前言
当一个设备动态的加入到系统时候（比如常见的将U盘插入到PC机器上）， 设备驱动程序就需要动态的检测到有设备插入了系统，就需要将此事件通知到用户层，然后用户层对这一事件做响应的处理，比如加载USB驱动，更新UI等。而将此事件通知到用户层就需要某种机制，典型的就是mdev hotplug和udev。关于udev和mdev hotplug可以在上篇文章有解释。Linux系统对uevent机制的具体实现是建立在设备模型的基础上的，通过kobject_uevent函数实现。
在前面的kset小节中提到了注册一个kset的接口，可以在这里习复下。
代码语言：javascript复制/**
* kset_register - initialize and add a kset.
* @k: kset.
*/
int kset_register(struct kset *k)
{
int err;
if (!k)
return -EINVAL;
kset_init(k);
err = kobject_add_internal(&k->kobj);
if (err)
return err;
kobject_uevent(&k->kobj, KOBJ_ADD);
return 0;
}可以看到这里调用了kobject_uevent接口，发送一个action为: KOBJ_ADD的事件。而kobject和kset的主要区别就是，将一个kset注册到系统的时候，就需要将此事件通过kobject_uevent发送到用户空间，而kobject如果是单独的，没有依赖kset，则无法通过uevent机制发送事件到用户空间。
数据结构
代码语言：javascript复制struct kset_uevent_ops {
int (* const filter)(struct kset *kset, struct kobject *kobj);
const char *(* const name)(struct kset *kset, struct kobject *kobj);
int (* const uevent)(struct kset *kset, struct kobject *kobj,
struct kobj_uevent_env *env);
};kset_uevent_ops代表意思是Kset事件处理函数集合。
filter: 当上报uevent的时候，kset会通过filter接口去过滤，阻止不希望上报的uevent。
name: 返回kset的名称，如果此kset没有名称，也是不允许上报event
uevent: 通常会调用此回调处理一些Kset的私有事情。
代码语言：javascript复制struct kobj_uevent_env {
char *argv[3];
char *envp[UEVENT_NUM_ENVP];
int envp_idx;
char buf[UEVENT_BUFFER_SIZE];
int buflen;
};envp: 用户保存每个环境变量的地址，最大支持32个。
envp_idx: 用户访问envp。
buf: 保存环境的buffer,最大支持2048
buflen: 用于访问buf。
代码分析
代码语言：javascript复制/**
* kobject_uevent - notify userspace by sending 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);
}可以看到注释： 通过发送一个uevent通知事件到用户层， action就是当前发生的事件类型，如下action是个枚举类型
代码语言：javascript复制enum kobject_action {
KOBJ_ADD,
KOBJ_REMOVE,
KOBJ_CHANGE,
KOBJ_MOVE,
KOBJ_ONLINE,
KOBJ_OFFLINE,
KOBJ_MAX
};KOBJ_ADD/KOBJ_REMOVE代表添加或者移除
KOBJ_ONLINE/KOBJ_OFFLINE代表上线或这下线
代码语言：javascript复制/**
* 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_env() 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;
const struct kset_uevent_ops *uevent_ops;
int i = 0;
int retval = 0;
#ifdef CONFIG_NET
struct uevent_sock *ue_sk;
#endif
pr_debug("kobject: '%s' (%p): %sn",
kobject_name(kobj), kobj, __func__);
/* search the kset we belong to */
top_kobj = kobj;
while (!top_kobj->kset && top_kobj->parent) //听过while循环找到kobj所属的顶层kset
top_kobj = top_kobj->parent;
if (!top_kobj->kset) { //发送一个envet必须存在kset
pr_debug("kobject: '%s' (%p): %s: attempted to send uevent "
"without kset!n", kobject_name(kobj), kobj,
__func__);
return -EINVAL;
}
kset = top_kobj->kset; //得到最顶层kset的uevent_ops
uevent_ops = kset->uevent_ops;
/* skip the event, if uevent_suppress is set*/
if (kobj->uevent_suppress) { //如果uevnet_suppress=1，则不发送uevent
pr_debug("kobject: '%s' (%p): %s: uevent_suppress "
"caused the event to drop!n",
kobject_name(kobj), kobj, __func__);
return 0;
}
/* skip the event, if the filter returns zero. */
if (uevent_ops && uevent_ops->filter) //通过filter函数过滤，如果返回0，则说明顶层的kset过滤了此event
if (!uevent_ops->filter(kset, kobj)) {
pr_debug("kobject: '%s' (%p): %s: filter function "
"caused the event to drop!n",
kobject_name(kobj), kobj, __func__);
return 0;
}
/* originating subsystem */
if (uevent_ops && uevent_ops->name) //通过name函数设置subsystem
subsystem = uevent_ops->name(kset, kobj);
else
subsystem = kobject_name(&kset->kobj);
if (!subsystem) {
pr_debug("kobject: '%s' (%p): %s: unset subsystem caused the "
"event to drop!n", kobject_name(kobj), kobj,
__func__);
return 0;
}
/* environment buffer */
env = kzalloc(sizeof(struct kobj_uevent_env), GFP_KERNEL); //分配环境变量buff
if (!env)
return -ENOMEM;
/* complete object path */
devpath = kobject_get_path(kobj, GFP_KERNEL); //得到此obj的路径
if (!devpath) {
retval = -ENOENT;
goto exit;
}
/* default keys */
retval = add_uevent_var(env, "ACTION=%s", action_string); //添加环境变量，ACTION， DEVPATH， SUBSYSTEM到环境变量buff中
if (retval)
goto exit;
retval = add_uevent_var(env, "DEVPATH=%s", devpath);
if (retval)
goto exit;
retval = add_uevent_var(env, "SUBSYSTEM=%s", subsystem);
if (retval)
goto exit;
/* keys passed in from the caller */
if (envp_ext) { //添加调用者提供的参数
for (i = 0; envp_ext[i]; i ) {
retval = add_uevent_var(env, "%s", envp_ext[i]);
if (retval)
goto exit;
}
}
/* let the kset specific function add its stuff */ //让Kset完成一些自己的私人处理
if (uevent_ops && uevent_ops->uevent) {
retval = uevent_ops->uevent(kset, kobj, env);
if (retval) {
pr_debug("kobject: '%s' (%p): %s: uevent() returned "
"%dn", kobject_name(kobj), kobj,
__func__, retval);
goto exit;
}
}
/*
* Mark "add" and "remove" events in the object to ensure proper
* events to userspace during automatic cleanup. If the object did
* send an "add" event, "remove" will automatically generated by
* the core, if not already done by the caller.
*/
if (action == KOBJ_ADD)
kobj->state_add_uevent_sent = 1;
else if (action == KOBJ_REMOVE)
kobj->state_remove_uevent_sent = 1;
mutex_lock(&uevent_sock_mutex);
/* we will send an event, so request a new sequence number */ //更新uevent seq number
retval = add_uevent_var(env, "SEQNUM=%llu", (unsigned long long) uevent_seqnum);
if (retval) {
mutex_unlock(&uevent_sock_mutex);
goto exit;
}
#if defined(CONFIG_NET) //如果开启了CONFIG_NET就使用netlink发送Uevent
/* send netlink message */
list_for_each_entry(ue_sk, &uevent_sock_list, list) {
struct sock *uevent_sock = ue_sk->sk;
struct sk_buff *skb;
size_t len;
if (!netlink_has_listeners(uevent_sock, 1))
continue;
/* allocate message with the maximum possible size */
len = strlen(action_string) strlen(devpath) 2;
skb = alloc_skb(len env->buflen, GFP_KERNEL);
if (skb) {
char *scratch;
/* add header */
scratch = skb_put(skb, len);
sprintf(scratch, "%s@%s", action_string, devpath);
/* copy keys to our continuous event payload buffer */
for (i = 0; i < env->envp_idx; i ) {
len = strlen(env->envp[i]) 1;
scratch = skb_put(skb, len);
strcpy(scratch, env->envp[i]);
}
NETLINK_CB(skb).dst_group = 1;
retval = netlink_broadcast_filtered(uevent_sock, skb,
0, 1, GFP_KERNEL,
kobj_bcast_filter,
kobj);
/* ENOBUFS should be handled in userspace */
if (retval == -ENOBUFS || retval == -ESRCH)
retval = 0;
} else
retval = -ENOMEM;
}
#endif
mutex_unlock(&uevent_sock_mutex);
#ifdef CONFIG_UEVENT_HELPER //如果开启了就是用uevent_helper发送uevent。
/* call uevent_helper, usually only enabled during early boot */
if (uevent_helper[0] && !kobj_usermode_filter(kobj)) {
struct subprocess_info *info;
retval = add_uevent_var(env, "HOME=/");
if (retval)
goto exit;
retval = add_uevent_var(env,
"PATH=/sbin:/bin:/usr/sbin:/usr/bin");
if (retval)
goto exit;
retval = init_uevent_argv(env, subsystem);
if (retval)
goto exit;
retval = -ENOMEM;
info = call_usermodehelper_setup(env->argv[0], env->argv,
env->envp, GFP_KERNEL,
NULL, cleanup_uevent_env, env);
if (info) {
retval = call_usermodehelper_exec(info, UMH_NO_WAIT);
env = NULL; /* freed by cleanup_uevent_env */
}
}
#endif
exit:
kfree(devpath);
kfree(env);
return retval;
}uevent_helper机制
目前内核支持两种方式，netlink和uevent_helper，本节重点分析uevent_helper的实现。
uevent_helper的定义如下
代码语言：javascript复制#ifdef CONFIG_UEVENT_HELPER
char uevent_helper[UEVENT_HELPER_PATH_LEN] = CONFIG_UEVENT_HELPER_PATH;
#endifCONFIG_UEVENT_HELPER_PATH可以在内核的config文件中找到。
代码语言：javascript复制CONFIG_UEVENT_HELPER_PATH="/sbin/hotplug"对应的/sbin/hotplug, 而此hotplug对应的程序是什么？ 如果是嵌入式设备，会在etc目录下看到这样的配置：
代码语言：javascript复制echo /sbin/mdev >/proc/sys/kernel/hotplug
/sbin/mdev -s也就是说uevent_helper最终调用到/sbin/mdev.
接着会到kobject_uevent函数中，继续分析。
在开是调用userhelper之前的准备工作。
代码语言：javascript复制struct subprocess_info *call_usermodehelper_setup(char *path, char **argv,
char **envp, gfp_t gfp_mask,
int (*init)(struct subprocess_info *info, struct cred *new),
void (*cleanup)(struct subprocess_info *info),
void *data)
{
struct subprocess_info *sub_info;
sub_info = kzalloc(sizeof(struct subprocess_info), gfp_mask);
if (!sub_info)
goto out;
INIT_WORK(&sub_info->work, __call_usermodehelper); //初始化一个工作队列。
sub_info->path = path; //通过参数初始化subprocess_info
sub_info->argv = argv;
sub_info->envp = envp;
sub_info->cleanup = cleanup;
sub_info->init = init;
sub_info->data = data;
out:
return sub_info;
}接着调用call_usermodehelper_exec函数开启一个用户模式的应用程序。
代码语言：javascript复制int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
{
DECLARE_COMPLETION_ONSTACK(done); //初始化一个完成对象
int retval = 0;
if (!sub_info->path) { //如果没有path变量，就执行cleanup
call_usermodehelper_freeinfo(sub_info);
return -EINVAL;
}
helper_lock(); //原子变量running_helpers加1
if (!khelper_wq || usermodehelper_disabled) { //如果不存在khelper_wq，或者usermodehelper已经disabled
retval = -EBUSY;
goto out;
}
/*
* Worker thread must not wait for khelper thread at below
* wait_for_completion() if the thread was created with CLONE_VFORK
* flag, for khelper thread is already waiting for the thread at
* wait_for_completion() in do_fork().
*/
if (wait != UMH_NO_WAIT && current == kmod_thread_locker) {
retval = -EBUSY;
goto out;
}
/*
* Set the completion pointer only if there is a waiter.
* This makes it possible to use umh_complete to free
* the data structure in case of UMH_NO_WAIT.
*/
sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &done;
sub_info->wait = wait;
queue_work(khelper_wq, &sub_info->work); //提交工作节点到工作队列。
if (wait == UMH_NO_WAIT) /* task has freed sub_info */ //如果wait等于NO_WAIT则就返回。
goto unlock;
if (wait & UMH_KILLABLE) {
retval = wait_for_completion_killable(&done); //如果支持可kill的
if (!retval)
goto wait_done;
/* umh_complete() will see NULL and free sub_info */
if (xchg(&sub_info->complete, NULL))
goto unlock;
/* fallthrough, umh_complete() was already called */
}
wait_for_completion(&done); //如果wait不是上述的两种，就一直等待，那等待什么？ 当然是等待有人解放它。
wait_done:
retval = sub_info->retval;
out:
call_usermodehelper_freeinfo(sub_info);
unlock:
helper_unlock();
return retval;
}那什么时候会唤醒等待？ 当然是工作队列上的任务完成之后，就会触发complete，唤醒等待。
代码语言：javascript复制/* This is run by khelper thread */
static void __call_usermodehelper(struct work_struct *work)
{
struct subprocess_info *sub_info =
container_of(work, struct subprocess_info, work);
int wait = sub_info->wait & ~UMH_KILLABLE;
pid_t pid;
/* CLONE_VFORK: wait until the usermode helper has execve'd
* successfully We need the data structures to stay around
* until that is done. */
if (wait == UMH_WAIT_PROC)
pid = kernel_thread(wait_for_helper, sub_info,
CLONE_FS | CLONE_FILES | SIGCHLD);
else {
pid = kernel_thread(call_helper, sub_info,
CLONE_VFORK | SIGCHLD);
/* Worker thread stopped blocking khelper thread. */
kmod_thread_locker = NULL;
}
if (pid < 0) {
sub_info->retval = pid;
umh_complete(sub_info);
}
}此处通过创建一个内核线程，当调度到call_helper函数，此函数调用到____call_usermodehelper。在此函数中最终调用
代码语言：javascript复制 retval = do_execve(getname_kernel(sub_info->path),
(const char __user *const __user *)sub_info->argv,
(const char __user *const __user *)sub_info->envp);在内核空间执行应用程序。最终在umh_complete函数中调用complete，唤醒等待。
代码语言：javascript复制static void umh_complete(struct subprocess_info *sub_info)
{
struct completion *comp = xchg(&sub_info->complete, NULL);
/*
* See call_usermodehelper_exec(). If xchg() returns NULL
* we own sub_info, the UMH_KILLABLE caller has gone away
* or the caller used UMH_NO_WAIT.
*/
if (comp)
complete(comp);
else
call_usermodehelper_freeinfo(sub_info);
}至此就分析完毕。
