ipv6: Do not consider link down nexthops in path selection

Nexthops whose link is down are not supposed to be considered during
path selection when the "ignore_routes_with_linkdown" sysctl is set.
This is done by assigning them a negative region boundary.

However, when comparing the computed hash (unsigned) with the region
boundary (signed), the negative region boundary is treated as unsigned,
resulting in incorrect nexthop selection.

Fix by treating the computed hash as signed. Note that the computed hash
is always in range of [0, 2^31 - 1].

Fixes: 3d709f6 ("ipv6: Use hash-threshold instead of modulo-N")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250402114224.293392-3-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: idosch

net/ipv6/route.c

@@ -442,6 +442,7 @@ void fib6_select_path(const struct net *net, struct fib6_result *res,
 {
 	struct fib6_info *first, *match = res->f6i;
 	struct fib6_info *sibling;
+	int hash;
 
 	if (!match->nh && (!match->fib6_nsiblings || have_oif_match))
 		goto out;
@@ -468,7 +469,8 @@ void fib6_select_path(const struct net *net, struct fib6_result *res,
 	if (!first)
 		goto out;
 
-	if (fl6->mp_hash <= atomic_read(&first->fib6_nh->fib_nh_upper_bound) &&
+	hash = fl6->mp_hash;
+	if (hash <= atomic_read(&first->fib6_nh->fib_nh_upper_bound) &&
 	    rt6_score_route(first->fib6_nh, first->fib6_flags, oif,
 			    strict) >= 0) {
 		match = first;
@@ -481,7 +483,7 @@ void fib6_select_path(const struct net *net, struct fib6_result *res,
 		int nh_upper_bound;
 
 		nh_upper_bound = atomic_read(&nh->fib_nh_upper_bound);
-		if (fl6->mp_hash > nh_upper_bound)
+		if (hash > nh_upper_bound)
 			continue;
 		if (rt6_score_route(nh, sibling->fib6_flags, oif, strict) < 0)
 			break;