[NET]: Warn in __skb_trim if skb is paged

It's better to warn and fail rather than rarely triggering BUG on paths
that incorrectly call skb_trim/__skb_trim on a non-linear skb.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -975,15 +975,16 @@ static inline void skb_reserve(struct sk_buff *skb, int len)
 #define NET_SKB_PAD	16
 #endif
 
-extern int ___pskb_trim(struct sk_buff *skb, unsigned int len, int realloc);
+extern int ___pskb_trim(struct sk_buff *skb, unsigned int len);
 
 static inline void __skb_trim(struct sk_buff *skb, unsigned int len)
 {
-	if (!skb->data_len) {
+	if (unlikely(skb->data_len)) {
+		WARN_ON(1);
+		return;
+	}
 	skb->len  = len;
 	skb->tail = skb->data + len;
-	} else
-		___pskb_trim(skb, len, 0);
 }
 
 /**
@@ -993,6 +994,7 @@ static inline void __skb_trim(struct sk_buff *skb, unsigned int len)
  *
  *	Cut the length of a buffer down by removing data from the tail. If
  *	the buffer is already under the length specified it is not modified.
+ *	The skb must be linear.
  */
 static inline void skb_trim(struct sk_buff *skb, unsigned int len)
 {
@@ -1003,12 +1005,10 @@ static inline void skb_trim(struct sk_buff *skb, unsigned int len)
 
 static inline int __pskb_trim(struct sk_buff *skb, unsigned int len)
 {
-	if (!skb->data_len) {
-		skb->len  = len;
-		skb->tail = skb->data+len;
+	if (skb->data_len)
+		return ___pskb_trim(skb, len);
+	__skb_trim(skb, len);
 	return 0;
-	}
-	return ___pskb_trim(skb, len, 1);
 }
 
 static inline int pskb_trim(struct sk_buff *skb, unsigned int len)

net/core/skbuff.c

@@ -801,12 +801,10 @@ struct sk_buff *skb_pad(struct sk_buff *skb, int pad)
 	return nskb;
 }	
  
-/* Trims skb to length len. It can change skb pointers, if "realloc" is 1.
- * If realloc==0 and trimming is impossible without change of data,
- * it is BUG().
+/* Trims skb to length len. It can change skb pointers.
  */
 
-int ___pskb_trim(struct sk_buff *skb, unsigned int len, int realloc)
+int ___pskb_trim(struct sk_buff *skb, unsigned int len)
 {
 	int offset = skb_headlen(skb);
 	int nfrags = skb_shinfo(skb)->nr_frags;
@@ -816,7 +814,6 @@ int ___pskb_trim(struct sk_buff *skb, unsigned int len, int realloc)
 		int end = offset + skb_shinfo(skb)->frags[i].size;
 		if (end > len) {
 			if (skb_cloned(skb)) {
-				BUG_ON(!realloc);
 				if (pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
 					return -ENOMEM;
 			}