netdev - Can we rely on ethernet header padding?

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Message-ID: <20130319150545.GA18218@unicorn.suse.cz>
Date:	Tue, 19 Mar 2013 16:05:45 +0100
From:	Michal Kubecek <mkubecek@...e.cz>
To:	netdev@...r.kernel.org
Cc:	netfilter-devel@...r.kernel.org
Subject: Can we rely on ethernet header padding?

Hello,

a customer of ours ran into

  http://bugzilla.netfilter.org/show_bug.cgi?id=765

They checked that commit a504b86e prevents the crash but I'm not sure it
is sufficient.

The crash happens when br_nf_pre_routing_finish_bridge() calls
neigh_hh_bridge() which copies not only destination MAC address but also
the padding with it. IIUC this is for performance reasons (so that
aligned 8 bytes are copied rather than 6).

But I wonder whether we can rely on the fact that every skb on an
ethernet-like device has ethernet header padded at least to the 16 bytes
expected by neigh_hh_bridge() and neigh_hh_output() or whether the
bridge code should make sure. I tried to look for such test but couldn't
find any, even if commit a504b86e description mentions reallocating the
skb rather than a crash.

Thanks in advance,
                                                          Michal Kubecek

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