mac80211: warn when receiving frames with unaligned data

This patch makes mac80211 warn (once) when the driver passes up a
frame in which the payload data is not aligned on a four-byte
boundary, with a long comment for people who run into the condition
and need to know what to do.
Signed-off-by: NJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/mac80211/rx.c

@@ -1443,6 +1443,7 @@ void __ieee80211_rx(struct ieee80211_hw *hw, struct sk_buff *skb,
 	struct ieee80211_sub_if_data *prev = NULL;
 	struct sk_buff *skb_new;
 	u8 *bssid;
+	int hdrlen;
 
 	/*
 	 * key references and virtual interfaces are protected using RCU
@@ -1472,6 +1473,18 @@ void __ieee80211_rx(struct ieee80211_hw *hw, struct sk_buff *skb,
 	rx.fc = le16_to_cpu(hdr->frame_control);
 	type = rx.fc & IEEE80211_FCTL_FTYPE;
 
+	/*
+	 * Drivers are required to align the payload data to a four-byte
+	 * boundary, so the last two bits of the address where it starts
+	 * may not be set. The header is required to be directly before
+	 * the payload data, padding like atheros hardware adds which is
+	 * inbetween the 802.11 header and the payload is not supported,
+	 * the driver is required to move the 802.11 header further back
+	 * in that case.
+	 */
+	hdrlen = ieee80211_get_hdrlen(rx.fc);
+	WARN_ON_ONCE(((unsigned long)(skb->data + hdrlen)) & 3);
+
 	if (type == IEEE80211_FTYPE_DATA || type == IEEE80211_FTYPE_MGMT)
 		local->dot11ReceivedFragmentCount++;