[IPV6]: Find option offset by type.

This is a helper to search option offset from extension header which
can carry TLV option like destination options header.

Mobile IPv6 home address option will use it.

Based on MIPL2 kernel patch.

Signed-off-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/exthdrs.c

@@ -49,6 +49,49 @@
 
 #include <asm/uaccess.h>
 
+int ipv6_find_tlv(struct sk_buff *skb, int offset, int type)
+{
+	int packet_len = skb->tail - skb->nh.raw;
+	struct ipv6_opt_hdr *hdr;
+	int len;
+
+	if (offset + 2 > packet_len)
+		goto bad;
+	hdr = (struct ipv6_opt_hdr*)(skb->nh.raw + offset);
+	len = ((hdr->hdrlen + 1) << 3);
+
+	if (offset + len > packet_len)
+		goto bad;
+
+	offset += 2;
+	len -= 2;
+
+	while (len > 0) {
+		int opttype = skb->nh.raw[offset];
+		int optlen;
+
+		if (opttype == type)
+			return offset;
+
+		switch (opttype) {
+		case IPV6_TLV_PAD0:
+			optlen = 1;
+			break;
+		default:
+			optlen = skb->nh.raw[offset + 1] + 2;
+			if (optlen > len)
+				goto bad;
+			break;
+		}
+		offset += optlen;
+		len -= optlen;
+	}
+	/* not_found */
+	return -1;
+ bad:
+	return -1;
+}
+
 /*
  *	Parsing tlv encoded headers.
  *