linux-kernel - Re: [PATCH v3 1/3] siphash: add cryptographically secure hashtable function

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Date:   Wed, 14 Dec 2016 15:30:55 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Tom Herbert <tom@...bertland.com>, Netdev <netdev@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>,
        "Daniel J . Bernstein" <djb@...yp.to>,
        Eric Biggers <ebiggers3@...il.com>,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH v3 1/3] siphash: add cryptographically secure hashtable function

On Wed, Dec 14, 2016 at 2:56 PM, Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> So actually jhash_Nwords makes no sense, since it takes dwords
> (32-bits) not words (16-bits). The siphash analog should be called
> siphash24_Nqwords.

No. The bug is talking about "words" in the first place.

Depending on your background, a "word" can be generally be either 16
bits or 32 bits (or, in some cases, 18 bits).

In theory, a 64-bit entity can be a "word" too, but pretty much nobody
uses that. Even architectures that started out with a 64-bit register
size and never had any smaller historical baggage (eg alpha) tend to
call 32-bit entities "words".

So 16 bits can be a word, but some people/architectures will call it a
"half-word".

To make matters even more confusing, a "quadword" is generally always
64 bits, regardless of the size of "word".

So please try to avoid the use of "word" entirely. It's too ambiguous,
and it's not even helpful as a "size of the native register". It's
almost purely random.

For the kernel, we tend use

 - uX for types that have specific sizes (X being the number of bits)

 - "[unsigned] long" for native register size

But never "word".

           Linus