linux-kernel - Re: [PATCH v2] net/ipv6: allow device-only routes via the multipath API

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Message-ID: <19ac5a2ee05.c5da832c80393.3479213523717146821@azey.net>
Date: Thu, 27 Nov 2025 15:06:20 +0100
From: azey <me@...y.net>
To: "nicolasdichtel" <nicolas.dichtel@...nd.com>
Cc: "Jakub Kicinski" <kuba@...nel.org>, "David Ahern" <dsahern@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	"Eric Dumazet" <edumazet@...gle.com>,
	"Paolo Abeni" <pabeni@...hat.com>, "Simon Horman" <horms@...nel.org>,
	"netdev" <netdev@...r.kernel.org>,
	"linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] net/ipv6: allow device-only routes via the multipath
 API

On 2025-11-27 08:58:59 +0100  Nicolas Dichtel <nicolas.dichtel@...nd.com> wrote:
> I still think that there could be regressions because this commit changes the
> default behavior.

I don't think it should - my reasoning is that any routes created via
ip6_route_multipath_add() would always pass rt6_qualify_for_ecmp()
before this patch anyway:
- RAs get added as single routes via ip6_route_add(), so RTF_ADDRCONF
  wouldn't be set
- f6i->nh wouldn't be set, since:
  - ip6_route_info_create_nh() only sets nh if cfg->fc_nh_id is set,
    otherwise sets fib6_nh
  - rtm_to_fib6_config() prevents RTA_NH_ID and RTA_MULTIPATH from being
    set at the same time, and only sets fc_nh_id if RTA_NH_ID is set
- f6i->fib6_nh->fib_nh_gw_family would always be set, as dev-only routes
  were stopped by the check in rtm_to_fib6_multipath_config()

Did I get anything wrong? I should've probably included this in the commit
message, sorry.

> As stated for v1, having device-only multipath routes is already possible via
> the nexthop API.

I understand, however I still think it would be worth it to add this
to reconcile the v6/v4 APIs a bit better. If my reasoning is correct
and this doesn't cause regressions, it's a pretty trivial patch, and
FWIW as a user the feature would be very useful to me.