Re: [BUG/PATCH] kernel RNG and its secrets

[Stephan Mueller <smueller@chronox.de>]

Am Freitag, 10. April 2015, 16:26:00 schrieb Hannes Frederic Sowa:

Hi Hannes,

>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...

Do you really think this is a compiler issue? I would rather think it is how 
to interpret the pure "memory" asm option. Thus, I would rather think that 
both, gcc and clang are right and we just need to use the code that fits both.
>
>Thanks,
>Hannes
>
>
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Ciao
Stephan