[PATCH] add skb_pad/skb_padto functionality

This is the stuff in 2.4 which DaveM has been over so I hope is ok for 2.5.
It provides a simple API for driver writers with hardware that doesn't
do packet padding to say 'make sure n bytes beyond the data is all clear'
for DMA etc.

include/linux/skbuff.h

@@ -287,6 +287,7 @@ extern struct sk_buff *skb_realloc_headroom(struct sk_buff *skb,
 extern struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
 				       int newheadroom, int newtailroom,
 				       int priority);
+extern struct sk_buff *		skb_pad(struct sk_buff *skb, int pad);
 #define dev_kfree_skb(a)	kfree_skb(a)
 extern void	      skb_over_panic(struct sk_buff *skb, int len,
 				     void *here);
@@ -1087,6 +1088,26 @@ static inline int skb_cow(struct sk_buff *skb, unsigned int headroom)
 	return 0;
 }
 
+/**
+ *	skb_padto	- pad an skbuff up to a minimal size
+ *	@skb: buffer to pad
+ *	@len: minimal length
+ *
+ *	Pads up a buffer to ensure the trailing bytes exist and are
+ *	blanked. If the buffer already contains sufficient data it
+ *	is untouched. Returns the buffer, which may be a replacement
+ *	for the original, or NULL for out of memory - in which case
+ *	the original buffer is still freed.
+ */
+ 
+static inline struct sk_buff *skb_padto(struct sk_buff *skb, unsigned int len)
+{
+	unsigned int size = skb->len + skb->data_len;
+	if (likely(size >= len))
+		return skb;
+	return skb_pad(skb, len-size);
+}
+
 /**
  *	skb_linearize - convert paged skb to linear one
  *	@skb: buffer to linarize

net/core/skbuff.c

@@ -761,6 +761,35 @@ struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
 	return n;
 }
 
+/**
+ *	skb_pad			-	zero pad the tail of an skb
+ *	@skb: buffer to pad
+ *	@pad: space to pad
+ *
+ *	Ensure that a buffer is followed by a padding area that is zero
+ *	filled. Used by network drivers which may DMA or transfer data
+ *	beyond the buffer end onto the wire.
+ *
+ *	May return NULL in out of memory cases.
+ */
+ 
+struct sk_buff *skb_pad(struct sk_buff *skb, int pad)
+{
+	struct sk_buff *nskb;
+	
+	/* If the skbuff is non linear tailroom is always zero.. */
+	if (skb_tailroom(skb) >= pad) {
+		memset(skb->data+skb->len, 0, pad);
+		return skb;
+	}
+	
+	nskb = skb_copy_expand(skb, skb_headroom(skb), skb_tailroom(skb) + pad, GFP_ATOMIC);
+	kfree_skb(skb);
+	if (nskb)
+		memset(nskb->data+nskb->len, 0, 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().