In article <20120830170609.14dd160...@jupiter.mumble.net>,
Taylor R Campbell <campbell+netbsd-source-change...@mumble.net> wrote:
> Date: Thu, 30 Aug 2012 11:44:41 -0400
> From: chris...@zoulas.com (Christos Zoulas)
>
> memcmp() does not promise alphabetical sorting. It just promises to do the
> byte comparison as unsigned so that the results are consistent. It is not
> complicated to do this at all, for example:
>
> int
> consttime_memcmp(const void *s1, const void *s2, size_t n)
> {
> int rv = 0, sv = 0;
> const unsigned char *p1 = s1, *p2 = s2;
>
> do
> if (rv == 0)
> rv = *p1++ - *p2++;
>
>Data-dependent branches are totally unacceptable for a routine whose
>sole purpose is to avoid timing side channels.
>
>Are there any applications that both want memcmp semantics and need to
>avoid timing side channels?
How can you tell the difference if both branches execute code that does
exactly the same work?
christos
