Re: [PATCH v2] netfilter: Make xt_cgroup independent from net_cls

[Pablo Neira Ayuso]

On Mon, Mar 24, 2025 at 01:56:07PM +0100, Michal Koutný wrote:
> Hello Pablo.
> 
> On Sun, Mar 23, 2025 at 10:20:10AM +0100, Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx> wrote:
> > why classid != 0 is accepted for cgroup_mt_check_v0()?
> 
> It is opposite, only classid == 0 is accepted (that should be same for
> all of v0..v2). (OTOH, there should be no change in validation with
> CONFIG_CGROUP_NET_CLASSID.)

Thanks for clarifying this, more questions below.

> > cgroup_mt_check_v0 represents revision 0 of this match, and this match
> > only supports for clsid (groupsv1).
> > 
> > History of revisions of cgroupsv2:
> > 
> > - cgroup_mt_check_v0 added to match on clsid (initial version of this match)
> > - cgroup_mt_check_v1 is added to support cgroupsv2 matching 
> > - cgroup_mt_check_v2 is added to make cgroupsv2 matching more flexible
>  
> > I mean, if !IS_ENABLED(CONFIG_CGROUP_NET_CLASSID) then xt_cgroup
> > should fail for cgroup_mt_check_v0.
> 
> 
> I considered classid == 0 valid (regardless of CONFIG_*) as counterpart
> to implementation of sock_cgroup_classid() that collapses to 0 when
> !CONFIG_CGROUP_NET_CLASSID (thus at least rules with classid=0 remain
> acceptable).

That is, 0 is the default value when !CONFIG_CGROUP_NET_CLASSID.

> > But a more general question: why this check for classid == 0 in
> > cgroup_mt_check_v1 and cgroup_mt_check_v2?
> 
> cgroup_mt_check_v1 is for cgroupv2 OR classid matching. Similar with
> cgroup_mt_check_v2.

Yes, and cgroup_mt_check_v0 only supports for classid matching.

> IOW, all three versions accept classid=0 with !CONFIG_CGROUP_NET_CLASSID
> equally because that is the value that sockets reported classid falls
> back to.

If !CONFIG_CGROUP_NET_CLASSID, then no classid matching is possible.

So why allow a rule to match on cgroup with classid == 0?

Maybe simply do this instead?

static bool possible_classid(u32 classid)
{
       return IS_ENABLED(CONFIG_CGROUP_NET_CLASSID);
}

Thanks.