skbuff: introduce skb_expand_head()

Like skb_realloc_headroom(), new helper increases headroom of specified skb.
Unlike skb_realloc_headroom(), it does not allocate a new skb if possible;
copies skb->sk on new skb when as needed and frees original skb in case
of failures.

This helps to simplify ip[6]_finish_output2() and a few other similar cases.

Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 2bcdc8c..783cc23 100644 (file)
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1183,6 +1183,7 @@ static inline struct sk_buff *__pskb_copy(struct sk_buff *skb, int headroom,
 int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail, gfp_t gfp_mask);
 struct sk_buff *skb_realloc_headroom(struct sk_buff *skb,
                                     unsigned int headroom);
+struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom);
 struct sk_buff *skb_copy_expand(const struct sk_buff *skb, int newheadroom,
                                int newtailroom, gfp_t priority);
 int __must_check skb_to_sgvec_nomark(struct sk_buff *skb, struct scatterlist *sg,

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index fcbd977..8bac7a1 100644 (file)
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1790,6 +1790,48 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom)
 EXPORT_SYMBOL(skb_realloc_headroom);
 
 /**
+ *     skb_expand_head - reallocate header of &sk_buff
+ *     @skb: buffer to reallocate
+ *     @headroom: needed headroom
+ *
+ *     Unlike skb_realloc_headroom, this one does not allocate a new skb
+ *     if possible; copies skb->sk to new skb as needed
+ *     and frees original skb in case of failures.
+ *
+ *     It expect increased headroom and generates warning otherwise.
+ */
+
+struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
+{
+       int delta = headroom - skb_headroom(skb);
+
+       if (WARN_ONCE(delta <= 0,
+                     "%s is expecting an increase in the headroom", __func__))
+               return skb;
+
+       /* pskb_expand_head() might crash, if skb is shared */
+       if (skb_shared(skb)) {
+               struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
+
+               if (likely(nskb)) {
+                       if (skb->sk)
+                               skb_set_owner_w(nskb, skb->sk);
+                       consume_skb(skb);
+               } else {
+                       kfree_skb(skb);
+               }
+               skb = nskb;
+       }
+       if (skb &&
+           pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
+               kfree_skb(skb);
+               skb = NULL;
+       }
+       return skb;
+}
+EXPORT_SYMBOL(skb_expand_head);
+
+/**
  *     skb_copy_expand -       copy and expand sk_buff
  *     @skb: buffer to copy
  *     @newheadroom: new free bytes at head