http://blog.csdn.net/UsSam/article/details/18040871
最近需要对OpenWRT内核中的beacon帧做修改
要修改beacon帧,就需要了解帧的写入和发送的过程
beacon帧发送机制:
beacon帧的发送是通过tasklet机制实现的,tasklet是软中断实现的下半部处理机制,用于中断处理流程的下半部。核心函数是beacon.c中的ath9k_beacon_tasklet函数,(将该函数的指针传递给tasklet_init()即可实现tasklet_struct的动态创建,当tasklet被调度以后,ath9k_beacon_tasklet函数会被执行)。函数体如下所示:
- void ath9k_beacon_tasklet(unsigned long data)
- {
- struct ath_softc *sc = (struct ath_softc *)data;
- struct ath_hw *ah = sc->sc_ah;
- struct ath_common *common = ath9k_hw_common(ah);
- struct ath_buf *bf = NULL;
- ...
- bf = ath9k_beacon_generate(sc->hw, vif);
- ...
- if (bf) {
- ath9k_reset_beacon_status(sc);
- ath_dbg(common, BEACON, "Transmitting beacon for slot: %d\n", slot);
- /* NB: cabq traffic should already be queued and primed */
- ath9k_hw_puttxbuf(ah, sc->beacon.beaconq, bf->bf_daddr);
- if (!edma)
- {
- ath9k_hw_txstart(ah, sc->beacon.beaconq);
- }
- }
- }
研究收发机制是为了修改beacon帧,因此接下来我们看一看beacon帧是如何产生的。这就要研究刚才提到的ath9k_beacon_generate函数了。ath9k_beacon_generate函数体如下所示:
- static struct ath_buf *ath9k_beacon_generate(struct ieee80211_hw *hw, struct ieee80211_vif *vif)
- {
- struct ath_softc *sc = hw->priv;
- struct ath_common *common = ath9k_hw_common(sc->sc_ah);
- struct ath_buf *bf;
- struct ath_vif *avp = (void *)vif->drv_priv;
- struct sk_buff *skb;
- struct ath_txq *cabq = sc->beacon.cabq;
- struct ieee80211_tx_info *info;
- struct ieee80211_mgmt *mgmt_hdr;
- int cabq_depth;
- if (avp->av_bcbuf == NULL)
- return NULL;
- bf = avp->av_bcbuf;
- skb = bf->bf_mpdu;
- if (skb) {
- dma_unmap_single(sc->dev, bf->bf_buf_addr, skb->len, DMA_TO_DEVICE);
- dev_kfree_skb_any(skb);
- bf->bf_buf_addr = 0;
- bf->bf_mpdu = NULL; /*清空缓存*/
- }
- skb = ieee80211_beacon_get(hw, vif);<strong>
- </strong>
- if (skb == NULL)
- return NULL; /*skb生成失败退出*/
- bf->bf_mpdu = skb; /*将生成的beacon帧缓存赋给bf结构体,此处是指针赋值,可以只用任一指针对对象进行修改*/
- mgmt_hdr = (struct ieee80211_mgmt *)skb->data;
- mgmt_hdr->u.beacon.timestamp = avp->tsf_adjust; /*用ieee80211_mgmt结构体将skb->data中的前面若干个字段提取出来(包括frame_control,duration,da,sa,bssid,seqctrl,以及beacon帧特有的timestamp,beacon interval,capability information,variable字段),并对其timestamp字段进行数据的写入*/
- ...
- return bf;
- }