ipv6: Align behavior across nexthops during path selection

JIRA: https://issues.redhat.com/browse/RHEL-96599

commit 6933cd4
Author: Ido Schimmel <[email protected]>
Date:   Tue Apr 8 11:43:16 2025 +0300

    ipv6: Align behavior across nexthops during path selection

    A nexthop is only chosen when the calculated multipath hash falls in the
    nexthop's hash region (i.e., the hash is smaller than the nexthop's hash
    threshold) and when the nexthop is assigned a non-negative score by
    rt6_score_route().

    Commit 4d0ab3a ("ipv6: Start path selection from the first
    nexthop") introduced an unintentional difference between the first
    nexthop and the rest when the score is negative.

    When the first nexthop matches, but has a negative score, the code will
    currently evaluate subsequent nexthops until one is found with a
    non-negative score. On the other hand, when a different nexthop matches,
    but has a negative score, the code will fallback to the nexthop with
    which the selection started ('match').

    Align the behavior across all nexthops and fallback to 'match' when the
    first nexthop matches, but has a negative score.

    Fixes: 3d709f6 ("ipv6: Use hash-threshold instead of modulo-N")
    Fixes: 4d0ab3a ("ipv6: Start path selection from the first nexthop")
    Reported-by: Willem de Bruijn <[email protected]>
    Closes: https://lore.kernel.org/netdev/[email protected]/
    Signed-off-by: Ido Schimmel <[email protected]>
    Reviewed-by: Willem de Bruijn <[email protected]>
    Reviewed-by: David Ahern <[email protected]>
    Link: https://patch.msgid.link/[email protected]
    Signed-off-by: Jakub Kicinski <[email protected]>

Signed-off-by: CKI Backport Bot <[email protected]>

Author: CKI Backport Bot

net/ipv6/route.c

@@ -470,10 +470,10 @@ void fib6_select_path(const struct net *net, struct fib6_result *res,
 		goto out;
 
 	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;
+	if (hash <= atomic_read(&first->fib6_nh->fib_nh_upper_bound)) {
+		if (rt6_score_route(first->fib6_nh, first->fib6_flags, oif,
+				    strict) >= 0)
+			match = first;
 		goto out;
 	}