ++++++++++++
 kernel/ksysfs.c                                | 18 ++++++++++++++++++
 2 files changed, 30 insertions(+)
 create mode 100644 Documentation/ABI/testing/sysfs-kernel-byteorder

diff --git a/Documentation/ABI/testing/sysfs-kernel-byteorder b/Documentation/ABI/testing/sysfs-kernel-byteorder
new file mode 100644
index 000000000000..4c45016d78ae
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-kernel-byteorder
@@ -0,0 +1,12 @@
+What:		/sys/kernel/byteorder
+Date:		February 2023
+KernelVersion:	6.2
+Contact:	linux-fsdevel@vger.kernel.org
+Description:
+		The current endianness of the running kernel.
+
+		Access: Read
+
+		Valid values:
+			"little", "big"
+Users:		util-linux
diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
index 65dba9076f31..7c7cb2c96ac0 100644
--- a/kernel/ksysfs.c
+++ b/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 BYTEORDER_STRING	"little"
+#elif defined(__BIG_ENDIAN)
+#define 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);
 
+/* kernel byteorder */
+static ssize_t byteorder_show(struct kobject *kobj,
+			      struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%s\n", BYTEORDER_STRING);
+}
+KERNEL_ATTR_RO(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,
+	&byteorder_attr.attr,
 #ifdef CONFIG_UEVENT_HELPER
 	&uevent_helper_attr.attr,
 #endif

base-commit: 5aaef24b5c6d4246b2cac1be949869fa36577737
-- 
2.38.1