本文主要是介绍PAM从入门到精通（二十三），希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
接前一篇文章：PAM从入门到精通（二十二）
本文参考：
《The Linux-PAM Application Developers' Guide》
先再来重温一下PAM系统架构：
更加形象的形式：
七、PAM-API各函数源码详解
前边的文章讲解了各PAM-API函数以及总体流程，但是也只是从接口层面介绍的，并没有深入到代码层面。从本篇文章开始，将对于各个接口函数从源码级进行讲解，以使大家不但知其然，还要知其所以然。
1. pam_start函数
上回讲到_pam_start_internal函数的第三部分，本文继续往下进行讲解。为了便于理解，再次贴出_pam_start_internal函数源码。在libpam/pam_start.c中，如下所示：
static int _pam_start_internal (const char *service_name,const char *user,const struct pam_conv *pam_conversation,const char *confdir,pam_handle_t **pamh)
{D(("called pam_start: [%s] [%s] [%p] [%p]",service_name, user, pam_conversation, pamh));if (pamh == NULL) {pam_syslog(NULL, LOG_CRIT,"pam_start: invalid argument: pamh == NULL");return (PAM_SYSTEM_ERR);}if (service_name == NULL) {pam_syslog(NULL, LOG_CRIT,"pam_start: invalid argument: service == NULL");return (PAM_SYSTEM_ERR);}if (pam_conversation == NULL) {pam_syslog(NULL, LOG_CRIT,"pam_start: invalid argument: conv == NULL");return (PAM_SYSTEM_ERR);}if ((*pamh = calloc(1, sizeof(**pamh))) == NULL) {pam_syslog(NULL, LOG_CRIT, "pam_start: calloc failed for *pamh");return (PAM_BUF_ERR);}/* All service names should be files below /etc/pam.d and nothingelse. Forbid paths. */if (strrchr(service_name, '/') != NULL)service_name = strrchr(service_name, '/') + 1;/* Mark the caller as the application - permission to do certainthings is limited to a module or an application */__PAM_TO_APP(*pamh);if (((*pamh)->service_name = _pam_strdup(service_name)) == NULL) {pam_syslog(*pamh, LOG_CRIT,"pam_start: _pam_strdup failed for service name");_pam_drop(*pamh);return (PAM_BUF_ERR);} else {char *tmp;for (tmp=(*pamh)->service_name; *tmp; ++tmp)*tmp = tolower(*tmp); /* require lower case */}if (user) {if (((*pamh)->user = _pam_strdup(user)) == NULL) {pam_syslog(*pamh, LOG_CRIT,"pam_start: _pam_strdup failed for user");_pam_drop((*pamh)->service_name);_pam_drop(*pamh);return (PAM_BUF_ERR);}} else(*pamh)->user = NULL;if (confdir) {if (((*pamh)->confdir = _pam_strdup(confdir)) == NULL) {pam_syslog(*pamh, LOG_CRIT,"pam_start: _pam_strdup failed for confdir");_pam_drop((*pamh)->service_name);_pam_drop((*pamh)->user);_pam_drop(*pamh);return (PAM_BUF_ERR);}} else(*pamh)->confdir = NULL;(*pamh)->tty = NULL;(*pamh)->prompt = NULL; /* prompt for pam_get_user() */(*pamh)->ruser = NULL;(*pamh)->rhost = NULL;(*pamh)->authtok = NULL;(*pamh)->oldauthtok = NULL;(*pamh)->fail_delay.delay_fn_ptr = NULL;(*pamh)->former.choice = PAM_NOT_STACKED;(*pamh)->former.substates = NULL;
#ifdef HAVE_LIBAUDIT(*pamh)->audit_state = 0;
#endif(*pamh)->xdisplay = NULL;(*pamh)->authtok_type = NULL;(*pamh)->authtok_verified = 0;memset (&((*pamh)->xauth), 0, sizeof ((*pamh)->xauth));if (((*pamh)->pam_conversation = (struct pam_conv *)malloc(sizeof(struct pam_conv))) == NULL) {pam_syslog(*pamh, LOG_CRIT, "pam_start: malloc failed for pam_conv");_pam_drop((*pamh)->service_name);_pam_drop((*pamh)->user);_pam_drop((*pamh)->confdir);_pam_drop(*pamh);return (PAM_BUF_ERR);} else {memcpy((*pamh)->pam_conversation, pam_conversation,sizeof(struct pam_conv));}(*pamh)->data = NULL;if ( _pam_make_env(*pamh) != PAM_SUCCESS ) {pam_syslog(*pamh,LOG_ERR,"pam_start: failed to initialize environment");_pam_drop((*pamh)->pam_conversation);_pam_drop((*pamh)->service_name);_pam_drop((*pamh)->user);_pam_drop((*pamh)->confdir);_pam_drop(*pamh);return PAM_ABORT;}_pam_reset_timer(*pamh); /* initialize timer support */_pam_start_handlers(*pamh); /* cannot fail *//* According to the SunOS man pages, loading modules and resolving* symbols happens on the first call from the application. */if ( _pam_init_handlers(*pamh) != PAM_SUCCESS ) {pam_syslog(*pamh, LOG_ERR, "pam_start: failed to initialize handlers");_pam_drop_env(*pamh); /* purge the environment */_pam_drop((*pamh)->pam_conversation);_pam_drop((*pamh)->service_name);_pam_drop((*pamh)->user);_pam_drop((*pamh)->confdir);_pam_drop(*pamh);return PAM_ABORT;}D(("exiting pam_start successfully"));return PAM_SUCCESS;
}接下来来到以下代码片段：
if (user) {if (((*pamh)->user = _pam_strdup(user)) == NULL) {pam_syslog(*pamh, LOG_CRIT,"pam_start: _pam_strdup failed for user");_pam_drop((*pamh)->service_name);_pam_drop(*pamh);return (PAM_BUF_ERR);}} else(*pamh)->user = NULL;user也是由pam_start()传过来的参数。一般调用pam_start函数的时候，传入的参数都是这样：
pam_handle_t *pamh = NULL;/* 初始化，并提供一个回调函数 */if ((pam_start("login", user_name, &conv, &pamh)) != PAM_SUCCESS)exit(1);因此，此处的user就是上边代码中的user_name，即用户名，也就是/home/xxx的xxx。
如果user传入的是NULL，则直接将(*pamh)->user赋值为NULL；如果user传入的不是NULL，比如上边的user_name，则处理与上一回中讲解的对于service_name的处理相似。在此就不赘述了。
接下来来到以下代码段：
if (confdir) {if (((*pamh)->confdir = _pam_strdup(confdir)) == NULL) {pam_syslog(*pamh, LOG_CRIT,"pam_start: _pam_strdup failed for confdir");_pam_drop((*pamh)->service_name);_pam_drop((*pamh)->user);_pam_drop(*pamh);return (PAM_BUF_ERR);}} else(*pamh)->confdir = NULL;由于我们是从pam_start()调用下来的，而pam_start函数（代码如下）在调用_pam_start_internal函数时，传入const char *confdir参数的值固定为NULL，因此此处直接走else分支，将(*pamh)->confdir设置为NULL。
int pam_start (const char *service_name,const char *user,const struct pam_conv *pam_conversation,pam_handle_t **pamh)
{return _pam_start_internal(service_name, user, pam_conversation,NULL, pamh);
}至于何时传入传入const char *confdir参数的值不是NULL，那得是同文件（libpam/pam_start.c）中的pam_start_confdir函数，代码如下：
int pam_start_confdir (const char *service_name,const char *user,const struct pam_conv *pam_conversation,const char *confdir,pam_handle_t **pamh)
{return _pam_start_internal(service_name, user, pam_conversation,confdir, pamh);
}在此种情况下，处理方式也和前边的service_name和user相似。
_pam_start_internal函数的其余部分将在后续文章中继续讲解。
这篇关于PAM从入门到精通（二十三）的文章就介绍到这儿，希望我们推荐的文章对编程师们有所帮助！
