netdev - Re: [PATCH v3 net-next 1/5] net: Get skb hash over flow

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Message-ID: <CALx6S36JxtWZP+b=UGhuyBpgNvi4B9YggZ_+s1zSRrk74cGNdw@mail.gmail.com>
Date:	Wed, 13 May 2015 15:37:50 -0400
From:	Tom Herbert <tom@...bertland.com>
To:	David Miller <davem@...emloft.net>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH v3 net-next 1/5] net: Get skb hash over flow_keys structure

On Wed, May 13, 2015 at 3:30 PM, David Miller <davem@...emloft.net> wrote:
> From: Tom Herbert <tom@...bertland.com>
> Date: Tue, 12 May 2015 08:22:58 -0700
>
>> @@ -15,6 +15,13 @@
>>   * All the members, except thoff, are in network byte order.
>>   */
>>  struct flow_keys {
>> +     u16     thoff;
>> +     u16     padding1;
>> +#define FLOW_KEYS_HASH_START_FIELD   n_proto
>> +     __be16  n_proto;
>> +     u8      ip_proto;
>> +     u8      padding;
>> +
>
> This padding works if everyone consistently zero initializes the whole
> key structure, but for whatever reason (performance, unintentional
> oversight, etc.) not all paths do.
>
> So, for example, inet_set_txhash() is going to have random crap in
> keys.padding, so the hashes computed are not stable for a given flow
> key tuple.
>
> That's just the first code path I found with this issue, there are
> probably several others.

memset zero is in the second patch for inet_set_txhash and
ip6_set_txhash. I can respin so those are in the first patch.

Tom
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