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

[Johannes Berg <johannes@sipsolutions.net>]

On Sun, 2026-03-08 at 16:45 +0000, 傅继晗 wrote:
> 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).

It actually broke the others too, as you note later.

> 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.

I did have pretty much the same attempt at a fix:

https://lore.kernel.org/linux-wireless/20251216111909.25076-2-johannes@sipsolutions.net/

but it was reported to cause crashes on certain devices, so we didn't
think it was very safe at the time.

Is that no longer an issue?

johannes