[ieee80211] [patch] BPF taps not working for ieee80211 interfaces in monitor mode

[Patrick Kelsey]

Hi,

A bug was introduced in r254082 that results in BPF taps never being
enabled for ieee80211 interfaces that are in monitor mode.

Before r254082, bpf_track() in sys/net80211/ieee80211_freebsd.c was
identifying ieee80211 interfaces by checking to see if the value of
the ifp->if_start pointer was equal to ieee80211_start.  r254082 was a
move away from using if_start to using if_transmit in the ieee80211
stack, and bpf_track() was correspondingly updated to check the value
of ifp->if_transmit against ieee80211_vap_transmit.  The problem is
that ifp->if_transmit is set to null_transmit by
ieee80211_vap_attach() in sys/net80211/ieee80211.c for interfaces that
are in monitor mode (code that has been in place since r195846).

One fix that resolves the issue is to use what is likely to be a more
stable signature in the check in bpf_track().

A patch against r256155 is attached.

-Patrick
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