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Message-ID: <4f6ti2orkpa2c5upawpaj63jyhdx3uxeobaxjhd2tjnuzgucqz@odfw5wacuwjt>
Date: Fri, 4 Jul 2025 11:37:10 +0200
From: Gabriel Goller <g.goller@...xmox.com>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
Cc: corbet@....net, davem@...emloft.net, dsahern@...nel.org,
edumazet@...gle.com, horms@...nel.org, kuba@...nel.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, netdev@...r.kernel.org,
nicolas.dichtel@...nd.com, pabeni@...hat.com, shuah@...nel.org
Subject: Re: [PATCH net-next v4] ipv6: add `force_forwarding` sysctl to
enable per-interface forwarding
On 04.07.2025 08:00, Kuniyuki Iwashima wrote:
>From: Gabriel Goller <g.goller@...xmox.com>
>Date: Thu, 3 Jul 2025 18:01:53 +0200
>> diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
>> index 0f1251cce314..ec7fa1e890f1 100644
>> --- a/Documentation/networking/ip-sysctl.rst
>> +++ b/Documentation/networking/ip-sysctl.rst
>> @@ -2292,6 +2292,11 @@ conf/all/forwarding - BOOLEAN
>> proxy_ndp - BOOLEAN
>> Do proxy ndp.
>>
>> +force_forwarding - BOOLEAN
>> + Enable forwarding on this interface only -- regardless of the setting on
>> + ``conf/all/forwarding``. When setting ``conf.all.forwarding`` to 0,
>> + the ``force_forwarding`` flag will be reset on all interfaces.
>
>Please update conf/all/forwarding too as it will be stale after
>this patch.
I'll change the conf/all/forwarding section from:
IPv4 and IPv6 work differently here; e.g. netfilter must be used
to control which interfaces may forward packets and which not.
to:
IPv4 and IPv6 work differently here; the ``force_forwarding`` flag must
be used to control which interfaces may forward packets.
I hope that's all right.
>> +
>> fwmark_reflect - BOOLEAN
>> Controls the fwmark of kernel-generated IPv6 reply packets that are not
>> associated with a socket for example, TCP RSTs or ICMPv6 echo replies).
>> diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
>> index 5aeeed22f35b..5380107e466c 100644
>> --- a/include/linux/ipv6.h
>> +++ b/include/linux/ipv6.h
>> @@ -19,6 +19,7 @@ struct ipv6_devconf {
>> __s32 forwarding;
>> __s32 disable_policy;
>> __s32 proxy_ndp;
>> + __s32 force_forwarding;
>
>nit: place force_forwarding just after forwarding.
Agree.
>> __cacheline_group_end(ipv6_devconf_read_txrx);
>>
>> __s32 accept_ra;
>> @@ -857,6 +859,15 @@ static void addrconf_forward_change(struct net *net, __s32 newf)
>> idev = __in6_dev_get_rtnl_net(dev);
>> if (idev) {
>> int changed = (!idev->cnf.forwarding) ^ (!newf);
>> + /*
>> + * With the introduction of force_forwarding, we need to be backwards
>> + * compatible, so that means we need to set the force_forwarding flag
>
>Strictly backward compatibility is not related I think because
>the per iface conf is disabled by default, and this is a new
>behaviour. Maybe simply say
>
>/* Disabling all.forwarding sets 0 to force_forwarding for all interfaces */
Agree.
>> + * on every interface to 0 if net.ipv6.conf.all.forwarding is set to 0.
>> + * This allows the global forwarding flag to disable forwarding for
>> + * all interfaces.
>> + */
>> + if (newf == 0)
>> + WRITE_ONCE(idev->cnf.force_forwarding, newf);
>>
>> WRITE_ONCE(idev->cnf.forwarding, newf);
>> if (changed)
>> @@ -5719,6 +5730,7 @@ static void ipv6_store_devconf(const struct ipv6_devconf *cnf,
>> array[DEVCONF_ACCEPT_UNTRACKED_NA] =
>> READ_ONCE(cnf->accept_untracked_na);
>> array[DEVCONF_ACCEPT_RA_MIN_LFT] = READ_ONCE(cnf->accept_ra_min_lft);
>> + array[DEVCONF_FORCE_FORWARDING] = READ_ONCE(cnf->force_forwarding);
>> }
>>
>[...]
>> static inline size_t inet6_ifla6_size(void)
>> @@ -6747,6 +6759,78 @@ static int addrconf_sysctl_disable_policy(const struct ctl_table *ctl, int write
>> return ret;
>> }
>>
>> +static void addrconf_force_forward_change(struct net *net, __s32 newf)
>> +{
>> + ASSERT_RTNL();
>
>__in6_dev_get_rtnl_net() has the same check so this is not needed.
Agree.
>> + struct net_device *dev;
>> + struct inet6_dev *idev;
>> +
>> + for_each_netdev(net, dev) {
>> + idev = __in6_dev_get_rtnl_net(dev);
>> + if (idev) {
>> + int changed = (!idev->cnf.force_forwarding) ^ (!newf);
>> +
>> + WRITE_ONCE(idev->cnf.force_forwarding, newf);
>> + if (changed) {
>> + inet6_netconf_notify_devconf(dev_net(dev), RTM_NEWNETCONF,
>> + NETCONFA_FORCE_FORWARDING,
>> + dev->ifindex, &idev->cnf);
>> + }
>> + }
>> + }
>> +}
>> +
>> +static int addrconf_sysctl_force_forwarding(const struct ctl_table *ctl, int write,
>> + void *buffer, size_t *lenp, loff_t *ppos)
>> +{
>> + struct inet6_dev *idev = ctl->extra1;
>> + struct net *net = ctl->extra2;
>> + int *valp = ctl->data;
>> + loff_t pos = *ppos;
>
>nit: Please keep the reverse xmas tree order.
>https://www.kernel.org/doc/html/latest/process/maintainer-netdev.html#local-variable-ordering-reverse-xmas-tree-rcs
>
>> + int new_val = *valp;
>> + int old_val = *valp;
>> + int ret;
>> +
>> + struct ctl_table tmp_ctl = *ctl;
>
>same here.
Will do; moved this up.
>> +
>> + tmp_ctl.extra1 = SYSCTL_ZERO;
>> + tmp_ctl.extra2 = SYSCTL_ONE;
>
>As you are copying *ctl, please specify this in addrconf_sysctl[].
Umm how would I do that? Do you want me to add a comment explaining it?
I need extra1 and extra2 to be the network device so that I can set
NETCONFA_FORCE_FORWARDING but I also want to use proc_douintvec_minmax.
>> + tmp_ctl.data = &new_val;
>> +
>> + ret = proc_douintvec_minmax(&tmp_ctl, write, buffer, lenp, ppos);
>> +
>> + if (write && old_val != new_val) {
>> + if (!rtnl_net_trylock(net))
>> + return restart_syscall();
>> +
>> + if (valp == &net->ipv6.devconf_dflt->force_forwarding) {
>> + inet6_netconf_notify_devconf(net, RTM_NEWNETCONF,
>> + NETCONFA_FORCE_FORWARDING,
>> + NETCONFA_IFINDEX_DEFAULT,
>> + net->ipv6.devconf_dflt);
>> + } else if (valp == &net->ipv6.devconf_all->force_forwarding) {
>> + inet6_netconf_notify_devconf(net, RTM_NEWNETCONF,
>> + NETCONFA_FORCE_FORWARDING,
>> + NETCONFA_IFINDEX_ALL,
>> + net->ipv6.devconf_all);
>> +
>> + addrconf_force_forward_change(net, new_val);
>> + } else {
>> + inet6_netconf_notify_devconf(net, RTM_NEWNETCONF,
>> + NETCONFA_FORCE_FORWARDING,
>> + idev->dev->ifindex,
>> + &idev->cnf);
>> + }
>> + rtnl_net_unlock(net);
>> + }
>> +
>> + if (write)
>> + WRITE_ONCE(*valp, new_val);
>> + if (ret)
>> + *ppos = pos;
>> + return ret;
>> +}
>> +
>> static int minus_one = -1;
>> static const int two_five_five = 255;
>> static u32 ioam6_if_id_max = U16_MAX;
>> @@ -7217,6 +7301,13 @@ static const struct ctl_table addrconf_sysctl[] = {
>> .extra1 = SYSCTL_ZERO,
>> .extra2 = SYSCTL_TWO,
>> },
>> + {
>> + .procname = "force_forwarding",
>> + .data = &ipv6_devconf.force_forwarding,
>> + .maxlen = sizeof(int),
>> + .mode = 0644,
>> + .proc_handler = addrconf_sysctl_force_forwarding,
>
>Here for extra{1,2}.
See above.
>> + },
>> };
>> static int __addrconf_sysctl_register(struct net *net, char *dev_name,
>> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
>> index 7bd29a9ff0db..440b9efced72 100644
>> --- a/net/ipv6/ip6_output.c
>> +++ b/net/ipv6/ip6_output.c
>> @@ -509,7 +509,8 @@ int ip6_forward(struct sk_buff *skb)
>> u32 mtu;
>>
>> idev = __in6_dev_get_safely(dev_get_by_index_rcu(net, IP6CB(skb)->iif));
>> - if (READ_ONCE(net->ipv6.devconf_all->forwarding) == 0)
>> + if (idev && !READ_ONCE(idev->cnf.force_forwarding) &&
>> + !READ_ONCE(net->ipv6.devconf_all->forwarding))
>
>Now this ignores devconf_all when !idev whose dev was not
>found or has not had a valid mtu.
>
>if (!READ_ONCE(net->ipv6.devconf_all->forwarding) &&
> (!idev || !READ_ONCE(idev->cnf.force_forwarding)))
Oof yeah, you're right this is my mistake. I'll use your solution.
Thanks for the review!
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