wifi: mt76: mt7996: Set proper link destination address in mt7996_tx()

Hello Lorenzo Bianconi,

Commit f940c9b7aef6 ("wifi: mt76: mt7996: Set proper link destination address in mt7996_tx()") from Jul 31, 2025 (linux-next), leads to the following Smatch static checker warning:

	drivers/net/wireless/mediatek/mt76/mt7996/main.c:1344 mt7996_tx()
	error: testing array offset 'link_id' after use.

drivers/net/wireless/mediatek/mt76/mt7996/main.c

    1288 static void mt7996_tx(struct ieee80211_hw *hw,
    1289                       struct ieee80211_tx_control *control,
    1290                       struct sk_buff *skb)
    1291 {
    1292         struct ieee80211_hdr *hdr = (struct ieee80211_hdr *)skb->data;
    1293         struct mt7996_dev *dev = mt7996_hw_dev(hw);
    1294         struct ieee80211_sta *sta = control->sta;
    1295         struct mt7996_sta *msta = sta ? (void *)sta->drv_priv : NULL;
    1296         struct mt76_phy *mphy = hw->priv;
    1297         struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
    1298         struct ieee80211_vif *vif = info->control.vif;
    1299         struct mt7996_vif *mvif = vif ? (void *)vif->drv_priv : NULL;
    1300         struct mt76_wcid *wcid = &dev->mt76.global_wcid;
    1301         u8 link_id = u32_get_bits(info->control.flags,
    1302                                   IEEE80211_TX_CTRL_MLO_LINK);
    1303 
    1304         rcu_read_lock();
    1305 
    1306         /* Use primary link_id if the value from mac80211 is set to
    1307          * IEEE80211_LINK_UNSPECIFIED.
    1308          */
    1309         if (link_id == IEEE80211_LINK_UNSPECIFIED) {
    1310                 if (msta)
    1311                         link_id = msta->deflink_id;
    1312                 else if (mvif)
    1313                         link_id = mvif->mt76.deflink_id;

Can link_id be IEEE80211_LINK_UNSPECIFIED after this if statement?

    1314         }
    1315 
    1316         if (vif && ieee80211_vif_is_mld(vif)) {
    1317                 struct ieee80211_bss_conf *link_conf;
    1318 
    1319                 if (msta) {
    1320                         struct ieee80211_link_sta *link_sta;
    1321 
    1322                         link_sta = rcu_dereference(sta->link[link_id]);

Some unchecked uses. IEEE80211_LINK_UNSPECIFIED would be off-by-one.

    1323                         if (!link_sta)
    1324                                 link_sta = rcu_dereference(sta->link[msta->deflink_id]);
    1325 
    1326                         if (link_sta) {
    1327                                 memcpy(hdr->addr1, link_sta->addr, ETH_ALEN);
    1328                                 if (ether_addr_equal(sta->addr, hdr->addr3))
    1329                                         memcpy(hdr->addr3, link_sta->addr, ETH_ALEN);
    1330                         }
    1331                 }
    1332 
    1333                 link_conf = rcu_dereference(vif->link_conf[link_id]);

Here too.

    1334                 if (link_conf) {
    1335                         memcpy(hdr->addr2, link_conf->addr, ETH_ALEN);
    1336                         if (ether_addr_equal(vif->addr, hdr->addr3))
    1337                                 memcpy(hdr->addr3, link_conf->addr, ETH_ALEN);
    1338                 }
    1339         }
    1340 
    1341         if (mvif) {
    1342                 struct mt76_vif_link *mlink = &mvif->deflink.mt76;
    1343 

--> 1344 if (link_id < IEEE80211_LINK_UNSPECIFIED)

Is this checker required?

    1345                         mlink = rcu_dereference(mvif->mt76.link[link_id]);
    1346 
    1347                 if (mlink->wcid)
    1348                         wcid = mlink->wcid;
    1349 
    1350                 if (mvif->mt76.roc_phy &&
    1351                     (info->flags & IEEE80211_TX_CTL_TX_OFFCHAN)) {
    1352                         mphy = mvif->mt76.roc_phy;
    1353                         if (mphy->roc_link)
    1354                                 wcid = mphy->roc_link->wcid;
    1355                 } else {
    1356                         mphy = mt76_vif_link_phy(mlink);
    1357                 }
    1358         }
    1359 
    1360         if (!mphy) {
    1361                 ieee80211_free_txskb(hw, skb);
    1362                 goto unlock;
    1363         }
    1364 
    1365         if (msta && link_id < IEEE80211_LINK_UNSPECIFIED) {

And this?

    1366                 struct mt7996_sta_link *msta_link;
    1367 
    1368                 msta_link = rcu_dereference(msta->link[link_id]);
    1369                 if (msta_link)
    1370                         wcid = &msta_link->wcid;
    1371         }
    1372         mt76_tx(mphy, control->sta, wcid, skb);
    1373 unlock:
    1374         rcu_read_unlock();
    1375 }

regards,
dan carpenter

Hi Dan,

I agree, we can get rid of this condition since if mvif (or msta) is not NULL, link_id can't be set to IEEE80211_LINK_UNSPECIFIED. I will post a fix for it.

Regards,
Lorenzo