Linux设备模型(六) - Android ueventd浅析

在linux2.6之后，udev取代了devfs，但是在android中却没有udev或者mdev[1]，而是由ueventd进程实现了类似功能（管理设备节点权限、创建设备节点）。

一，ueventd启动过程

system/core/rootdir/init.rc

on early-init
start ueventd## Daemon processes to be run by init.
##
service ueventd /system/bin/ueventdclass corecriticalseclabel u:r:ueventd:s0shutdown critical

在运行环境中查看命令“/sbin/ueventd”，其实它是"/init"的软链接：

lynkco:/system $ ls -l /system/bin/ueventd
lrwxr-xr-x 1 root shell 4 2009-01-01 00:00 /system/bin/ueventd -> init

在文件init.c的main()函数中有一个巧妙的处理：可以通过判断第一个运行参数来启动不同的进程：

如果执行“./ueventd”，进入第一个条件分支，执行uevent_main()函数；

如果执行“./watchdog”，进入第二个条件分支，执行watchdogd_main()函数；

如果执行"./init"，跳过所有分支条件，继续执行main()函数。

system/core/init/init.c :

int main(int argc, char** argv) {if (!strcmp(basename(argv[0]), "ueventd")) {return ueventd_main(argc, argv);}... ...
}

因此，脚本init.rc中的命令“start ueventd”最终执行的是ueventd_main()函数。

二，组成和功能

1，ueventd 的主要组成部分

devices.cpp

devices.h

uevent.h

uevent_listener.cpp

uevent_listener.h

ueventd.cpp

ueventd.h

ueventd_parser.cpp

ueventd_parser.h

2，主要实现的功能

解析ueventd相关rc文件，管理设备节点权限；
Cold boot，递归扫描/sys目录，根据uevent文件，静态创建设备节点；
Hot plug，获取内核uevent事件，动态创建设备节点。

int ueventd_main(int argc, char** argv) {LOG(INFO) << "ueventd started!";auto ueventd_configuration = GetConfiguration(); //解析ueventd相关rc文件uevent_handlers.emplace_back(std::make_unique<DeviceHandler>(std::move(ueventd_configuration.dev_permissions),std::move(ueventd_configuration.sysfs_permissions),std::move(ueventd_configuration.subsystems), android::fs_mgr::GetBootDevices(), true));uevent_handlers.emplace_back(std::make_unique<FirmwareHandler>(std::move(ueventd_configuration.firmware_directories),std::move(ueventd_configuration.external_firmware_handlers)));if (ueventd_configuration.enable_modalias_handling) {std::vector<std::string> base_paths = {"/odm/lib/modules", "/vendor/lib/modules"};uevent_handlers.emplace_back(std::make_unique<ModaliasHandler>(base_paths));}UeventListener uevent_listener(ueventd_configuration.uevent_socket_rcvbuf_size);if (!android::base::GetBoolProperty(kColdBootDoneProp, false)) {ColdBoot cold_boot(uevent_listener, uevent_handlers,ueventd_configuration.enable_parallel_restorecon);cold_boot.Run(); //静态创建设备节点}for (auto& uevent_handler : uevent_handlers) {uevent_handler->ColdbootDone();}// We use waitpid() in ColdBoot, so we can't ignore SIGCHLD until now.signal(SIGCHLD, SIG_IGN);// Reap and pending children that exited between the last call to waitpid() and setting SIG_IGN// for SIGCHLD above.while (waitpid(-1, nullptr, WNOHANG) > 0) {}// Restore prio before main loopsetpriority(PRIO_PROCESS, 0, 0);uevent_listener.Poll([&uevent_handlers](const Uevent& uevent) {for (auto& uevent_handler : uevent_handlers) {uevent_handler->HandleUevent(uevent); //动态创建设备节点}return ListenerAction::kContinue;});return 0;
}}  // namespace init
}  // namespace android

三，主要功能解析

1，解析ueventd相关rc文件，管理设备节点权限

ueventd_configuration = GetConfiguration();----ParseConfig({"/system/etc/ueventd.rc"});--------parser.AddSectionParser("import", std::make_unique<ImportParser>(&parser));------------parser.AddSingleLineParser("uevent_socket_rcvbuf_size", std::bind(ParseUeventSocketRcvbufSizeLine, _1, &ueventd_configuration.uevent_socket_rcvbuf_size));

这里请注意，GetConfiguration并不创建设备节点，它的作用是提供数据库，当有设备节点生成的时候，eventd会参考这个数据库设置设备节点的权限。

2，Cold boot，递归扫描/sys目录，根据uevent文件，静态创建设备节点

当ueventd启动时，它会遍历所有在/sys注册的设备并将'add'写入它找到的每个'uevent'文件，这会导致内核生成并重新发送所有当前注册设备的uevent消息。这样做是因为当这些设备注册到sysfs时，ueventd根本还没有开始运行，没能接收他们的uevent消息并妥善处理，这样Android系统也是无法正常运行的，所以需要重新生成其uevent以便ueventd对其做处理。这个过程被称为

'冷启动'，即cold_boot，cold_boot也是ueventd在开机过程中的最主要的工作。

netlink socket的创建
uevent的监听与接收
uevent的处理

UeventListener(size_t uevent_socket_rcvbuf_size)
----device_fd_.reset(uevent_open_socket(uevent_socket_rcvbuf_size, true)); //netlink socket的创建
----fcntl(device_fd_, F_SETFL, O_NONBLOCK); //设置为非阻塞
ColdBoot cold_boot(uevent_listener, uevent_handlers, ueventd_configuration.enable_parallel_restorecon);
cold_boot.Run();
--------RegenerateUevents(); //it writes 'add' to every 'uevent' file that it finds in `/sys/class`, `/sys/block`, and `/sys/devices`
------------RegenerateUeventsForPath(path, callback)
----------------RegenerateUeventsForDir(d.get(), callback);
--------------------fd = openat(dfd, "uevent", O_WRONLY | O_CLOEXEC); //打开搜索到的uevent节点并写入add，重新触发kernel发送uevent msg
--------------------write(fd, "add\n", 4);
--------------------close(fd);
--------------------result = ReadUevent(&uevent)
------------------------uevent_kernel_multicast_recv(device_fd_, msg, UEVENT_MSG_LEN); //uevent的监听与接收
------------------------ParseEvent(msg, uevent); //解析uevent msg，结果存在uevent结构体中，所有的uevent最后保存在uevent_queue_链表
----------------------------uevent->action = msg;
----------------------------uevent->path = msg;
--------ForkSubProcesses() //fork子进程并行处理保存在uevent_queue_链表中的uevent
------------UeventHandlerMain(unsigned int process_num, unsigned int total_processes)
------------uevent_handler->HandleUevent(uevent); //uevent的处理
----------------DeviceHandler::HandleUevent(const Uevent& uevent)
----------------HandleDevice(uevent.action, devpath, block, uevent.major, uevent.minor, links);
--------------------MakeDevice(devpath, block, major, minor, links); //创建设备节点
------------------------auto[mode, uid, gid] = GetDevicePermissions(path, links); //在数据库中查找指定的节点权限，如果没有指定就用默认的
------------------------dev_t dev = makedev(major, minor);
--------------------mkdir_recursive(Dirname(link), 0755) //创建目录
------------------------make_dir(path, mode);
----------------------------rc = mkdir(path.c_str(), mode);

3，Hot plug，获取内核uevent事件，动态创建设备节点

uevent_listener.Poll([&uevent_handlers](const Uevent& uevent)
----ufd.fd = device_fd_;
----poll(&ufd, 1, timeout_ms);
----result = ReadUevent(&uevent)
uevent_handler->HandleUevent(uevent);

四，ueventd相关rc文件的规则

system/core/init/init.c/README.ueventd.md文件中有介绍rc文件的规则。

1，ueventd rc文件路径

## Importing configuration files

--------------------------------

Ueventd reads /system/etc/ueventd.rc, all other files are imported via the `import` command, which takes the format of

    import <path>

This command parses an ueventd config file, extending the current configuration. If _path_ is a

directory, each file in the directory is parsed as a config file. It is not recursive, nested

directories will not be parsed. Imported files are parsed after the current file has been parsed.

2，/dev规则

The permissions can be modified using a ueventd.rc script and a line that beings with `/dev`. These lines take the format of

    devname mode uid gid [options]

For example

    /dev/null 0666 root root

When `/dev/null` is created, its mode will be set to `0666`, its user to `root` and its group to

`root`.

3，/sys规则

Ueventd by default takes no action for `/sys`, however it can be instructed to set permissions for

certain files in `/sys` when matching uevents are generated. This is done using a ueventd.rc script and a line that begins with `/sys`. These lines take the format of

    nodename attr mode uid gid [options]

For example

    /sys/devices/system/cpu/cpu* cpufreq/scaling_max_freq 0664 system system

When a uevent that matches the pattern `/sys/devices/system/cpu/cpu*` is sent, the matching sysfs attribute, `cpufreq/scaling_max_freq`, will have its mode set to `0664`, its user to to `system` and its group set to `system`.

具体内容请查看README.ueventd.md。

参考链接：

AndroidP之 Ueventd - 简书