Subject: Re: [PATCH] wifi: mt76: mt7996: fix reading zeroed info->control.flags after mt76_tx_status_skb_add()

From: "[email-masked]" <[email-masked]>
Date: Sun, 31 May 2026 09:09:52 +0200

> On Sat, 2026-05-30 at 17:25 +0200, Lorenzo Bianconi wrote:
> > mt76_tx_status_skb_add() zeroes the mt76_tx_cb struct stored at
> > info->status.status_driver_data via memset(). Since info->control and
> > info->status are members of the same union in ieee80211_tx_info,
> > this overwrites info->control.flags.
> > In mt7996_tx_prepare_skb(), mt76_tx_status_skb_add() is called before
> > mt7996_mac_write_txwi(), which re-reads info->control.flags to
> > extract
> > IEEE80211_TX_CTRL_MLO_LINK. Because the field has been zeroed, the
> > link_id always resolves to 0 for frames using global_wcid, leading to
> > incorrect TXWI configuration.
> > Fix this by passing link_id as an explicit parameter to
> > mt7996_mac_write_txwi(). In mt7996_tx_prepare_skb(), the link_id is
> > already extracted from info->control.flags before the destructive
> > mt76_tx_status_skb_add() call. For the beacon and inband discovery
> > callers in mcu.c, use link_conf->link_id directly.
> > 
> > Fixes: f0b0b239b8f36 ("wifi: mt76: mt7996: rework
> > mt7996_mac_write_txwi() for MLO support")
> > Signed-off-by: Lorenzo Bianconi <[email-masked]>
> > ---
> >  drivers/net/wireless/mediatek/mt76/mt7996/mac.c    | 9 +++------
> >  drivers/net/wireless/mediatek/mt76/mt7996/mcu.c    | 5 +++--
> >  drivers/net/wireless/mediatek/mt76/mt7996/mt7996.h | 3 ++-
> >  3 files changed, 8 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> > b/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> > index c98446057282..2d3f80b3e41a 100644
> > --- a/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> > +++ b/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> > @@ -856,7 +856,8 @@ mt7996_mac_write_txwi_80211(struct mt7996_dev
> > *dev, __le32 *txwi,
> >  void mt7996_mac_write_txwi(struct mt7996_dev *dev, __le32 *txwi,
> >  			   struct sk_buff *skb, struct mt76_wcid
> > *wcid,
> >  			   struct ieee80211_key_conf *key, int pid,
> > -			   enum mt76_txq_id qid, u32 changed)
> > +			   enum mt76_txq_id qid, u32 changed,
> > +			   unsigned int link_id)
> >  {
> >  	struct ieee80211_hdr *hdr = (struct ieee80211_hdr *)skb-
> > >data;
> >  	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
> > @@ -866,7 +867,6 @@ void mt7996_mac_write_txwi(struct mt7996_dev
> > *dev, __le32 *txwi,
> >  	bool is_8023 = info->flags &
> > IEEE80211_TX_CTL_HW_80211_ENCAP;
> >  	struct mt76_vif_link *mlink = NULL;
> >  	struct mt7996_vif *mvif;
> > -	unsigned int link_id;
> >  	u16 tx_count = 15;
> >  	u32 val;
> >  	bool inband_disc = !!(changed &
> > (BSS_CHANGED_UNSOL_BCAST_PROBE_RESP |
> > @@ -876,9 +876,6 @@ void mt7996_mac_write_txwi(struct mt7996_dev
> > *dev, __le32 *txwi,
> >  
> >  	if (wcid != &dev->mt76.global_wcid)
> >  		link_id = wcid->link_id;
> > -	else
> > -		link_id = u32_get_bits(info->control.flags,
> > -				       IEEE80211_TX_CTRL_MLO_LINK);
> >  
> >  	mvif = vif ? (struct mt7996_vif *)vif->drv_priv : NULL;
> >  	if (mvif) {
> > @@ -1096,7 +1093,7 @@ int mt7996_tx_prepare_skb(struct mt76_dev
> > *mdev, void *txwi_ptr,
> >  	/* Transmit non qos data by 802.11 header and need to fill
> > txd by host*/
> >  	if (!is_8023 || pid >= MT_PACKET_ID_FIRST)
> >  		mt7996_mac_write_txwi(dev, txwi_ptr, tx_info->skb,
> > wcid, key,
> > -				      pid, qid, 0);
> > +				      pid, qid, 0, link_id);
> >  
> >  	/* MT7996 and MT7992 require driver to provide the MAC TXP
> > for AddBA
> >  	 * req
> > diff --git a/drivers/net/wireless/mediatek/mt76/mt7996/mcu.c
> > b/drivers/net/wireless/mediatek/mt76/mt7996/mcu.c
> > index 8be40d60ad29..a14c63438923 100644
> > --- a/drivers/net/wireless/mediatek/mt76/mt7996/mcu.c
> > +++ b/drivers/net/wireless/mediatek/mt76/mt7996/mcu.c
> > @@ -3103,7 +3103,7 @@ mt7996_mcu_beacon_cont(struct mt7996_dev *dev,
> >  
> >  	buf = (u8 *)bcn + sizeof(*bcn);
> >  	mt7996_mac_write_txwi(dev, (__le32 *)buf, skb, wcid, NULL,
> > 0, 0,
> > -			      BSS_CHANGED_BEACON);
> > +			      BSS_CHANGED_BEACON, link_conf-
> > >link_id);
> >  
> >  	memcpy(buf + MT_TXD_SIZE, skb->data, skb->len);
> >  }
> > @@ -3249,7 +3249,8 @@ int mt7996_mcu_beacon_inband_discov(struct
> > mt7996_dev *dev,
> >  
> >  	buf = (u8 *)tlv + sizeof(*discov);
> >  
> > -	mt7996_mac_write_txwi(dev, (__le32 *)buf, skb, wcid, NULL,
> > 0, 0, changed);
> > +	mt7996_mac_write_txwi(dev, (__le32 *)buf, skb, wcid, NULL,
> > 0, 0,
> > +			      changed, link_conf->link_id);
> >  
> >  	memcpy(buf + MT_TXD_SIZE, skb->data, skb->len);
> >  
> > diff --git a/drivers/net/wireless/mediatek/mt76/mt7996/mt7996.h
> > b/drivers/net/wireless/mediatek/mt76/mt7996/mt7996.h
> > index 0dc4198fcf8b..0d6488522ba7 100644
> > --- a/drivers/net/wireless/mediatek/mt76/mt7996/mt7996.h
> > +++ b/drivers/net/wireless/mediatek/mt76/mt7996/mt7996.h
> > @@ -874,7 +874,8 @@ void mt7996_mac_enable_nf(struct mt7996_dev *dev,
> > u8 band);
> >  void mt7996_mac_write_txwi(struct mt7996_dev *dev, __le32 *txwi,
> >  			   struct sk_buff *skb, struct mt76_wcid
> > *wcid,
> >  			   struct ieee80211_key_conf *key, int pid,
> > -			   enum mt76_txq_id qid, u32 changed);
> > +			   enum mt76_txq_id qid, u32 changed,
> > +			   unsigned int link_id);
> >  void mt7996_mac_update_beacons(struct mt7996_phy *phy);
> >  void mt7996_mac_set_coverage_class(struct mt7996_phy *phy);
> >  void mt7996_mac_work(struct work_struct *work);
> 
> The reason we didn't make the same change is because we use other
> control flags (IEEE80211_TX_CTRL*) of info->control.flags not just MLO
> one. So with this change we still need to copy over the other flags and
> pass them in as well.

Do you mean you are using info->control.flags in mt7996_mac_write_txwi() in
some downstream code? If so, I guess you can use a similar approach and
just pass the required field. Copy the full ieee80211_tx_info struct on
per-packet basis seems unnecessary.

Regards,
Lorenzo

> 
> Ryder
> > 
> > ---
> > base-commit: 4913f44167cf35a9536e9eec7352e15b2de0c573
> > change-id: 20260530-mt76_tx_status_skb_add-overwrite-fix-85818a9bb31f
> > 
> > Best regards,
>