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

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Message-ID: <d90c29af-d614-43ea-8bcd-f2c8ced779df@6wind.com>
Date: Fri, 4 Jul 2025 10:07:24 +0200
From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
To: Gabriel Goller <g.goller@...xmox.com>,
 "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>
Cc: 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

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.


> +	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.

> +
> +	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.

Nicolas