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 <linux@xxxxxxxxxxxxxx> --- Development of userspace part: https://github.com/util-linux/util-linux/pull/1872 Changelog: v1: https://lore.kernel.org/lkml/20221101005043.1791-1-linux@xxxxxxxxxxxxxx/ v1->v2: * Move file to /sys/kernel/byteorder v2: https://lore.kernel.org/lkml/20221101130401.1841-1-linux@xxxxxxxxxxxxxx/ 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 <linux@xxxxxxxxxxxxxx> +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 <kay.sievers@xxxxxxxx> */ +#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
