【Busybox】Busybox源码分析-04

文章目录

一、ash程序入口分析
二、ash_main总结
三、login进程
四、login程序入口分析
五、login_main总结

busybox版本：1.35.0

一、ash程序入口分析

ash程序是linux内核启动后期进入busybox后，在busybox中启动的默认shell，用于响应和执行命令输入。ash的操作入口由ash_main()函数代表，定义在/shell/ash.c文件中。

贴上ash_main函数的完整代码（出自/shell/ash.c）：

int ash_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
#if NUM_SCRIPTS > 0
int ash_main(int argc, char **argv)
#else
int ash_main(int argc UNUSED_PARAM, char **argv)
#endif
/* note: 'argc' is used only if embedded scripts are enabled */
{volatile smallint state;struct jmploc jmploc;struct stackmark smark;int login_sh;/* Initialize global data */INIT_G_misc();INIT_G_memstack();INIT_G_var();
#if ENABLE_ASH_ALIASINIT_G_alias();
#endifINIT_G_cmdtable();#if PROFILEmonitor(4, etext, profile_buf, sizeof(profile_buf), 50);
#endifstate = 0;if (setjmp(jmploc.loc)) {smallint e;smallint s;exitreset();e = exception_type;s = state;if (e == EXEND || e == EXEXIT || s == 0 || iflag == 0 || shlvl) {exitshell();}reset();if (e == EXINT) {newline_and_flush(stderr);}popstackmark(&smark);FORCE_INT_ON; /* enable interrupts */if (s == 1)goto state1;if (s == 2)goto state2;if (s == 3)goto state3;goto state4;}exception_handler = &jmploc;rootpid = getpid();init();setstackmark(&smark);#if NUM_SCRIPTS > 0if (argc < 0)/* Non-NULL minusc tells procargs that an embedded script is being run */minusc = get_script_content(-argc - 1);
#endiflogin_sh = procargs(argv);
#if DEBUGTRACE(("Shell args: "));trace_puts_args(argv);
#endifif (login_sh) {const char *hp;state = 1;read_profile("/etc/profile");state1:state = 2;hp = lookupvar("HOME");if (hp)read_profile("$HOME/.profile");}state2:state = 3;if (iflag
#ifndef linux&& getuid() == geteuid() && getgid() == getegid()
#endif) {const char *shinit = lookupvar("ENV");if (shinit != NULL && *shinit != '\0')read_profile(shinit);}popstackmark(&smark);state3:state = 4;if (minusc) {/* evalstring pushes parsefile stack.* Ensure we don't falsely claim that 0 (stdin)* is one of stacked source fds.* Testcase: ash -c 'exec 1>&0' must not complain. */// if (!sflag) g_parsefile->pf_fd = -1;// ^^ not necessary since now we special-case fd 0// in save_fd_on_redirect()lineno = 0; // bash compat// dash: evalstring(minusc, sflag ? 0 : EV_EXIT);// The above makes//  ash -sc 'echo $-'// continue reading input from stdin after running 'echo'.// bash does not do this: it prints "hBcs" and exits.evalstring(minusc, EV_EXIT);}if (sflag || minusc == NULL) {
#if MAX_HISTORY > 0 && ENABLE_FEATURE_EDITING_SAVEHISTORYif (line_input_state) {const char *hp = lookupvar("HISTFILE");if (!hp) {hp = lookupvar("HOME");if (hp) {INT_OFF;hp = concat_path_file(hp, ".ash_history");setvar0("HISTFILE", hp);free((char*)hp);INT_ON;hp = lookupvar("HISTFILE");}}if (hp)line_input_state->hist_file = xstrdup(hp);
# if ENABLE_FEATURE_SH_HISTFILESIZEhp = lookupvar("HISTFILESIZE");line_input_state->max_history = size_from_HISTFILESIZE(hp);
# endif}
#endifstate4: /* XXX ??? - why isn't this before the "if" statement */cmdloop(1);}
#if PROFILEmonitor(0);
#endif
#ifdef GPROF{extern void _mcleanup(void);_mcleanup();}
#endifTRACE(("End of main reached\n"));exitshell();/* NOTREACHED */
}

下文将分段描述该函数。

首先是调用几个INIT_G_XXX命名的宏定义：

 INIT_G_misc();INIT_G_memstack();INIT_G_var();
#if ENABLE_ASH_ALIASINIT_G_alias();
#endifINIT_G_cmdtable();

以上几个函数用于初始化全局变量。

然后将state变量值设置为0：

state = 0;

接着是调用一个C语言标准库中的setjmp()函数实现异常处理机制：

    if (setjmp(jmploc.loc)) {smallint e;smallint s;exitreset();e = exception_type;s = state;if (e == EXEND || e == EXEXIT || s == 0 || iflag == 0 || shlvl) {exitshell();}reset();if (e == EXINT) {newline_and_flush(stderr);}popstackmark(&smark);FORCE_INT_ON; /* enable interrupts */printf("s : %d\r\n",s);if (s == 1)goto state1;if (s == 2)goto state2;if (s == 3)goto state3;goto state4;}exception_handler = &jmploc;

在接下来的代码中，会调用procargs(argv)处理命令行参数；调用read_profile("/etc/profile")读取配置文件，该文件正是在busybox中需要我们自己添加的用于配置shell的描述文件。

在最后，则会调用cmdloop(1)函数用于执行命令行循环操作。该函数用于读取执行命令。

二、ash_main总结

ash_main()函数用于初始化，解析参数，执行/etc/profile配置文件，然后调用cmdloop()来执行命令。setjmp()函数是一个C语言库函数，用于设置当事件发生时跳转到的位置。当异常发生时，变量"state"是用于计算跳转的位置。

三、login进程

在busybox运行后，可在命令行下输入login命令则可以运行login程序，默认busybox配置下，在启动busybox后，会执行ash程序而不是login程序。在实际应用需求中，我们可以将login设置为busybox启动后的运行程序。方法如下：

（1）使用make menuconfig编译构建出busybox的图形配置界面，选择下列选项：

（2）进入Login/Password Management Utilities选项，将该配置下的所有项目都配置上：

（3）使用make -j12编译构建busybox。

（4）安装busybox

通过以上步骤，这时候的busybox是支持login程序的，接下来，在/etc/inittab文件中设置启动项：

::respawn:-/bin/login
或者
::respawn:/sbin/getty 115200 console

注：上述配置任选一种

配置/etc/group，/etc/passwd，/etc/shadow三个文件（如果在busybox中没有，则需要自己创建
）

在/etc/group文件中，添加如下配置：

root:x:0:root

在/etc/passwd文件需要有root用户的口令信息：

root:x:0:0:root:/root:/bin/sh

小生这里/etc/passed文件中内容如下：

在/etc/shadow文件中配置用户（这里是root用户）口令：

root:DH9Ade75qXIdI:1:0:99999:7:::

DH9Ade75qXIdI表示设置的口令

该串密文可使用mkpasswd命令生成，在命令行终端输入mkpasswd后会提示输入口令，这时候输入我们想要设置的明文口令，完成后按下回车键即可生成crypt格式的字符串：

上述操作就将login登录的口令设置为iriczhao，用户名为root。

至此，通过上述步骤，就完成了login的配置，运行busybox后，即可进入login程序，如下图所示：

键入root和密码（本文是iriczhao）后，即可进入shell。

四、login程序入口分析

根据busybox的工具特征，知道login程序对应的入口则是login_main()，本小节将分析该函数：

当在busybox中运行login程序后，会提示输入登录名，然后会提示输入口令，按下Enter键后，将会去验证登录口令是否正确，这一系列的操作是由login_main函数中的while(1){}结构完成的，代码如下（出自/loginutils/login.c）：

while (1) {/* flush away any type-ahead (as getty does) */tcflush(0, TCIFLUSH);if (!username[0])get_username_or_die(username, sizeof(username));#if ENABLE_PAMpamret = pam_start("login", username, &conv, &pamh);if (pamret != PAM_SUCCESS) {failed_msg = "start";goto pam_auth_failed;}/* set TTY (so things like securetty work) */pamret = pam_set_item(pamh, PAM_TTY, short_tty);if (pamret != PAM_SUCCESS) {failed_msg = "set_item(TTY)";goto pam_auth_failed;}/* set RHOST */if (opt_host) {pamret = pam_set_item(pamh, PAM_RHOST, opt_host);if (pamret != PAM_SUCCESS) {failed_msg = "set_item(RHOST)";goto pam_auth_failed;}}if (!(opt & LOGIN_OPT_f)) {pamret = pam_authenticate(pamh, 0);if (pamret != PAM_SUCCESS) {failed_msg = "authenticate";goto pam_auth_failed;/* TODO: or just "goto auth_failed"* since user seems to enter wrong password* (in this case pamret == 7)*/}}/* check that the account is healthy */pamret = pam_acct_mgmt(pamh, 0);if (pamret == PAM_NEW_AUTHTOK_REQD) {pamret = pam_chauthtok(pamh, PAM_CHANGE_EXPIRED_AUTHTOK);}if (pamret != PAM_SUCCESS) {failed_msg = "acct_mgmt";goto pam_auth_failed;}/* read user back */pamuser = NULL;/* gcc: "dereferencing type-punned pointer breaks aliasing rules..."* thus we cast to (void*) */if (pam_get_item(pamh, PAM_USER, (void*)&pamuser) != PAM_SUCCESS) {failed_msg = "get_item(USER)";goto pam_auth_failed;}if (!pamuser || !pamuser[0])goto auth_failed;safe_strncpy(username, pamuser, sizeof(username));/* Don't use "pw = getpwnam(username);",* PAM is said to be capable of destroying static storage* used by getpwnam(). We are using safe(r) function */pw = NULL;getpwnam_r(username, &pwdstruct, pwdbuf, sizeof(pwdbuf), &pw);if (!pw)goto auth_failed;pamret = pam_open_session(pamh, 0);if (pamret != PAM_SUCCESS) {failed_msg = "open_session";goto pam_auth_failed;}pamret = pam_setcred(pamh, PAM_ESTABLISH_CRED);if (pamret != PAM_SUCCESS) {failed_msg = "setcred";goto pam_auth_failed;}break; /* success, continue login process */pam_auth_failed:/* syslog, because we don't want potential attacker* to know _why_ login failed */syslog(LOG_WARNING, "pam_%s call failed: %s (%d)", failed_msg,pam_strerror(pamh, pamret), pamret);login_pam_end(pamh);safe_strncpy(username, "UNKNOWN", sizeof(username));
#else /* not PAM */pw = getpwnam(username);if (!pw) {strcpy(username, "UNKNOWN");goto fake_it;}if (pw->pw_passwd[0] == '!' || pw->pw_passwd[0] == '*')goto auth_failed;if (opt & LOGIN_OPT_f)break; /* -f USER: success without asking passwd */if (pw->pw_uid == 0 && !is_tty_secure(short_tty))goto auth_failed;/* Don't check the password if password entry is empty (!) */if (!pw->pw_passwd[0])break;fake_it:/* Password reading and authorization takes place here.* Note that reads (in no-echo mode) trash tty attributes.* If we get interrupted by SIGALRM, we need to restore attrs.*/if (ask_and_check_password(pw) > 0)break;
#endif /* ENABLE_PAM */auth_failed:opt &= ~LOGIN_OPT_f;pause_after_failed_login();/* TODO: doesn't sound like correct English phrase to me */puts("Login incorrect");syslog(LOG_WARNING, "invalid password for '%s'%s",username, fromhost);if (++count == 3) {if (ENABLE_FEATURE_CLEAN_UP)free(fromhost);return EXIT_FAILURE;}username[0] = '\0';} /* while (1) */

在login_main函数中操作的重要数据结构是pw，pw是一个指向struct passwd的结构指针，其定义如下：

#include <sys/types.h>
#include <pwd.h>
struct passwd {char *pw_name;                /* 用户登录名 */char *pw_passwd;              /* 密码(加密后) */__uid_t pw_uid;               /* 用户ID */__gid_t pw_gid;               /* 组ID */char *pw_gecos;               /* 详细用户名 */char *pw_dir;                 /* 用户目录 */char *pw_shell;               /* Shell程序名 */
};

在login_main()函数中使用：

pw = getpwnam(username);

根据用户名获取用户信息。

如果输入正确，将会在while(1)中使用break跳出循环，继续执行后续代码；如果验证失败，则会跳转到auth_failed标签处，返回EXIT_FAILURE。

在login_main函数的最后，调用exec_login_shell(pw->pw_shell);登录shell。本质上则是execv()系统调用：

五、login_main总结

在命令行输入login命令后，则会执行login程序；也可以将login程序设置为busybox启动后执行的程序，实现带用户名和口令的登录方式。在buildroot构建工具中则自动实现了login机制，只需要在图形配置界面中开启并配置登录口令即可。