ath9k: Do not remove header padding on RX from short frames

The 802.11 header is only padded to 32-bit boundary when the frame has
a non-zero length payload. In other words, control frames (e.g., ACK)
do not have a padding and we should not try to remove it. This fixes
monitor mode for short control frames. In addition, the hdrlen&3 use
is described in more detail to make it easier to understand how the
padding length is calculated.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath9k/recv.c

@@ -571,8 +571,16 @@ int ath_rx_tasklet(struct ath_softc *sc, int flush)
 		hdr = (struct ieee80211_hdr *)skb->data;
 		hdrlen = ieee80211_get_hdrlen_from_skb(skb);
 
-		if (hdrlen & 3) {
-			padsize = hdrlen % 4;
+		/* The MAC header is padded to have 32-bit boundary if the
+		 * packet payload is non-zero. The general calculation for
+		 * padsize would take into account odd header lengths:
+		 * padsize = (4 - hdrlen % 4) % 4; However, since only
+		 * even-length headers are used, padding can only be 0 or 2
+		 * bytes and we can optimize this a bit. In addition, we must
+		 * not try to remove padding from short control frames that do
+		 * not have payload. */
+		padsize = hdrlen & 3;
+		if (padsize && hdrlen >= 24) {
 			memmove(skb->data + padsize, skb->data, hdrlen);
 			skb_pull(skb, padsize);
 		}