Linux内核模块的自动加载及request_module系统调用

automatic kernel-module loading

Linux提供一种模块自动加载的机制，是通过request_module()系统调用实现的：
当内核发现一个需要的module不在内核中时，会调用request_module去用户空间创建进程去加载这个缺失的module;
For example, if an application opens a char device with a given major and minor number and no driver exists for those numbers, the char device code will attempt to locate a driver by calling:

request_module("char-major-%d-%d", MAJOR(dev), MINOR(dev));

request通过module名字匹配的方式，自动加载module;

request_module

request_module调用的是__request_module()，源码如下：
/kernel/kmod.c

/**
 * __request_module - try to load a kernel module
 * @wait: wait (or not) for the operation to complete
 * @fmt: printf style format string for the name of the module
 * @...: arguments as specified in the format string
 *
 * Load a module using the user mode module loader. The function returns
 * zero on success or a negative errno code or positive exit code from
 * "modprobe" on failure. Note that a successful module load does not mean
 * the module did not then unload and exit on an error of its own. Callers
 * must check that the service they requested is now available not blindly
 * invoke it.
 *
 * If module auto-loading support is disabled then this function
 * simply returns -ENOENT.
 */
int __request_module(bool wait, const char *fmt, ...)
{
	va_list args;
	char module_name[MODULE_NAME_LEN];
	int ret;

	/*
	 * We don't allow synchronous module loading from async.  Module
	 * init may invoke async_synchronize_full() which will end up
	 * waiting for this task which already is waiting for the module
	 * loading to complete, leading to a deadlock.
	 */
	WARN_ON_ONCE(wait && current_is_async());

	if (!modprobe_path[0])
		return -ENOENT;

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

	ret = security_kernel_module_request(module_name);
	if (ret)
		return ret;

	if (atomic_dec_if_positive(&kmod_concurrent_max) < 0) {
		pr_warn_ratelimited("request_module: kmod_concurrent_max (%u) close to 0 (max_modprobes: %u), for module %s, throttling...",
				    atomic_read(&kmod_concurrent_max),
				    MAX_KMOD_CONCURRENT, module_name);
		ret = wait_event_killable_timeout(kmod_wq,
						  atomic_dec_if_positive(&kmod_concurrent_max) >= 0,
						  MAX_KMOD_ALL_BUSY_TIMEOUT * HZ);
		if (!ret) {
			pr_warn_ratelimited("request_module: modprobe %s cannot be processed, kmod busy with %d threads for more than %d seconds now",
					    module_name, MAX_KMOD_CONCURRENT, MAX_KMOD_ALL_BUSY_TIMEOUT);
			return -ETIME;
		} else if (ret == -ERESTARTSYS) {
			pr_warn_ratelimited("request_module: sigkill sent for modprobe %s, giving up", module_name);
			return ret;
		}
	}

	trace_module_request(module_name, wait, _RET_IP_);

	ret = call_modprobe(module_name, wait ? UMH_WAIT_PROC : UMH_WAIT_EXEC);

	atomic_inc(&kmod_concurrent_max);
	wake_up(&kmod_wq);

	return ret;
}
EXPORT_SYMBOL(__request_module);

request_module的实现其实是调用内核提供的call_usermodehelper的API，这类API帮助内核进程在用户空间创建进程并执行相关指令;

如何禁用模块的自动加载

打开：

vim /etc/modprobe.d/blacklist.conf 

添加：

blacklist driver-name

把指定module加入黑名单blacklist，之后就不会再去自动加载它;