Linux userspace api

有时候需要在内核态执行用户态应用，提供了一套usermode-helper API。

参考：

https://developer.ibm.com/articles/l-user-space-apps/

参考：linux里的UEVENT_HELPER机制

在linux kernel的Kconfig里有两个配置：CONFIG_UEVENT_HELPER和CONFIG_UEVENT_HELPER_PATH，前者确定是否开启功能，后者指定路径，功能是当hotplug发生时执行这个路径对应的用户空间的脚本（程序）。

这个路径也可以在运行期间通过写入/proc/sys/kernel/hotplug或者/sys/kernel/uevent_helper来动态修改。

以下是Kconfig里的说明：

config UEVENT_HELPER
    bool "Support for uevent helper"
    help
      The uevent helper program is forked by the kernel for
      every uevent.
      Before the switch to the netlink-based uevent source, this was
      used to hook hotplug scripts into kernel device events. It
      usually pointed to a shell script at /sbin/hotplug.
      This should not be used today, because usual systems create
      many events at bootup or device discovery in a very short time
      frame. One forked process per event can create so many processes
      that it creates a high system load, or on smaller systems
      it is known to create out-of-memory situations during bootup.

config UEVENT_HELPER_PATH
    string "path to uevent helper"
    depends on UEVENT_HELPER
    default ""
    help
      To disable user space helper program execution at by default
      specify an empty string here. This setting can still be altered
      via /proc/sys/kernel/hotplug or via /sys/kernel/uevent_helper

以下是相关实现，github传送门：

...
...
#ifdef CONFIG_UEVENT_HELPER
char uevent_helper[UEVENT_HELPER_PATH_LEN] = CONFIG_UEVENT_HELPER_PATH;
#endif
...
...
#ifdef CONFIG_UEVENT_HELPER
static int kobj_usermode_filter(struct kobject *kobj)
{
    const struct kobj_ns_type_operations *ops;

    ops = kobj_ns_ops(kobj);
    if (ops) {
        const void *init_ns, *ns;

        ns = kobj->ktype->namespace(kobj);
        init_ns = ops->initial_ns();
        return ns != init_ns;
    }

    return 0;
}

static int init_uevent_argv(struct kobj_uevent_env *env, const char *subsystem)
{
    int len;

    len = strlcpy(&env->buf[env->buflen], subsystem,
              sizeof(env->buf) - env->buflen);
    if (len >= (sizeof(env->buf) - env->buflen)) {
        WARN(1, KERN_ERR "init_uevent_argv: buffer size too small\n");
        return -ENOMEM;
    }

    env->argv[0] = uevent_helper;
    env->argv[1] = &env->buf[env->buflen];
    env->argv[2] = NULL;

    env->buflen += len + 1;
    return 0;
}

static void cleanup_uevent_env(struct subprocess_info *info)
{
    kfree(info->data);
}
#endif
...
...
int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
               char *envp_ext[])
{
...
...
#ifdef CONFIG_UEVENT_HELPER
    /* call uevent_helper, usually only enabled during early boot */
    if (uevent_helper[0] && !kobj_usermode_filter(kobj)) {
        struct subprocess_info *info;

        retval = add_uevent_var(env, "HOME=/");
        if (retval)
            goto exit;
        retval = add_uevent_var(env,
                    "PATH=/sbin:/bin:/usr/sbin:/usr/bin");
        if (retval)
            goto exit;
        retval = init_uevent_argv(env, subsystem);
        if (retval)
            goto exit;

        retval = -ENOMEM;
        info = call_usermodehelper_setup(env->argv[0], env->argv,
                         env->envp, GFP_KERNEL,
                         NULL, cleanup_uevent_env, env);
        if (info) {
            retval = call_usermodehelper_exec(info, UMH_NO_WAIT);
            env = NULL; /* freed by cleanup_uevent_env */
        }
    }
#endif
...
...
}