mac80211: drop frames too short for FCS earlier

Instead of dropping such frames only when removing the
monitor info, drop them earlier (keeping the warning)
and simplify removing monitor info. While at it, make
that function return void.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rx.c

@@ -95,24 +95,13 @@ static u8 *ieee80211_get_bssid(struct ieee80211_hdr *hdr, size_t len,
  * This function cleans up the SKB, i.e. it removes all the stuff
  * only useful for monitoring.
  */
-static struct sk_buff *remove_monitor_info(struct ieee80211_local *local,
-					   struct sk_buff *skb,
-					   unsigned int rtap_vendor_space)
+static void remove_monitor_info(struct sk_buff *skb,
+				unsigned int present_fcs_len,
+				unsigned int rtap_vendor_space)
 {
-	if (ieee80211_hw_check(&local->hw, RX_INCLUDES_FCS)) {
-		if (likely(skb->len > FCS_LEN))
-			__pskb_trim(skb, skb->len - FCS_LEN);
-		else {
-			/* driver bug */
-			WARN_ON(1);
-			dev_kfree_skb(skb);
-			return NULL;
-		}
-	}
-
+	if (present_fcs_len)
+		__pskb_trim(skb, skb->len - present_fcs_len);
 	__pskb_pull(skb, rtap_vendor_space);
-
-	return skb;
 }
 
 static inline bool should_drop_frame(struct sk_buff *skb, int present_fcs_len,
@@ -534,8 +523,15 @@ ieee80211_rx_monitor(struct ieee80211_local *local, struct sk_buff *origskb,
 	 * the SKB because it has a bad FCS/PLCP checksum.
 	 */
 
-	if (ieee80211_hw_check(&local->hw, RX_INCLUDES_FCS))
+	if (ieee80211_hw_check(&local->hw, RX_INCLUDES_FCS)) {
+		if (unlikely(origskb->len <= FCS_LEN)) {
+			/* driver bug */
+			WARN_ON(1);
+			dev_kfree_skb(origskb);
+			return NULL;
+		}
 		present_fcs_len = FCS_LEN;
+	}
 
 	/* ensure hdr->frame_control and vendor radiotap data are in skb head */
 	if (!pskb_may_pull(origskb, 2 + rtap_vendor_space)) {
@@ -550,7 +546,9 @@ ieee80211_rx_monitor(struct ieee80211_local *local, struct sk_buff *origskb,
 			return NULL;
 		}
 
-		return remove_monitor_info(local, origskb, rtap_vendor_space);
+		remove_monitor_info(origskb, present_fcs_len,
+				    rtap_vendor_space);
+		return origskb;
 	}
 
 	/* room for the radiotap header based on driver features */
@@ -580,9 +578,8 @@ ieee80211_rx_monitor(struct ieee80211_local *local, struct sk_buff *origskb,
 		 * and FCS from the original.
 		 */
 		skb = skb_copy_expand(origskb, needed_headroom, 0, GFP_ATOMIC);
-
-		origskb = remove_monitor_info(local, origskb,
-					      rtap_vendor_space);
+		remove_monitor_info(origskb, present_fcs_len,
+				    rtap_vendor_space);
 
 		if (!skb)
 			return origskb;