Android(安卓)ueventd解析

16lz 2021-12-11

android 中的ueventd是一个守护进程，主要作用是接收uevent来创建或删除/dev/xxx(设备节点)，ueventd代码不多，下面我们直接针对代码分析。

int ueventd_main(int argc, char **argv)

{

struct pollfd ufd;

int nr;

char tmp[32];

/*

* init sets the umask to 077 for forked processes. We need to

* create files with exact permissions, without modification by

* the umask.

*/

umask(000);

/* Prevent fire-and-forget children from becoming zombies.

* If we should need to wait() for some children in the future

* (as opposed to none right now), double-forking here instead

* of ignoring SIGCHLD may be the better solution.

*/

signal(SIGCHLD, SIG_IGN);

open_devnull_stdio();

klog_init();

INFO("starting ueventd\n");

/* Respect hardware passed in through the kernel cmd line. Here we willlook

* for androidboot.hardware param in kernel cmdline, and save its valuein

* hardware[]. */

import_kernel_cmdline(0, import_kernel_nv);

get_hardware_name(hardware, &revision);

//解析ueventd.rc和ueventd.xx.rc,将文件中的每一行信息存放到一个perm_node结构体，然后将这些结构体组成一个双向链表，每一行包含文件名、uid、gid、权限等

ueventd_parse_config_file("/ueventd.rc");

snprintf(tmp, sizeof(tmp), "/ueventd.%s.rc", hardware);

ueventd_parse_config_file(tmp);

//1.建立接收uevent的socket 2.如果没有做过coldboot,做一次coldboot

//所谓coldboot是指让kernel重新生成deviceadd事件

device_init();

ufd.events = POLLIN;

ufd.fd = get_device_fd();

//循环poll ufd的POLLIN事件, POLLIN事件表示有数据可读，具体可参考linux poll系统调用的相关资料

while(1) {

ufd.revents = 0;

nr = poll(&ufd, 1, -1);

if (nr <= 0)

continue;

if (ufd.revents == POLLIN)

handle_device_fd(); //详见下文

}

}

#define UEVENT_MSG_LEN 1024

void handle_device_fd()

{

char msg[UEVENT_MSG_LEN+2];

int n;

while ((n = uevent_kernel_multicast_recv(device_fd, msg,UEVENT_MSG_LEN)) > 0) {

if(n >= UEVENT_MSG_LEN) /*overflow -- discard */

continue;

msg[n] = '\0';

msg[n+1] = '\0';

struct uevent uevent;

//解析uevent消息，保存到uevent变量中

parse_event(msg, &uevent);

handle_device_event(&uevent); //详见下文

//处理subsystem是firmware的uevent,详见下文

handle_firmware_event(&uevent);

}

}

static void handle_device_event(structuevent *uevent)

{

//针对action为add和changer,修复一下文件权限

if (!strcmp(uevent->action,"add") ||!strcmp(uevent->action, "change"))

fixup_sys_perms(uevent->path);

//针对block、platform等设备进行处理

if (!strncmp(uevent->subsystem, "block", 5)) {

handle_block_device_event(uevent);

}else if (!strncmp(uevent->subsystem, "platform", 8)) {

handle_platform_device_event(uevent);

}else {

handle_generic_device_event(uevent);

}

}

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