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SubjectRe: [BUG/PATCH] kernel RNG and its secrets
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On Fr, 2015-04-10 at 16:09 +0200, Stephan Mueller wrote:
> Am Freitag, 10. April 2015, 16:00:03 schrieb Hannes Frederic Sowa:
>
> Hi Hannes,
>
> >On Fr, 2015-04-10 at 15:25 +0200, Stephan Mueller wrote:
> >> I would like to bring up that topic again as I did some more analyses:
> >>
> >> For testing I used the following code:
> >>
> >> static inline void memset_secure(void *s, int c, size_t n)
> >> {
> >>
> >> memset(s, c, n);
> >>
> >> BARRIER
> >>
> >> }
> >>
> >> where BARRIER is defined as:
> >>
> >> (1) __asm__ __volatile__("" : "=r" (s) : "0" (s));
> >>
> >> (2) __asm__ __volatile__("": : :"memory");
> >>
> >> (3) __asm__ __volatile__("" : "=r" (s) : "0" (s) : "memory");
> >
> >Hm, I wonder a little bit...
> >
> >Could you quickly test if you replace (s) with (n) just for the fun of
> >it? I don't know if we should ask clang people about that, at least it
> >is their goal to be as highly compatible with gcc inline asm.
>
> Using
>
> __asm__ __volatile__("" : "=r" (n) : "0" (n) : "memory");
>
> clang O2/3: no mov
>
> gcc O2/3: mov present
>
> ==> not good

I suspected a problem in how volatile with non-present output args could
be different, but this seems not to be the case.

I would contact llvm/clang mailing list and ask. Maybe there is a
problem? It seems kind of strange to me...

Thanks,
Hannes




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