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

[Michal Koutný]

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.)

> 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).

> 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.

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.

But please correct me if I misunderstood the logic.

Thanks,
Michal

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