netdev - Re: [PATCH net-next 2/2] net: sched: add em_ipt ematch for calling xtables matches

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Date:   Fri, 26 Jan 2018 06:24:12 +0200
From:   Eyal Birger <eyal.birger@...il.com>
To:     Pablo Neira Ayuso <pablo@...filter.org>
Cc:     David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        shmulik@...anetworks.com, Eyal Birger <eyal@...anetworks.com>
Subject: Re: [PATCH net-next 2/2] net: sched: add em_ipt ematch for calling
 xtables matches

On Thu, Jan 25, 2018 at 2:00 AM, Pablo Neira Ayuso <pablo@...filter.org> wrote:
> On Wed, Jan 24, 2018 at 04:37:16PM -0500, David Miller wrote:
>> From: Eyal Birger <eyal.birger@...il.com>
>> Date: Tue, 23 Jan 2018 11:17:32 +0200
>>
>> > +   network_offset = skb_network_offset(skb);
>> > +   skb_pull(skb, network_offset);
>> > +
>> > +   rcu_read_lock();
>> > +
>> > +   if (skb->skb_iif)
>> > +           indev = dev_get_by_index_rcu(em->net, skb->skb_iif);
>> > +
>> > +   nf_hook_state_init(&state, im->hook, im->nfproto, indev ?: skb->dev,
>> > +                      skb->dev, NULL, em->net, NULL);
>> > +
>> > +   acpar.match = im->match;
>> > +   acpar.matchinfo = im->match_data;
>> > +   acpar.state = &state;
>> > +
>> > +   ret = im->match->match(skb, &acpar);
>> > +
>> > +   rcu_read_unlock();
>> > +
>> > +   skb_push(skb, network_offset);
>>
>> If the SKB is shared in any way, this pull/push around the NF hook
>> invocation is illegal.
>
> At ingress, skb->data points to the network header, which is what the
> xtables matches expect, so these are actually noops, therefore,
> skb_pull() and skb_push() can be removed.

Right. I added those for completeness in supporting the xmit path.
In the xt_policy use-case it is irrelevant as tc is invoked after
encapsulation.

I will submit a v2 without these, asserting the ingress direction.

Note I have followed the example in em_ipset.c, so that might be a
problem there too...

Thanks!
Eyal.