Subject: Re: [PATCH v2] 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 15:12:32 +0200

On May 31, Ryder Lee wrote:
> On Sun, 2026-05-31 at 14:11 +0200, [email-masked] wrote:
> > > On Sun, 2026-05-31 at 10:55 +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]>
> > > > ---
> > > > Changes in v2:
> > > > - Do not use link_id in mt7996_mac_write_txwi if it is
> > > > IEEE80211_LINK_UNSPECIFIED
> > > > - In mt7996_mac_write_txwi() rely on link_id calculated in
> > > >   mt7996_tx_prepare_skb().
> > > > - Link to v1:
> > > > https://lore.kernel.org/r/[email-masked]
> > > >  
> > > > ---
> > > >  drivers/net/wireless/mediatek/mt76/mt7996/mac.c    | 14 ++++----
> > > > ----
> > > > --
> > > >  drivers/net/wireless/mediatek/mt76/mt7996/mcu.c    |  5 +++--
> > > >  drivers/net/wireless/mediatek/mt76/mt7996/mt7996.h |  3 ++-
> > > >  3 files changed, 9 insertions(+), 13 deletions(-)
> > > > 
> > > > diff --git a/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> > > > b/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> > > > index c98446057282..95b3078d9667 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 |
> > > > @@ -874,17 +874,11 @@ void mt7996_mac_write_txwi(struct
> > > > mt7996_dev
> > > > *dev, __le32 *txwi,
> > > >  	bool beacon = !!(changed & (BSS_CHANGED_BEACON |
> > > >  				    BSS_CHANGED_BEACON_ENABLED))
> > > > &&
> > > > (!inband_disc);
> > > >  
> > > > -	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) {
> > > >  		if (wcid->offchannel)
> > > >  			mlink = rcu_dereference(mvif-
> > > > > mt76.offchannel_link);
> > > > -		if (!mlink)
> > > > +		if (!mlink && link_id !=
> > > > IEEE80211_LINK_UNSPECIFIED)
> > > >  			mlink = rcu_dereference(mvif-
> > > > > mt76.link[link_id]);
> > > >  	}
> > > >  
> > > > @@ -1096,7 +1090,7 @@ 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);
> > > > 
> > > > ---
> > > > base-commit: 4913f44167cf35a9536e9eec7352e15b2de0c573
> > > > change-id: 20260530-mt76_tx_status_skb_add-overwrite-fix-
> > > > 85818a9bb31f
> > > > 
> > > > Best regards,
> > > > 
> > > > 
> > > We might expand flags further so this still doesn't solve the issue
> > > of
> > > flags being cleared - it only works for MLO flag. And the
> > > developers
> > > still won't easily notice that the flags are being cleared.
> > 
> > My opinion is we should consider just upstream code and then change
> > it as soon
> > as you post this new feature upstream, but I will let Felix comments
> > on it.
> > Moreover, the proposed approach aligns link_id used in
> > mt7996_tx_prepare_skb()
> > to the one used in mt7996_mac_write_txwi() and fix a possible OOB bug
> > in
> > mt7996_mac_write_txwi().
> > 
> > Regards,
> > Lorenzo
> > 
> > > 
> 
> Just to tie in with this patch subject - I'm just thinking of a way to
> solve this once and for all. If the problem is reading zeroed info-
> >control.flags, wouldn't it be better to just pass a u32 flags,
> something like this:
> 
> u32 flags = info->control.flags
> 
> mt7996_mac_write_txwi(dev, (__le32 *)buf, skb, wcid, NULL, 0, 0,
> 			      changed, flags);
> 
> We can use all flags then.

what about link_id? Should it be the same between mt7996_tx_prepare_skb()
and mt7996_mac_write_txwi()?

Regards,
Lorenzo

> Ryder