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: Thomas Weißschuh <[email protected]>
---
Development of userspace part: https://github.com/util-linux/util-linux/pull/1872
Changelog:
v1: https://lore.kernel.org/lkml/[email protected]/
v1->v2:
* Move file to /sys/kernel/byteorder
v2: https://lore.kernel.org/lkml/[email protected]/
v2->v3:
* Fix commit title to mention sysfs
* Use explicit cpu_byteorder name
* Use sysfs_emit
* Use myself as Contact
* Reword commit message
---
.../ABI/testing/sysfs-kernel-cpu_byteorder | 12 ++++++++++++
kernel/ksysfs.c | 18 ++++++++++++++++++
2 files changed, 30 insertions(+)
create mode 100644 Documentation/ABI/testing/sysfs-kernel-cpu_byteorder
diff --git a/Documentation/ABI/testing/sysfs-kernel-cpu_byteorder b/Documentation/ABI/testing/sysfs-kernel-cpu_byteorder
new file mode 100644
index 000000000000..f0e6ac1b5356
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-kernel-cpu_byteorder
@@ -0,0 +1,12 @@
+What: /sys/kernel/cpu_byteorder
+Date: February 2023
+KernelVersion: 6.2
+Contact: Thomas Weißschuh <[email protected]>
+Description:
+ The 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..2df00b789b90 100644
--- a/kernel/ksysfs.c
+++ b/kernel/ksysfs.c
@@ -6,6 +6,7 @@
* Copyright (C) 2004 Kay Sievers <[email protected]>
*/
+#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
base-commit: 8e5423e991e8cd0988d0c4a3f4ac4ca1af7d148a
--
2.38.1