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

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Date:   Wed, 14 Dec 2016 20:22:04 +0100
From:   Hannes Frederic Sowa <hannes@...essinduktion.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        David Miller <davem@...emloft.net>
Cc:     David Laight <David.Laight@...lab.com>,
        Netdev <netdev@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        Andi Kleen <ak@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>
Subject: Re: [PATCH v2 3/4] secure_seq: use siphash24 instead of md5_transform

On 14.12.2016 19:06, Jason A. Donenfeld wrote:
> Hi David,
> 
> On Wed, Dec 14, 2016 at 6:56 PM, David Miller <davem@...emloft.net> wrote:
>> 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.
> 
> Oh, jimminy cricket, I did not realize that it made assignments
> byte-by-byte *always*. So what options am I left with? What
> immediately comes to mind are:
> 
> 1)
> 
> struct {
>     u64 a;
>     u32 b;
>     u32 c;
>     u16 d;
>     u8 end[];

I don't think this helps. Did you test it? I don't see reason why
padding could be left out between `d' and `end' because of the flexible
array member?

Bye,
Hannes