Re: [PATCH v2 3/4] secure_seq: use siphash24 instead of md5_transform

Hannes Frederic Sowa
2016-12-14T20:27:17Z

Hey Jason,

On 14.12.2016 20:38, Jason A. Donenfeld wrote:
> On Wed, Dec 14, 2016 at 8:22 PM, Hannes Frederic Sowa
> <hannes@stressinduktion.org> wrote:
>> I don't think this helps. Did you test it? I don't see reason why
>> padding could be left out between `d' and `end' because of the flexible
>> array member?
>
> Because the type u8 doesn't require any alignment requirements, it can
> nestle right up there cozy with the u16:
>
> zx2c4@thinkpad ~ $ cat a.c
> #include <stdint.h>
> #include <stdio.h>
> #include <stddef.h>
> int main()
> {
> struct {
> uint64_t a;
> uint32_t b;
> uint32_t c;
> uint16_t d;
> char x[];
> } a;
> printf("%zu\n", sizeof(a));
> printf("%zu\n", offsetof(typeof(a), x));
> return 0;
> }
> zx2c4@thinkpad ~ $ gcc a.c
> zx2c4@thinkpad ~ $ ./a.out
> 24
> 18

Sorry, I misread the patch. You are using offsetof. In this case remove
the char x[] and just use offsetofend because it is misleading
otherwise. Should work like that though.

What I don't really understand is that the addition of this complexity
actually reduces the performance, as you have to take the "if (left)"
branch during hashing and causes you to make a load_unaligned_zeropad.

Bye,
Hannes