linux-kernel - Re: [PATCH v2] netfilter: Make xt_cgroup independent from net

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Message-ID: <Z-LMa9k9q_tJolr3@calendula>
Date: Tue, 25 Mar 2025 16:31:55 +0100
From: Pablo Neira Ayuso <pablo@...filter.org>
To: Michal Koutný <mkoutny@...e.com>
Cc: netfilter-devel@...r.kernel.org, coreteam@...filter.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jozsef Kadlecsik <kadlec@...filter.org>,
	"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>, cgroups@...r.kernel.org,
	Jan Engelhardt <ej@...i.de>, Florian Westphal <fw@...len.de>
Subject: Re: [PATCH v2] netfilter: Make xt_cgroup independent from net_cls

On Mon, Mar 24, 2025 at 07:01:14PM +0100, Michal Koutný wrote:
> On Mon, Mar 24, 2025 at 05:49:09PM +0100, Pablo Neira Ayuso <pablo@...filter.org> wrote:
> > If !CONFIG_CGROUP_NET_CLASSID, then no classid matching is possible.
> > 
> > So why allow a rule to match on cgroup with classid == 0?
> 
> It is conservative approach to supposed users who may have filtering
> rules with classid=0 but never mkdir any net_cls group. Only those who
> eventually need to mkdir would realize there's nowhere to mkdir on (with
> !CONFIG_CGROUP_NET_CLASSID). Admittedly, I have no idea if this helps to
> 5% of net_cls users or 0.05% or 0%. Do you have any insights into that?

I suspect this partial support will not help anyway, because user will
be most likely matching to classid != 0 in their rulesets, and the
ruleset loads via iptables-restore in an atomic fashion, ie. take it
all or nothing.

> > Maybe simply do this instead?
> > 
> > static bool possible_classid(u32 classid)
> > {
> >        return IS_ENABLED(CONFIG_CGROUP_NET_CLASSID);
> > }
> 
> Yes, if the above carefulness is unnecessary, I'd like to accompany this
> with complete removal of sock_cgroup_classid() function then (to have it
> compile-checked that it's really impossible to compare any classids w/o
> CONFIG_CGROUP_NET_CLASSID).

Go ahead remove this shim function and post v3.

Thanks.