libnids中多线程

冷不防 2022-08-08 09:23 128阅读 0赞

可以通过设置全局变量nids\_params.multiproc不为0来在libnids中使用多线程捕获数据包。libnids使用的是glib库的gthread-2.0线程函数（该函数库某些函数已经过时，例如线程创建函数；过时的api：[https://developer.gnome.org/glib/stable/glib-Deprecated-Thread-APIs.html][https_developer.gnome.org_glib_stable_glib-Deprecated-Thread-APIs.html]；新版thread接口：https://developer.gnome.org/glib/stable/glib-Threads.html）。

在libnids1.24源文件libnids.c中nids\_run会调用pcap\_loop函数进行循环捕获数据包的操作；pcap\_loop函数的回调函数nids\_pcap\_heandler在设置了全局变量multiproc时，会将捕获的数据包片段插入到异步捕获队列中（这里的异步队列是glib的接口，用于在不同线程之间通信，[https://developer.gnome.org/glib/stable/glib-Asynchronous-Queues.html][https_developer.gnome.org_glib_stable_glib-Asynchronous-Queues.html]）

int nids_run()
    {
        if (!desc) {
    	strcpy(nids_errbuf, "Libnids not initialized");
    	return 0;
        }
        START_CAP_QUEUE_PROCESS_THREAD(); /* threading... */
        pcap_loop(desc, -1, (pcap_handler) nids_pcap_handler, 0); //循环捕获数据包
        /* FIXME: will this code ever be called? Don't think so - mcree */
        STOP_CAP_QUEUE_PROCESS_THREAD(); 
        nids_exit();
        return 0;
    }

nids\_pcap\_handler部分代码：

#ifdef HAVE_LIBGTHREAD_2_0
         if(nids_params.multiproc) {  //如果系统有thread_2_0函数库并设置了使用多线程
            /* 
             * Insert received fragment into the async capture queue.
             * We hope that the overhead of memcpy 
             * will be saturated by the benefits of SMP - mcree
             */
            qitem=malloc(sizeof(struct cap_queue_item));
            if (qitem && (qitem->data=malloc(hdr->caplen - nids_linkoffset))) {
              qitem->caplen=hdr->caplen - nids_linkoffset;
              memcpy(qitem->data,data_aligned,qitem->caplen);
              g_async_queue_lock(cap_queue);  //为异步队列加锁
              /* ensure queue does not overflow，如果队列长度大于全局变量queue_limit的限制就说明overflow了*/
              if(g_async_queue_length_unlocked(cap_queue) > nids_params.queue_limit) {
    	    /* queue limit reached: drop packet - should we notify user via syslog? */
    	    free(qitem->data); //丢弃数据包数据
    	    free(qitem);  //销毁数据包
    	    } else {
    	    /* insert packet to queue */
    	    g_async_queue_push_unlocked(cap_queue,qitem);
              }
              g_async_queue_unlock(cap_queue); //解锁异步队列
    	}
         } else { /* user requested simple passthru - no threading */
            call_ip_frag_procs(data_aligned,hdr->caplen - nids_linkoffset);
         }
     #else
         call_ip_frag_procs(data_aligned,hdr->caplen - nids_linkoffset);
     #endif
    }

可以看出nids\_run函数中是通过宏START\_CAP\_QUEUE\_PROCESS\_THREAD()来创建新线程的，通过STOP\_CAP\_QUEUE\_PROCESS\_THREAD()来销毁线程的。

如下是宏 START\_CAP\_QUEUE\_PROCESS\_THREAD()和STOP\_CAP\_QUEUE\_PROCESS\_THREAD()的定义：

//g_thread_create_full从version 2.32开始被废弃了 and should not be used in newly-written code.
    //https://developer.gnome.org/glib/stable/glib-Deprecated-Thread-APIs.html#g-thread-create-full
    
    #define START_CAP_QUEUE_PROCESS_THREAD() \
        if(nids_params.multiproc) { /* threading... */ \
    	 if(!(g_thread_create_full((GThreadFunc)cap_queue_process_thread,NULL,0,FALSE,TRUE,G_THREAD_PRIORITY_LOW,&gerror))) { \
    	    strcpy(nids_errbuf, "thread: "); \
    	    strncat(nids_errbuf, gerror->message, sizeof(nids_errbuf) - 8); \
    	    return 0; \
    	 }; \
        }
    
    #define STOP_CAP_QUEUE_PROCESS_THREAD() \
        if(nids_params.multiproc) { /* stop the capture process thread */ \
    	 g_async_queue_push(cap_queue,&EOF_item); \
        }

g\_thread\_create\_full创建新线程，其中第一个参数cap\_queue\_process\_thread是要在新创建的线程里执行的函数，它的定义如下：

/* thread entry point 
     * pops capture queue items and feeds them to
     * the ip fragment processors - mcree
     */
    static void cap_queue_process_thread()
    {
         struct cap_queue_item *qitem;
         
         while(1) { /* loop "forever" */
    	  qitem=g_async_queue_pop(cap_queue);
    	  if (qitem==&EOF_item) break; /* EOF item received: we should exit */
    	  call_ip_frag_procs(qitem->data,qitem->caplen);
    	  free(qitem->data);
    	  free(qitem);
         }
         g_thread_exit(NULL);
    }

该函数从异步队列中不断取出pcap\_loop捕获的数据包，交给call\_ip\_frag\_procs函数进行处理。上面nids\_pcap\_handler函数的部分代码中可以看出，如果没有设置使用多线程，那么直接在nids\_pcap\_handler函数中就使用call\_ip\_frag\_procs函数处理数据包了。接下来处理过程参考 [http://blog.csdn.net/u013074465/article/details/45555837][http_blog.csdn.net_u013074465_article_details_45555837]

[https_developer.gnome.org_glib_stable_glib-Deprecated-Thread-APIs.html]: https://developer.gnome.org/glib/stable/glib-Deprecated-Thread-APIs.html
[https_developer.gnome.org_glib_stable_glib-Asynchronous-Queues.html]: https://developer.gnome.org/glib/stable/glib-Asynchronous-Queues.html
[http_blog.csdn.net_u013074465_article_details_45555837]: http://blog.csdn.net/u013074465/article/details/45555837