netdev - Re: ipv4: Simplify ARP hash function.

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Message-ID: <CAL1RGDWbCi_nE8tRp9zyX3Z1yBHKP9ygxNV6TNKp4Up+0g2EPA@mail.gmail.com>
Date:	Fri, 8 Jul 2011 12:26:17 -0700
From:	Roland Dreier <roland@...estorage.com>
To:	David Miller <davem@...emloft.net>
Cc:	johnwheffner@...il.com, mj@....cz, netdev@...r.kernel.org
Subject: Re: ipv4: Simplify ARP hash function.

On Fri, Jul 8, 2011 at 11:06 AM, David Miller <davem@...emloft.net> wrote:
> Ok, there was also an unintended bug in my original patch,
> I lost the bottom 8 bits in the fold, the hash function
> should instead be:
>
> +static inline u32 arp_hashfn(u32 key, const struct net_device *dev, u32 hash_rnd)
> +{
> +       u32 val = key ^ dev->ifindex ^ hash_rnd;
> +
> +       return val ^ (val >> 8) ^ (val >> 16) ^ (val >> 24);
> +}

Doesn't seem to matter much -- this is now equivalent to

      a = key ^ dev->ifindex
       return (a ^ (a >> 8) ^ (a >> 16) ^ (a >> 24))           // (1)
            ^ (rnd ^ (rnd >> 8) ^ (rnd >> 16) ^ (rnd >> 24))   // (2)

where again the attacker controls (1), and (2) is a constant.
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