When a monitor interface is present together with other interfaces, a received skb is copied and received on the monitor netdev. Before, the copied skb was allocated with exactly the amount of space needed for the radiotap header, resulting in an skb without any headroom at all being received on the monitor netdev. With the introduction of eBPF and XDP in the kernel, skbs may be processed by custom eBPF programs. However, since the skb cannot be reallocated in the eBPF program, no more data or headers can be pushed. The old code made sure the final headroom was zero regardless of the value of NET_SKB_PAD, so increasing that constant would have no effect. Now we allocate monitor skb copies with a headroom of NET_SKB_PAD bytes before the radiotap header. Monitor interfaces now behave in the same way as other netdev interfaces that honor the NET_SKB_PAD constant. Signed-off-by: Johan Almbladh <johan.almbladh@xxxxxxxxxxxxxxxxx> --- net/mac80211/rx.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/net/mac80211/rx.c b/net/mac80211/rx.c index 771921c057e8..2563473b5cf1 100644 --- a/net/mac80211/rx.c +++ b/net/mac80211/rx.c @@ -730,7 +730,8 @@ ieee80211_make_monitor_skb(struct ieee80211_local *local, * Need to make a copy and possibly remove radiotap header * and FCS from the original. */ - skb = skb_copy_expand(*origskb, needed_headroom, 0, GFP_ATOMIC); + skb = skb_copy_expand(*origskb, needed_headroom + NET_SKB_PAD, + 0, GFP_ATOMIC); if (!skb) return NULL; -- 2.25.1
