linux-kernel - Re: kernel v4.8: iptables logs are truncated with the 4.8 kernel?

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Date:   Tue, 11 Oct 2016 08:58:57 +0800
From:   Liping Zhang <zlpnobody@...il.com>
To:     Chris Caputo <ccaputo@....net>
Cc:     Vishwanath Pai <vpai@...mai.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Justin Piszcz <jpiszcz@...idpixels.com>,
        linux-kernel@...r.kernel.org,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: kernel v4.8: iptables logs are truncated with the 4.8 kernel?

2016-10-11 2:33 GMT+08:00 Chris Caputo <ccaputo@....net>:
>>
>> What numbers did you specify after --nflog-size option?
>> --nflog-size 0 or ...? If you want log the whole packet to
>> the ulogd, please do not specify this nflog-size option.
>
> Not specifying nflog-size does not appear to log the whole packet...
>
> If "--nflog-size" is unspecified, and the iptables config is left
> unchanged when the kernel is upgraded to 4.8, ulogd-2.0.5 crashes.
>
> If "--nflog-size 0" is used, ulogd-2.0.5 crashes.
>
> If "--nflog-size" is used with size 1 or greater, ulogd-2.0.5 is fine.
>
>> > I'm surprised to see a kernel change cause unexpected userspace segfaults,
>> > so further investigation into a kernel fix would seem a good idea.
>>
>> According to the original user's manual, nflog-range option was
>> designed to be the number of bytes copied to userspace, but
>> unfortunately there's a bug from the beginning and it never works,
>> i.e. in kernel, it just ignored this option.
>>
>> Try to change the current nflog-range option's semantics may
>> cause unexpected results(maybe like this ulogd crash) ...
>>
>> In order to keep compatibility, Vishwanath introduce a new
>> nflog-size option and keep nflog-range unchanged. If you just
>> upgrade the kernel, and do not change iptables rules, this
>> problem will not happen.
>
> I am reporting that the problem does happen simply with an upgrade to
> kernel 4.8 and no other changes.  When "--nflog-size" is unspecified or
> set to 0, the bug in ulogd-2.0.5 gets triggered.
>
> I agree there is a bug in ulogd-2.0.5 that this kernel change exposed, but
> I am trying to explain that all ulogd users risk this segfault if they
> upgrade to kernel 4.8 and don't either update to a fixed ulogd (possibly
> using your patch below) or an unreleased iptables with iptables config
> changes to implement nflog-size on each NFLOG target.

Yes, thanks for clarifying this. There's a bug in kernel, can you try
this patch:

diff --git a/net/netfilter/xt_NFLOG.c b/net/netfilter/xt_NFLOG.c
index 018eed7..8c069b4 100644
--- a/net/netfilter/xt_NFLOG.c
+++ b/net/netfilter/xt_NFLOG.c
@@ -32,6 +32,7 @@ nflog_tg(struct sk_buff *skb, const struct
xt_action_param *par)
        li.u.ulog.copy_len   = info->len;
        li.u.ulog.group      = info->group;
        li.u.ulog.qthreshold = info->threshold;
+       li.u.ulog.flags      = 0;

        if (info->flags & XT_NFLOG_F_COPY_LEN)
                li.u.ulog.flags |= NF_LOG_F_COPY_LEN;

Thanks