linux-kernel - Re: [PATCH] netfilter: Force fake conntrack entry to be at least 8 bytes aligned

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Date:   Sat, 11 Mar 2017 10:12:22 +0100
From:   Florian Westphal <fw@...len.de>
To:     "Steven Rostedt (VMware)" <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        coreteam@...filter.org, Florian Westphal <fw@...len.de>
Subject: Re: [PATCH] netfilter: Force fake conntrack entry to be at least 8
 bytes aligned

Steven Rostedt (VMware) <rostedt@...dmis.org> wrote:
> Since the nfct and nfctinfo have been combined, the nf_conn structure
> must be at least 8 bytes aligned, as the 3 LSB bits are used for the
> nfctinfo. But there's a fake nf_conn structure to denote untracked
> connections, which is created by a PER_CPU construct. This does not
> guarantee that it will be 8 bytes aligned and can break the logic in
> determining the correct nfctinfo.
> 
> I triggered this on a 32bit machine with the following error:
[..]

Ugh.  Originally I had planned to also submit followup changes
to get rid of the untracked objects but that part got delayed.

> By using DEFINE/DECLARE_PER_CPU_ALIGNED we can enforce at least 8 byte
> alignment as all cache line sizes are at least 8 bytes or more.

Thanks for fixing this!

Acked-by: Florian Westphal <fw@...len.de>