netdev - Re: act_mirred: Fix bogus header when redirecting from VLAN

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Message-ID: <20150417024029.GA6718@Alexeis-MacBook-Pro.local>
Date:	Thu, 16 Apr 2015 19:40:30 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	netdev@...r.kernel.org, jamal <hadi@...erus.ca>
Subject: Re: act_mirred: Fix bogus header when redirecting from VLAN

On Fri, Apr 17, 2015 at 10:15:01AM +0800, Herbert Xu wrote:
> > seems the cleaner fix will be to push skb->mac_len instead?
> 
> No skb->mac_len is the same as skb2->dev->hard_header_len.

hmm. please help me understand the problem then.
In the commit log you mentioned that your vlan dev and ifb
have unequal hard header length. I think that can only happen if
your master dev used to create vlan, didn't have vlan offload,
so vlandev->hhl = 18 and ifb->hhl = 14. Then when tagged packet
arrives on master device we call skb_vlan_untag() and
skb->mac_len becomes 14, then vlan_do_receive() will do
another_round, ingress qdisc will trigger and act_mirred
eventually will be called. So existing act_mirred will be pushing
18 bytes, whereas only 14 bytes are valid and the lowest 4 have junk.
By 'fixing it' using ifb's 14 byte the patch is only masking the
problem, no?

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