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Date: Thu, 15 Dec 2016 19:50:36 +0100
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Hannes Frederic Sowa <hannes@...essinduktion.org>, David Laight <David.Laight@...lab.com>
Cc: 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 David & Hannes,
This conversation is veering off course. I think this doesn't really
matter at all. Gcc converts u64 into essentially a pair of u32 on
32-bit platforms, so the alignment requirements for 32-bit is at a
maximum 32 bits. On 64-bit platforms the alignment requirements are
related at a maximum to the biggest register size, so 64-bit
alignment. For this reason, no matter the behavior of __aligned(8),
we're okay. Likewise, even without __aligned(8), if gcc aligns structs
by their biggest member, then we get 4 byte alignment on 32-bit and 8
byte alignment on 64-bit, which is fine. There's no 32-bit platform
that will trap on a 64-bit unaligned access because there's no such
thing as a 64-bit access there. In short, we're fine.
(The reason in6_addr aligns itself to 4 bytes on 64-bit platforms is
that it's defined as being u32 blah[4]. If we added a u64 blah[2],
we'd get 8 byte alignment, but that's not in the header. Feel free to
start a new thread about this issue if you feel this ought to be added
for whatever reason.)
One optimization that's been suggested on this list is that instead of
u8 key[16] and requiring the alignment attribute, I should just use
u64 key[2]. This seems reasonable to me, and it will also save the
endian conversion call. These keys generally aren't transmitted over a
network, so I don't think a byte-wise encoding is particularly
important.
The other suggestion I've seen is that I make the functions take a
const void * instead of a const u8 * for the data, in order to save
ugly casts. I'll do this too.
Meanwhile Linus has condemned our 4dwords/2qwords naming, and I'll
need to think of something different. The best I can think of right
now is siphash_4_u32/siphash_2_u64, but I don't find it especially
pretty. Open to suggestions.
Regards,
Jason
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