Man page 9 : skb_copy_expand

skb_copy_expand - copy and expand sk_buff

struct sk_buff * skb_copy_expand(const struct sk_buff *  skb , int  newheadroom , int  newtailroom , gfp_t  gfp_mask );

skb
    buffer to copy

newheadroom
    new free bytes at head

newtailroom
    new free bytes at tail

gfp_mask
    allocation priority

Make a copy of both an sk_buff and its data and while doing so allocate additional space.

This is used when the caller wishes to modify the data and needs a private copy of the data to alter as well as more space for new fields. Returns NULL on failure or the pointer to the buffer on success. The returned buffer has a reference count of 1.

You must pass GFP_ATOMIC as the allocation priority if this function is called from an interrupt.