netdev - Re: [PATCH nf-next] netfilter: xt

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Date:   Tue, 6 Dec 2016 17:44:05 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Willem de Bruijn <willemb@...gle.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        netfilter-devel <netfilter-devel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH nf-next] netfilter: xt_bpf: support ebpf

On Mon, Dec 5, 2016 at 7:20 PM, Florian Westphal <fw@...len.de> wrote:
> Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
>> While we're discussing the patch, another question, about revisions: I
>> tested both modified and original iptables binaries on both standard
>> and modified kernels. It all works as expected, except for the case
>> where both binaries are used on a single kernel. For instance:
>>
>>   iptables -A OUTPUT -m bpf --bytecode "`./nfbpf_compile RAW 'udp port
>> 8000'`" -j LOG
>>   ./iptables.new -L
>>
>> Here the new binary will interpret the object as xt_bpf_match_v1, but
>> iptables has inserted xt_bpf_match. The same problem happens the other
>> way around. A new binary can be made robust to detect old structs, but
>> not the other way around. Specific to bpf, the existing xt_bpf code
>> has an unfortunate bug that it always prints at least one line of
>> code, even if ->bpf_program_num_elems == 0.
>>
>> I notice that other extensions also do not necessarily only extend
>> struct vN in vN+1. Is the above a known issue?
>
> Yes, I guess noone ever bothered to fix this.
>
> The kernel blob should contain the match/target revision number,
> so userspace can in fact see that 'this is bpf v42', but iirc
> the netfilter userspace just loads the highest userspace revision
> supported by the kernel (which is then different for the 2 iptables
> binaries).

We can fall back on not parsing contents on mismatch:

diff --git a/iptables/iptables.c b/iptables/iptables.c
index 540d111..ada7c94 100644
--- a/iptables/iptables.c
+++ b/iptables/iptables.c
@@ -504,7 +504,8 @@ print_match(const struct xt_entry_match *m,
                xtables_find_match(m->u.user.name, XTF_TRY_LOAD, NULL);

        if (match) {
-               if (match->print)
+               if (match->print &&
+                   m->u.user.revision == match->revision)
                        match->print(ip, m, numeric);
                else
                        printf("%s ", match->name);

> But we *could* display message like 'kernel uses revision 2 but I can
> only find 0 and 1' or fall back to the lower supported revision without
> guess-the-struct-by-size games.

That's a good idea. A special case printf() with a notice, then.