linux-kernel - Re: [PATCH v2 3/4] secure_seq: use siphash24 instead of md5

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Message-Id: <20161214.125612.1361254098267633173.davem@davemloft.net>
Date:   Wed, 14 Dec 2016 12:56:12 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     Jason@...c4.com
Cc:     David.Laight@...lab.com, netdev@...r.kernel.org,
        kernel-hardening@...ts.openwall.com, ak@...ux.intel.com,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [PATCH v2 3/4] secure_seq: use siphash24 instead of
 md5_transform

From: "Jason A. Donenfeld" <Jason@...c4.com>
Date: Wed, 14 Dec 2016 13:53:10 +0100

> In all current uses of __packed in the code, I think the impact is
> precisely zero, because all structures have members in descending
> order of size, with each member being a perfect multiple of the one
> below it. The __packed is therefore just there for safety, in case
> somebody comes in and screws everything up by sticking a u8 in
> between.

Just marking the structure __packed, whether necessary or not, makes
the compiler assume that the members are not aligned and causes
byte-by-byte accesses to be performed for words.

Never, _ever_, use __packed unless absolutely necessary, it pessimizes
the code on cpus that require proper alignment of types.