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

Hannes Frederic Sowa
2016-12-15T12:23:30Z

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@aculab.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?