Linux kernel -- Uevent发送（热插拔）事件到用户空间

Uevent是一种在内核空间和用户空间之间通信的机制，主要用于热插拔事件（hotplug）。

uevent事件

根据include/linux/kobject.h中的定义，kobject对应的动作可分为以下几种：

enum kobject_action {
    KOBJ_ADD,
    KOBJ_REMOVE,
    KOBJ_CHANGE,
    KOBJ_MOVE,
    KOBJ_ONLINE,
    KOBJ_OFFLINE,
    KOBJ_MAX
};

这些动作对应的事件相应有以下几种：

static const char *kobject_actions[] = {
    [KOBJ_ADD] =        "add",
    [KOBJ_REMOVE] =     "remove",
    [KOBJ_CHANGE] =     "change",
    [KOBJ_MOVE] =       "move",
    [KOBJ_ONLINE] =     "online",
    [KOBJ_OFFLINE] =    "offline",
};

说明：本文所有的源代码均取自内核2.6.32。

uevent环境变量

Uevent事件具体以环境变量（即字符串）的形式发送到用户空间。

struct kobj_uevent_env {
    char *envp[UEVENT_NUM_ENVP]; /*环境变量指针数组*/
    int envp_idx; /*数组下标*/
    char buf[UEVENT_BUFFER_SIZE];
    int buflen;
};

发送事件方法

在内核中通过kobject_uevent_env函数将事件发送到用户空间，但是实际上发送事件的动作由调用函数kobject_uevent_env实现。

/**
 * kobject_uevent - notify userspace by ending an uevent
 *
 * @action: action that is happening
 * @kobj: struct kobject that the action is happening to
 *
 * Returns 0 if kobject_uevent() is completed with success or the
 * corresponding error when it fails.
 */
int kobject_uevent(struct kobject *kobj, enum kobject_action action)
{
    return kobject_uevent_env(kobj, action, NULL);
}
EXPORT_SYMBOL_GPL(kobject_uevent);

---

kobject_uevent_env函数主要做了两方面的工作：

1. 获取整理了与即将发送的事件相关的环境变量，如ACTION、DEVPATH和SUBSYSTE等。
2. 发送事件到用户空间。

/**
 * kobject_uevent_env - send an uevent with environmental data
 *
 * @action: action that is happening
 * @kobj: struct kobject that the action is happening to
 * @envp_ext: pointer to environmental data
 *
 * Returns 0 if kobject_uevent() is completed with success or the
 * corresponding error when it fails.
 */
int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
               char *envp_ext[])
{
    struct kobj_uevent_env *env;
    const char *action_string = kobject_actions[action];
    const char *devpath = NULL;
    const char *subsystem;
    struct kobject *top_kobj;
    struct kset *kset;
    struct kset_uevent_ops *uevent_ops;
    u64 seq;
    int i = 0;
    int retval = 0;

    pr_debug("kobject: '%s' (%p): %s\n",
         kobject_name(kobj), kobj, __func__);

    /* search the kset we belong to */
    top_kobj = kobj;
    while (!top_kobj->kset && top_kobj->