net: add function to allocate sk_buff head without data area

Add a function to allocate a sk_buff head without any data. This will
be used by memory mapped netlink to attach data from the mmaped area
to the skb.

Additionally change skb_release_all() to check whether the skb has a
data area to allow the skb destructor to clear the data pointer in case
only a head has been allocated.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 29 insertions, 1 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a92d9e7d10f7..898cf5c566f9 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -179,6 +179,33 @@ out:
  *
  */
 
+struct sk_buff *__alloc_skb_head(gfp_t gfp_mask, int node)
+{
+        struct sk_buff *skb;
+
+        /* Get the HEAD */
+        skb = kmem_cache_alloc_node(skbuff_head_cache,
+                                    gfp_mask & ~__GFP_DMA, node);
+        if (!skb)
+                goto out;
+
+        /*
+         * Only clear those fields we need to clear, not those that we will
+         * actually initialise below. Hence, don't put any more fields after
+         * the tail pointer in struct sk_buff!
+         */
+        memset(skb, 0, offsetof(struct sk_buff, tail));
+        skb->data = NULL;
+        skb->truesize = sizeof(struct sk_buff);
+        atomic_set(&skb->users, 1);
+
+#ifdef NET_SKBUFF_DATA_USES_OFFSET
+        skb->mac_header = ~0U;
+#endif
+out:
+        return skb;
+}
+
 /**
  *      __alloc_skb     -       allocate a network buffer
  *      @size: size to allocate
@@ -584,7 +611,8 @@ static void skb_release_head_state(struct sk_buff *skb)
 static void skb_release_all(struct sk_buff *skb)
 {
         skb_release_head_state(skb);
-        skb_release_data(skb);
+        if (likely(skb->data))
+                skb_release_data(skb);
 }
 
 /**