OpenWRT系统启动过程

水深无声 2023-02-24 07:51 18阅读 0赞

**OpenWrt系统启动脚本调用流程如下**：  
![openwrt启动流程图\#\# 标题][openwrt_]  
Linux在`start_kernel`函数执行的最后会调用`kernel_init`函数来启动用户空间的一号进程，之前都是我们熟悉的init进程，但在OpenWRT里面会执行`/etc/preinit`脚本:  
![start\_kernel\_fun][start_kernel_fun]

> 其中红框标注的代码为OpenWRT给kernel打的patch

`/etc/preinit`脚本如下：

#!/bin/sh
    # Copyright (C) 2006 OpenWrt.org
    # Copyright (C) 2010 Vertical Communications
    
    [ -z "$PREINIT" ] && exec /sbin/init
    
    export PATH=/bin:/sbin:/usr/bin:/usr/sbin
    
    pi_ifname=
    pi_ip=192.168.1.1
    pi_broadcast=192.168.1.255
    pi_netmask=255.255.255.0
    
    fs_failsafe_ifname=
    fs_failsafe_ip=192.168.2.20
    fs_failsafe_broadcast=192.168.2.255
    fs_failsafe_netmask=255.255.255.0
    
    fs_failsafe_wait_timeout=2
    
    pi_suppress_stderr="y"
    pi_init_suppress_stderr="y"
    pi_init_path="/bin:/sbin:/usr/bin:/usr/sbin"
    pi_init_cmd="/sbin/init"
    
    . /lib/functions.sh
    . /lib/functions/preinit.sh
    . /lib/functions/system.sh
    
    boot_hook_init preinit_essential
    boot_hook_init preinit_main
    boot_hook_init failsafe
    boot_hook_init initramfs
    boot_hook_init preinit_mount_root
    
    for pi_source_file in /lib/preinit/*; do
    	. $pi_source_file
    done
    
    boot_run_hook preinit_essential
    
    pi_mount_skip_next=false
    pi_jffs2_mount_success=false
    pi_failsafe_net_message=false
    
    boot_run_hook preinit_main

`PREINIT`变量当前没有值，所以执行`init`；使用`exec`，表明`init`替换进程上下文，pid保持不变，应该为1;

下面来看`init.c`函数:

int  main(int argc, char **argv)  
    {   
        pid_t pid;  
      
        sigaction(SIGTERM, &sa_shutdown, NULL);  
        sigaction(SIGUSR1, &sa_shutdown, NULL);  
        sigaction(SIGUSR2, &sa_shutdown, NULL);  
      
        early();//-------->early.c 
        cmdline();  
        watchdog_init(1); //------->../watchdog.c 
      
        pid = fork();  
        if (!pid) {   
            char *kmod[] = {  "/sbin/kmodloader", "/etc/modules-boot.d/", NULL };  
      
            if (debug < 3) {   
                int fd = open("/dev/null", O_RDWR);  
      
                if (fd > -1) {   
                    dup2(fd, STDIN_FILENO);  
                    dup2(fd, STDOUT_FILENO);  
                    dup2(fd, STDERR_FILENO);  
                    if (fd > STDERR_FILENO)  
                        close(fd);  
                }  
            }  
            execvp(kmod[0], kmod);  
            ERROR("Failed to start kmodloader\n");  
            exit(-1);  
        }  
        if (pid <= 0)  
            ERROR("Failed to start kmodloader instance\n");  
        uloop_init();  
        preinit();    //-------------->watchdog.c 
        uloop_run();  
      
        return 0;  
    }

首先做一些文件系统挂载，环境变量设置，watchdog初试化等操作，然后fork一个子进程执行kmodloader，下面来看`kmodloader.c`函数：

if (scan_loaded_modules())
            return -1;
    
        if (scan_module_folders())
            return -1;
            
        while (getline(&mod, &mod_len, fp) > 0) { 
            char *nl = strchr(mod, '\n');
            struct module *m;
            char *opts;
    
            if (nl)
                *nl = '\0';
    
            opts = strchr(mod, ' ');
            if (opts)
                *opts++ = '\0';
    
            m = find_module(get_module_name(mod));
            if (!m || (m->state == LOADED))
                continue;
    
            if (opts)
                m->opts = strdup(opts);
            m->state = PROBE;
            if (basename(gl.gl_pathv[j])[0] - '0' <= 9)
                load_modprobe();
    
        }

从`/proc/modules`里面遍历所有已经安装的模块，包括模块名称和大小等信息，插入到avl树中，同时将节点状态设置为`LOADED`  
；将`/lib/modules/x.x.x`路径保存到二维指针`module_folders`，遍历`/lib/modules/x.x.x/*.ko`，查看是否在avl树中，如果不在，添加到avl树中，同时将节点状态设置为`SCANNED`;遍历`/etc/modules-boot.d/`下所有文件并打开，判断当前模块是否在avl树中，如果存在且节点状态不为`LOADED`，则将此节点状态设置为`PROBE`，如果文件名第一个字符是0~9之间，则装载此模块，同时将此节点状态设置为`LOADED`;

回到`init.c`，来看`preinit`函数:

void
    preinit(void)
    { 
        char *init[] = {  "/bin/sh", "/etc/preinit", NULL };
        char *plug[] = {  "/sbin/procd", "-h", "/etc/hotplug-preinit.json", NULL };
    
        LOG("- preinit -\n");
    
        plugd_proc.cb = plugd_proc_cb;
        plugd_proc.pid = fork();
        if (!plugd_proc.pid) { 
            execvp(plug[0], plug);
            ERROR("Failed to start plugd\n");
            exit(-1);
        }
        if (plugd_proc.pid <= 0) { 
            ERROR("Failed to start new plugd instance\n");
            return;
        }
        uloop_process_add(&plugd_proc);
    
        setenv("PREINIT", "1", 1);
    
        preinit_proc.cb = spawn_procd;
        preinit_proc.pid = fork();
        if (!preinit_proc.pid) { 
            execvp(init[0], init);
            ERROR("Failed to start preinit\n");
            exit(-1);
        }
        if (preinit_proc.pid <= 0) { 
            ERROR("Failed to start new preinit instance\n");
            return;
        }
        uloop_process_add(&preinit_proc);
    
        DEBUG(4, "Launched preinit instance, pid=%d\n", (int) preinit_proc.pid);
    }

fork两个子进程分别执行`procd`和`/etc/preinit`，来看`procd.c`，因携带参数`-h /etc/hotplug-preinit.json`，所以进入`hotplug_run`:

void hotplug(char *rules)
    { 
        struct sockaddr_nl nls;
        int nlbufsize = 512 * 1024;
    
        rule_file = strdup(rules);
        memset(&nls,0,sizeof(struct sockaddr_nl));
        nls.nl_family = AF_NETLINK;
        nls.nl_pid = getpid();
        nls.nl_groups = -1;
    
        if ((hotplug_fd.fd = socket(PF_NETLINK, SOCK_DGRAM | SOCK_CLOEXEC, NETLINK_KOBJECT_UEVENT)) == -1) { 
            ERROR("Failed to open hotplug socket: %s\n", strerror(errno));
            exit(1);
        }
        
        if (bind(hotplug_fd.fd, (void *)&nls, sizeof(struct sockaddr_nl))) { 
            ERROR("Failed to bind hotplug socket: %s\n", strerror(errno));
            exit(1);
        }
    
        if (setsockopt(hotplug_fd.fd, SOL_SOCKET, SO_RCVBUFFORCE, &nlbufsize, sizeof(nlbufsize)))
            ERROR("Failed to resize receive buffer: %s\n", strerror(errno));
    
        json_script_init(&jctx);
        queue_proc.cb = queue_proc_cb;
        uloop_fd_add(&hotplug_fd, ULOOP_READ);
    }

这里是建立netlink监听内核的uevent事件，具体的实现之后再单独分析；回到`/etc/preinit`:

. /lib/functions.sh
    . /lib/functions/preinit.sh
    . /lib/functions/system.sh

在当前shell环境下调用三个脚本，里面包含一些函数定义,

boot_hook_init preinit_essential
    boot_hook_init preinit_main
    boot_hook_init failsafe
    boot_hook_init initramfs
    boot_hook_init preinit_mount_root

初试化hook链:

for pi_source_file in /lib/preinit/*; do
        . $pi_source_file
    done

依次执行/lib/preinit/下面的脚本，都是将函数调用添加到hook链上面:

boot_run_hook preinit_essential

执行preinit\_essential注册的hook链的所有函数:

boot_run_hook preinit_main

执行`preinit_main`注册的hook链的所有函数；`/etc/preinit`执行完毕后，init进程调用`spawn_procd`，procd内容较多，此处暂不讨论了，可参考第一个链接的末尾部分；

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

## 参考文章 ##

1.  [Openwrt学习笔记（四）——系统开机启动][Openwrt]
2.  [OPENWRT启动流程分析][OPENWRT]
3.  [Openwrt启动流程及启动脚本分析][Openwrt 1]
4.  [openwrt启动流程（一）][openwrt]
5.  [OpenWrt 开机流程分析][OpenWrt]

[openwrt_]: /images/20230209/0e2f3e95143b40ef9d5e2c6037582ca7.png
[start_kernel_fun]: /images/20230209/e52cb38aebc046f6a35dca542273f012.png
[Openwrt]: https://blog.csdn.net/lee244868149/article/details/57396776
[OPENWRT]: https://blog.csdn.net/fengfeng0328/article/details/83352459
[Openwrt 1]: https://blog.csdn.net/wwx0715/article/details/41725917
[openwrt]: https://www.cnblogs.com/openwrt/p/7454809.html
[OpenWrt]: http://www.openwrt.pro/post-39.html