[PATCH] wifi: mac80211: fix monitor mode frame capture for real chanctx drivers

[傅继晗 <fjhhz1997@gmail.com>]

Commit 0a44dfc07074 ("wifi: mac80211: simplify non-chanctx drivers")
removed the fallback path in ieee80211_monitor_start_xmit() for when
the monitor interface has no channel context assigned. This broke frame
capture and injection for drivers that implement real channel context
ops (as opposed to the ieee80211_emulate_* helpers), such as the mt76
family, when a monitor interface runs alongside another interface
(e.g. managed mode).

In that scenario the (virtual) monitor sdata does not get a chanctx of
its own, even though there is an active one from the other interface.
Before the simplification the code fell back to local->_oper_chandef;
after it, the code goes straight to fail_rcu and silently drops every
injected frame.

Commit d594cc6f2c58 ("wifi: mac80211: restore non-chanctx injection
behaviour") restored the fallback for drivers using emulate_chanctx,
but explicitly left real chanctx drivers unfixed.

Fix this by falling back to the first entry in local->chanctx_list
when the monitor vif has no chanctx and the driver uses real channel
contexts. This is analogous to how ieee80211_hw_conf_chan() already
uses the same pattern.

Tested on MT7921AU (mt76) USB adapter:
  - v6.13: managed + monitor coexistence restored (0 -> 37 frames/5s)
  - v6.19: managed + monitor coexistence restored (0 -> 39 frames/5s)
  - v7.0-rc2: managed + monitor coexistence restored (0 -> 33 frames/5s)

Cc: stable@vger.kernel.org
Fixes: 0a44dfc07074 ("wifi: mac80211: simplify non-chanctx drivers")
Link: https://github.com/morrownr/USB-WiFi/issues/682
Signed-off-by: 傅继晗 <fjhhz1997@gmail.com>
---
 net/mac80211/tx.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
index 8cdbd41..56eaf9a 100644
--- a/net/mac80211/tx.c
+++ b/net/mac80211/tx.c
@@ -2396,12 +2396,28 @@ netdev_tx_t ieee80211_monitor_start_xmit(struct sk_buff *skb,
 				rcu_dereference(tmp_sdata->vif.bss_conf.chanctx_conf);
 	}
 
-	if (chanctx_conf)
+	if (chanctx_conf) {
 		chandef = &chanctx_conf->def;
-	else if (local->emulate_chanctx)
+	} else if (local->emulate_chanctx) {
 		chandef = &local->hw.conf.chandef;
-	else
-		goto fail_rcu;
+	} else {
+		/*
+		 * For real chanctx drivers (e.g. mt76), the monitor
+		 * interface may not have a chanctx assigned when running
+		 * concurrently with another interface. Fall back to any
+		 * active chanctx so that injection can still work on the
+		 * operating channel.
+		 */
+		struct ieee80211_chanctx *ctx;
+
+		ctx = list_first_entry_or_null(&local->chanctx_list,
+					       struct ieee80211_chanctx,
+					       list);
+		if (ctx)
+			chandef = &ctx->conf.def;
+		else
+			goto fail_rcu;
+	}
 
 	/*
 	 * If driver/HW supports IEEE80211_CHAN_CAN_MONITOR we still
-- 
2.43.0