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From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH] clear skb cb on IP input
Date: Sun, 16 Jul 2006 18:18:09 -0700 (PDT)
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From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Mon, 17 Jul 2006 08:03:50 +1000

> The thing is qdiscs using cb means that this method of clearing cb
> before netif_rx doesn't work anymore.
> 
> In particular, even if loopback clears cb before calling netif_rx,
> some qdisc could come along between netif_rx and ip_rcv and put
> stuff in the cb.
> 
> The same thing can happen to any NIC in fact, as long as we allow
> qdiscs to use the cb area without clearing it, ip_rcv needs to
> clear it itself.

Ok, I'm convinced that IPv4 has to clear this out and shouldn't
assume it's contents are anything but garbage.

> With a little bit of effort we should be able to get away with
> clearing just optlen.  Whether this effort is worthwhile I don't
> know :)

It all sits in the same cacheline, so probably not.