[PATCH V3] ipv6: fix incorrect route 'expires' value passed to userspace

[Li Wei <lw@cn.fujitsu.com>]

When userspace use RTM_GETROUTE to dump route table, with an already
expired route entry, we always got an 'expires' value(2147157)
calculated base on INT_MAX.

The reason of this problem is in the following satement:
	rt->dst.expires - jiffies < INT_MAX
gcc promoted the type of both sides of '<' to unsigned long, thus
a small negative value would be considered greater than INT_MAX.

This patch fix this by use the same trick as time_after macro to
avoid the 'unsigned long' type promotion and deal with jiffies
wrapping.

Also we should do some fix in rtnl_put_cacheinfo() which use
jiffies_to_clock_t(which take an unsigned long as parameter) to
convert jiffies to clock_t to handle the negative expires.

With the help of David Laight, we can make the code a little clean.

Signed-off-by: Li Wei <lw@cn.fujitsu.com>
---
 net/core/rtnetlink.c |    3 ++-
 net/ipv6/route.c     |   11 ++++++-----
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 334b930..2e96396 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -626,7 +626,8 @@ int rtnl_put_cacheinfo(struct sk_buff *skb, struct dst_entry *dst, u32 id,
 	};
 
 	if (expires)
-		ci.rta_expires = jiffies_to_clock_t(expires);
+		ci.rta_expires = expires > 0 ? jiffies_to_clock_t(expires)
+			: -jiffies_to_clock_t(-expires);
 
 	return nla_put(skb, RTA_CACHEINFO, sizeof(ci), &ci);
 }
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index cf02cb9..6efeb28 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -2480,12 +2480,13 @@ static int rt6_fill_node(struct net *net,
 		goto nla_put_failure;
 	if (nla_put_u32(skb, RTA_PRIORITY, rt->rt6i_metric))
 		goto nla_put_failure;
-	if (!(rt->rt6i_flags & RTF_EXPIRES))
+	if (!(rt->rt6i_flags & RTF_EXPIRES)) {
 		expires = 0;
-	else if (rt->dst.expires - jiffies < INT_MAX)
-		expires = rt->dst.expires - jiffies;
-	else
-		expires = INT_MAX;
+	} else {
+		expires = (long)rt->dst.expires - (long)jiffies;
+		if (expires != (int)expires)
+			expires = expires > 0 ? INT_MAX : INT_MIN;
+	}
 
 	if (rtnl_put_cacheinfo(skb, &rt->dst, 0, expires, rt->dst.error) < 0)
 		goto nla_put_failure;
-- 
1.7.10.1