Man page 9 : __pskb_copy

__pskb_copy - create copy of an sk_buff with private head.

struct sk_buff * __pskb_copy(struct sk_buff *  skb , int  headroom , gfp_t  gfp_mask );

skb
    buffer to copy

headroom
    headroom of new skb

gfp_mask
    allocation priority

Make a copy of both an sk_buff and part of its data, located in header. Fragmented data remain shared. This is used when the caller wishes to modify only header of sk_buff and needs private copy of the header to alter. Returns NULL on failure or the pointer to the buffer on success. The returned buffer has a reference count of 1.