linux-kernel - Re: [PATCH v2 1/4] siphash: add cryptographically secure hashtable function

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Message-ID: <CAHmME9rF0pKxfc3p9ThD3LZU2+ZNPeR-4=9MVEE3AJQKBMNA+w@mail.gmail.com>
Date:   Fri, 16 Dec 2016 00:43:52 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:     David Laight <David.Laight@...lab.com>,
        Netdev <netdev@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        "Daniel J . Bernstein" <djb@...yp.to>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Eric Biggers <ebiggers3@...il.com>
Subject: Re: [PATCH v2 1/4] siphash: add cryptographically secure hashtable function

Hi Hannes,

Good news.

On Thu, Dec 15, 2016 at 10:45 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
>> How's that sound?
>
> I am still very much concerned about the API.

Thanks for pushing me and putting up with my daftness... the constant
folding works absolutely perfectly. I've run several tests. When gcc
knows that a struct is aligned (say, via __aligned(8)), then it erases
the branch and makes a direct jump to the aligned code. When it's
uncertain, it evaluates at runtime. So, now there is a single
siphash() function that chooses the best one automatically. Behind the
scene there's siphash_aligned and siphash_unaligned, but nobody needs
to call these directly. (Should I rename these to have a double
underscore prefix?) On platforms that have
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS, of course all of this
disappears and everything goes directly to the aligned version.

So, I think this assuages your concerns entirely. A single API entry
point that does the right thing.

Whew! Good thinking, and thanks again for the suggestion.

Jason