Man page 9 : skb_copy_bits

skb_copy_bits - copy bits from skb to kernel buffer

int skb_copy_bits(const struct sk_buff *  skb , int  offset , void *  to , int  len );

skb
    source skb

offset
    offset in source

to
    destination buffer

len
    number of bytes to copy

Copy the specified number of bytes from the source skb to the destination buffer.

CAUTION ! : If its prototype is ever changed, check arch/{*}/net/{*}.S files, since it is called from BPF assembly code.