netdev - Re: [PATCH net-next v4] ipv6: add `force_forwarding` sysctl to enable per-interface forwarding

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Message-ID: <pwrkjcem57hgbg7ptfbqofr42kqgfyac5eprptn2uw3e5tdfge@4rip2rl2my6b>
Date: Fri, 4 Jul 2025 11:41:16 +0200
From: Gabriel Goller <g.goller@...xmox.com>
To: Nicolas Dichtel <nicolas.dichtel@...nd.com>
Cc: "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Jonathan Corbet <corbet@....net>, David Ahern <dsahern@...nel.org>, Shuah Khan <shuah@...nel.org>, 
	netdev@...r.kernel.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net-next v4] ipv6: add `force_forwarding` sysctl to
 enable per-interface forwarding

On 04.07.2025 10:07, Nicolas Dichtel wrote:
>Le 03/07/2025 à 18:01, Gabriel Goller a écrit :
>> It is currently impossible to enable ipv6 forwarding on a per-interface
>> basis like in ipv4. To enable forwarding on an ipv6 interface we need to
>> enable it on all interfaces and disable it on the other interfaces using
>> a netfilter rule. This is especially cumbersome if you have lots of
>> interface and only want to enable forwarding on a few. According to the
>> sysctl docs [0] the `net.ipv6.conf.all.forwarding` enables forwarding
>> for all interfaces, while the interface-specific
>> `net.ipv6.conf.<interface>.forwarding` configures the interface
>> Host/Router configuration.
>>
>> Introduce a new sysctl flag `force_forwarding`, which can be set on every
>> interface. The ip6_forwarding function will then check if the global
>> forwarding flag OR the force_forwarding flag is active and forward the
>> packet.
>>
>> To preserver backwards-compatibility reset the flag (on all interfaces)
>> to 0 if the net.ipv6.conf.all.forwarding flag is set to 0.
>>
>> Add a short selftest that checks if a packet gets forwarded with and
>> without `force_forwarding`.
>>
>> [0]: https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
>>
>> Signed-off-by: Gabriel Goller <g.goller@...xmox.com>
>> ---
>>
>
>[snip]
>
>> @@ -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();
>> +	struct net_device *dev;
>> +	struct inet6_dev *idev;
>> +
>
>ASSERT_RTNL() is always put after variables declaration.

I removed ASSERT_RTNL completely, this is already checked by __in6_dev_get_rtnl_net.

>> +	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;
>> +	int new_val = *valp;
>> +	int old_val = *valp;
>> +	int ret;
>> +
>> +	struct ctl_table tmp_ctl = *ctl;
>This declaration should be put with other declarations.

Agree.

>> +
>> +	tmp_ctl.extra1 = SYSCTL_ZERO;
>> +	tmp_ctl.extra2 = SYSCTL_ONE;
>> +	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);
>Why not putting this in the above block?
>And maybe under the rtnl_lock to avoid race if two users change the value at the
>same time.

Yep, you're right.

>Nicolas

Thanks for the review!