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

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Message-Id: <20150513.153007.1923480989811217786.davem@davemloft.net>
Date:	Wed, 13 May 2015 15:30:07 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	tom@...bertland.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH v3 net-next 1/5] net: Get skb hash over flow_keys
 structure

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.
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