Date: Thu, 15 Dec 2016 13:23:18 +0100 From: Hannes Frederic Sowa <hannes@...essinduktion.org> To: David Laight <David.Laight@...LAB.COM>, "Jason A. Donenfeld" <Jason@...c4.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 On 15.12.2016 12:04, David Laight wrote: > From: Hannes Frederic Sowa >> Sent: 14 December 2016 22:03 >> On 14.12.2016 13:46, Jason A. Donenfeld wrote: >>> Hi David, >>> >>> On Wed, Dec 14, 2016 at 10:56 AM, David Laight <David.Laight@...lab.com> wrote: >>>> ... >>>>> +u64 siphash24(const u8 *data, size_t len, const u8 key[SIPHASH24_KEY_LEN]) >>>> ... >>>>> + u64 k0 = get_unaligned_le64(key); >>>>> + u64 k1 = get_unaligned_le64(key + sizeof(u64)); >>>> ... >>>>> + m = get_unaligned_le64(data); >>>> >>>> All these unaligned accesses are going to get expensive on architectures >>>> like sparc64. >>> >>> Yes, the unaligned accesses aren't pretty. Since in pretty much all >>> use cases thus far, the data can easily be made aligned, perhaps it >>> makes sense to create siphash24() and siphash24_unaligned(). Any >>> thoughts on doing something like that? >> >> I fear that the alignment requirement will be a source of bugs on 32 bit >> machines, where you cannot even simply take a well aligned struct on a >> stack and put it into the normal siphash(aligned) function without >> adding alignment annotations everywhere. Even blocks returned from >> kmalloc on 32 bit are not aligned to 64 bit. > > Are you doing anything that will require 64bit alignment on 32bit systems? > It is unlikely that the kernel can use any simd registers that have wider > alignment requirements. > > You also really don't want to request on-stack items have large alignments. > While gcc can generate code to do it, it isn't pretty. Hmm? Even the Intel ABI expects alignment of unsigned long long to be 8 bytes on 32 bit. Do you question that?
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