修改WiFi beacon帧
http://blog.iyunv.com/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;
[*]}
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