Subject: Re: [PATCHv4 net-next-2.6 1/5] XFRM,IPv6: Remove xfrm_spi_hash() dependency on destination address

From: [email-masked] (Arnaud Ebalard)
Date: Mon, 04 Oct 2010 22:51:38 +0200

Hi,

Herbert Xu <[email-masked]> writes:

> On Mon, Oct 04, 2010 at 08:25:07AM +0200, Arnaud Ebalard wrote:
>>
>> At the moment, Linux XFRM stack includes the address when computing
>> the hash to perform state lookup by SPI. This patch changes XFRM
>> state hash computation to prevent destination address to be
>> used. This will later allow finding states for packets w/ mangled
>> destination addresses.
>
> I'm fine with doing this for inbound SAs.  However, I can't see
> how we can do this for outbound SAs where the SPI is chosen by
> the remote end.

The change *does not* make the lookup in the hash table rely only on the
spi, i.e. __xfrm_state_lookup() is still passed the address. It only
removes the destination address from the computation of the hash. This
allows passing NULL to __xfrm_state_lookup() specifically for input path
and make the lookup only based on the SPI. The destination address check
is done later (possibly after IRO remapping).

Except if I really missed something, this has no impact on outbound SA
(other hashtables are used in that case). 

> Incidentally, it appears that our hash could do with some
> strengthening.

After the change, xfrm_spi_hash() would contain:

 	unsigned int h = (__force u32)spi ^ proto;
        return  ((h ^ (h >> 10) ^ (h >> 20)) & hmask)

which seems to spread the bits h correctly into hmask bits (I mean for
the effort ;-) ). Are you thinking about something like changing the
shifts based on the length of the mask?

Cheers,

a+