if (skb_shinfo(skb)->frag_list) { struct sk_buff *list = skb_shinfo(skb)->frag_list;  for (; list; list = list->next) { int end;  BUG_TRAP(start <= offset + len);  end = start + list->len; if ((copy = end - offset) > 0) { if (copy > len) copy = len; if (skb_copy_datagram_iovec(list, offset - start, to, copy)) goto fault; if ((len -= copy) == 0) return 0; offset += copy; } start = end; }
if (next) { skb = next; BUG_ON(skb_shinfo(skb)->frag_list); next = skb->next; goto next_skb;
int skb_copy_datagram_iovec(const struct sk_buff *skb, int offset,			    struct iovec *to, int len){	int start = skb_headlen(skb);	int i, copy = start - offset;	/* Copy header. */	if (copy > 0) {		if (copy > len)			copy = len;		if (memcpy_toiovec(to, skb->data + offset, copy))			goto fault;		if ((len -= copy) == 0)			return 0;		offset += copy;	}	/* Copy paged appendix. Hmm... why does this look so complicated? */	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {		int end;		BUG_TRAP(start <= offset + len);		end = start + skb_shinfo(skb)->frags[i].size;		if ((copy = end - offset) > 0) {			int err;			u8  *vaddr;			skb_frag_t *frag = &skb_shinfo(skb)->frags[i];			struct page *page = frag->page;			if (copy > len)				copy = len;			vaddr = kmap(page);			err = memcpy_toiovec(to, vaddr + frag->page_offset +					     offset - start, copy);			kunmap(page);			if (err)				goto fault;			if (!(len -= copy))				return 0;			offset += copy;		}		start = end;	}	if (skb_shinfo(skb)->frag_list) {		struct sk_buff *list = skb_shinfo(skb)->frag_list;		for (; list; list = list->next) {			int end;			BUG_TRAP(start <= offset + len);			end = start + list->len;			if ((copy = end - offset) > 0) {				if (copy > len)					copy = len;				if (skb_copy_datagram_iovec(list,							    offset - start,							    to, copy))					goto fault;				if ((len -= copy) == 0)					return 0;				offset += copy;			}			start = end;		}	}	if (!len)		return 0;fault:	return -EFAULT;}