linux-kernel - Re: [PATCH 1/2] random: use chacha20 for get_random

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Message-ID: <20170121001013.6vtxpuaqplwnvwaz@thunk.org>
Date:   Fri, 20 Jan 2017 19:10:13 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 1/2] random: use chacha20 for get_random_int/long

On Fri, Jan 20, 2017 at 04:38:59PM +0100, Jason A. Donenfeld wrote:
> I was thinking that the issue isn't merely cache line and a slow down,
> but that on some platforms, this could be an _illegal unaligned
> access_. That means we'd need to rewrite the code to use the unaligned
> access helpers or memcpy, and then it's really suboptimal, not to
> mention ugly, since just indexing into an array like we do now is so
> clean.

Why would there be an unaligned access?  What I was suggesting was an
array of u32, and we just do two separate u32 accesses with a shift in
the case of get_random_u64.  There's nothing illegal about that.

    u64 retval;

    retval = (array[pointer] << 32) + array[pointer+1];
    pointer += 2;

This is not terribly suboptimal nor terribly ugly.

						- Ted