Linux内核通信-基于sysfs

问题来源

写了个内核模块的demo，想在用户态跟它交互，如何把用户态数据发送给它？

相关概念

image.png

基于sysfs的通信

struct kobj_attribute 和 __ATTR 宏

struct kobj_attribute {
    struct attribute attr;
    ssize_t (*show)(struct kobject *kobj, struct kobj_attribute *attr, char *buf);
    ssize_t (*store)(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count);
};

#define __ATTR(_name, _mode, _show, _store) {               \
    .attr = {.name = __stringify(_name),                \
         .mode = VERIFY_OCTAL_PERMISSIONS(_mode) },     \
    .show   = _show,                        \
    .store  = _store,                       \
}

初始化一个kobj_attribute

static ssize_t kcom_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
{
    ...
    return 0;
}

static ssize_t kcom_store(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count)
{
    ...
    return count;
}

static struct kobj_attribute kcom_attribute = __ATTR(kcom, 0644, kcom_show, kcom_store);

kobject

struct kobject {
    const char      *name;
    struct list_head    entry;
    struct kobject      *parent;
    struct kset     *kset;
    struct kobj_type    *ktype;
    struct kernfs_node  *sd; /* sysfs directory entry */
    struct kref     kref;
#ifdef CONFIG_DEBUG_KOBJECT_RELEASE
    struct delayed_work release;
#endif
    unsigned int state_initialized:1;
    unsigned int state_in_sysfs:1;
    unsigned int state_add_uevent_sent:1;
    unsigned int state_remove_uevent_sent:1;
    unsigned int uevent_suppress:1;
};

kobject_create_and_add 和 kobject_put

Let’s first create sysfs with kobject_create_and_add function.
This function creates a kobject structure dynamically and registers it with sysfs.
If the kobject was not able to be created, NULL will be returned.

When you are finished with this structure, call kobject_put and the structure will be dynamically freed when it is no longer being used.

struct kobject *kobject_create_and_add(const char *name, struct kobject *parent);

Where, name is the name for the kobject, parent is the parent kobject of this kobject, if any.

If we pass kernel_kobj to the second argument, it will create the directory under /sys/kernel/, and under /sys/firmware/ for firmware_kobj, under /sys/fs for fs_kobj respectively. To create directly under /sys/ we may pass NULL.

static struct kobject *kcom_kobject;
kcom_kobject = kobject_create_and_add("kcom", kernel_kobj);
kobject_put(kcom_kobject);

sysfs_create_file 和 sysfs_remove_file

int sysfs_create_file(struct kobject *kobj, const struct attribute *attr);
void sysfs_remove_file(struct kobject *kobj, const struct attribute *attr);

sysfs_create_file(kcom_kobject, &kcom_attribute.attr);
sysfs_remove_file(kcom_kobject, &kcom_attribute.attr);

完整代码

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/kobject.h>
#include <linux/sysfs.h>
#include <linux/string.h>
#include <linux/vmalloc.h>

static struct kobject *kcom_kobject;
static char kcom_buffer[4096] = {0, };

static ssize_t kcom_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
{
    int ret = sprintf(buf, "Kernel response to msg: %s\n", kcom_buffer);
    return ret;
}

static ssize_t kcom_store(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count)
{
    snprintf(kcom_buffer, sizeof(kcom_buffer), "%s", buf);
    kcom_buffer[sizeof(kcom_buffer) - 1] = '\0';
    return count;
}

static struct kobj_attribute kcom_attribute = __ATTR(value, 0644, kcom_show, kcom_store);

static int __init kcom_init(void)
{
    int ret = 0;

    pr_info("kcom: Initialize module\n");

    kcom_kobject = kobject_create_and_add("kcom", kernel_kobj);
    if (!kcom_kobject) {
        pr_err("kcom: Failed to create kernel object\n");
        return -ENOMEM;
    }

    ret = sysfs_create_file(kcom_kobject, &kcom_attribute.attr);
    if (ret)
        pr_err("kcom: Failed to create sysfs file\n");

    return ret;
}

static void __exit kcom_exit(void)
{
    pr_info("kcom: Deinitialize module\n");
    sysfs_remove_file(kcom_kobject, &kcom_attribute.attr);
    kobject_put(kcom_kobject);
}

module_init(kcom_init);
module_exit(kcom_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("ikx");
MODULE_DESCRIPTION("Kernel Communication Module.");
MODULE_VERSION("0.1");

编译和测试

Makefile

obj-m += kcom.o
KDIR := /lib/modules/$(shell uname -r)/build

all:
    make -C $(KDIR) M=$(PWD) modules

clean:
    make -C $(KDIR) M=$(PWD) clean

$ make
$ sudo insmod kcom.ko
$ su
root@~# echo Hello > /sys/kernel/kcom/value
root@~# cat /sys/kernel/kcom/value 
Kernel response to msg: Hello

root@~# rmmod kcom.ko

参考

https://elmurod.net/blog/kernel-crypto-api-message-digest/