ipv4: Fix IP timestamp option (IPOPT_TS_PRESPEC) handling in ip_options_echo()

The current handling of echoed IP timestamp options with prespecified
addresses is rather broken since the 2.2.x kernels. As far as i understand
it, it should behave like when originating packets.

Currently it will only timestamp the next free slot if:
 - there is space for *two* timestamps
 - some random data from the echoed packet taken as an IP is *not* a local IP

This first is caused by an off-by-one error. 'soffset' points to the next
free slot and so we only need to have 'soffset + 7 <= optlen'.

The second bug is using sptr as the start of the option, when it really is
set to 'skb_network_header(skb)'. I just use dptr instead which points to
the timestamp option.

Finally it would only timestamp for non-local IPs, which we shouldn't do.
So instead we exclude all unicast destinations, similar to what we do in
ip_options_compile().

Signed-off-by: Jan Luebbe <jluebbe@debian.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_options.c

@@ -140,11 +140,11 @@ int ip_options_echo(struct ip_options * dopt, struct sk_buff * skb)
 				} else {
 					dopt->ts_needtime = 0;
 
-					if (soffset + 8 <= optlen) {
+					if (soffset + 7 <= optlen) {
 						__be32 addr;
 
-						memcpy(&addr, sptr+soffset-1, 4);
-						if (inet_addr_type(dev_net(skb_dst(skb)->dev), addr) != RTN_LOCAL) {
+						memcpy(&addr, dptr+soffset-1, 4);
+						if (inet_addr_type(dev_net(skb_dst(skb)->dev), addr) != RTN_UNICAST) {
 							dopt->ts_needtime = 1;
 							soffset += 8;
 						}