内核启动用户态的程序

内核启动用户态的应用程序是通过call_usermodehelper来调用的，比较常见的调用，比如modprobe. 

 static inline int
 call_usermodehelper(char *path, char **argv, char **envp, enum umh_wait wait)

参数说明; 

path : 应用程序的位置 

argv : 传递给用户态应用程序的参数 

envp: 传递给应用程序的环境变量 

wait : 调用用户态应用程序的内核程序是否等到用户态程序退出后才退出。 

使用的例子： 

我们来看kmod.c的源码，它通过调用用户态的modprobe来加载所需要的驱动模块。 

 int request_module(const char *fmt, ...)
 {
     va_list args;
     char module_name[MODULE_NAME_LEN];
     unsigned int max_modprobes;
     int ret;

 ///构造传递给modeprobe的参数
     char *argv[] = { modprobe_path, "-q", "--", module_name, NULL };
     static char *envp[] = { "HOME=/",
                 "TERM=linux",
                 "PATH=/sbin:/usr/sbin:/bin:/usr/bin",
                 NULL };
     static atomic_t kmod_concurrent = ATOMIC_INIT(0);
 #define MAX_KMOD_CONCURRENT 50  /* Completely arbitrary value - KAO */
     static int kmod_loop_msg;

     va_start(args, fmt);
     ret = vsnprintf(module_name, MODULE_NAME_LEN, fmt, args);
     va_end(args);
     if (ret >= MODULE_NAME_LEN)
         return -ENAMETOOLONG;

     /* If modprobe needs a service that is in a module, we get a recursive
      * loop.  Limit the number of running kmod threads to max_threads/2 or
      * MAX_KMOD_CONCURRENT, whichever is the smaller.  A cleaner method
      * would be to run the parents of this process, counting how many times
      * kmod was invoked.  That would mean accessing the internals of the
      * process tables to get the command line, proc_pid_cmdline is static
      * and it is not worth changing the proc code just to handle this case.
      * KAO.
      *
      * "trace the ppid" is simple, but will fail if someone's
      * parent exits.  I think this is as good as it gets. --RR
      */
     max_modprobes = min(max_threads/2, MAX_KMOD_CONCURRENT);
     atomic_inc(&kmod_concurrent);
     if (atomic_read(&kmod_concurrent) > max_modprobes) {
         /* We may be blaming an innocent here, but unlikely */
         if (kmod_loop_msg++ < 5)
             printk(KERN_ERR
                    "request_module: runaway loop modprobe %s\n",
                    module_name);
         atomic_dec(&kmod_concurrent);
         return -ENOMEM;
     }
 ///调用modeprobe.
     ret = call_usermodehelper(modprobe_path, argv, envp, 1);
     atomic_dec(&kmod_concurrent);
     return ret;
 }