Android: 启动init.rc 中service的权限问题

通过property_set("ctl.start", service_xx);

来启动init.rc中的service是一个很方便方法来调用某个可执行程序或某个脚本程序

service service_xx  /system/bin/xx

    disabled
    oneshot

 

但在非AID_ROOT、AID_SYSTEM 用户的进程中调用ctl.start ctl.stop会碰到权限问题:

system/core/init/property_service.c

 

 /*
  * White list of UID that are allowed to start/stop services.
  * Currently there are no user apps that require.
  */
 struct {
     const char *service;
     unsigned int uid;
     unsigned int gid;
 } control_perms[] = {
     { "dumpstate",AID_SHELL, AID_LOG },
      {NULL, 0, 0 }
 };

 /*
  * Checks permissions for starting/stoping system services.
  * AID_SYSTEM and AID_ROOT are always allowed.
  *
  * Returns 1 if uid allowed, 0 otherwise.
  */
 static int check_control_perms(const char *name, int uid, int gid) {
     int i;
     if (uid == AID_SYSTEM || uid == AID_ROOT)
         return 1;

     /* Search the ACL */
     for (i = 0; control_perms[i].service; i++) {
         if (strcmp(control_perms[i].service, name) == 0) {
             if ((uid && control_perms[i].uid == uid) ||
                 (gid && control_perms[i].gid == gid)) {
                 return 1;
             }
         }
     }
     return 0;
 }

 

只有uid == AID_SYSTEM || uid == AID_ROOT

或符合 control_perms[] = {
    { "dumpstate",AID_SHELL, AID_LOG },
     {NULL, 0, 0 }
}; 的uid进程才有权限star/stop services

 

因此，如果我们碰到了权限问题，根据log提示，在/system/core/include/private/android_filesystem_config.h

中查到进程定义，添加到control_perms[]列表

 

比如，uid ==AID_WIFI的某个程序需要权限启动service_xx

control_perms[] = {
    { "dumpstate",AID_SHELL, AID_LOG },

+  { "service_xx ",AID_WIFI, AID_WIFI},
     {NULL, 0, 0 }
};

很受启发，由此弄明白服务的加载过程。init.c的man函数中的 handle_property_set_fd();该函数（在property_service.c）中check_control_perms（）函数成功时handle_control_message（）执行。最终调用msg_start(arg);