[PATCH v1 1/2] random-bits: Factor out entropy generating function

[Noah Goldstein]

On Wed, Mar 30, 2022 at 10:37 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 29/03/2022 17:44, Noah Goldstein wrote:
> > On Tue, Mar 29, 2022 at 3:37 PM Adhemerval Zanella
> > <adhemerval.zanella@linaro.org> wrote:
> >>
> >>
> >>
> >> On 29/03/2022 16:56, Noah Goldstein wrote:
> >>> On Tue, Mar 29, 2022 at 2:51 PM Adhemerval Zanella
> >>> <adhemerval.zanella@linaro.org> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 28/03/2022 19:09, Noah Goldstein via Libc-alpha wrote:
> >>>>> On some architectures `clock_gettime` is undesirable as
> >>>>> it may use a syscall or there may be a faster alternative.
> >>>>> Future architecture specific functions can be added in
> >>>>> sysdeps/<arch>/random-bits-entropy.h to provide a version of
> >>>>> 'random_bits_entropy' that doesn't use 'clock_gettime'.
> >>>>> ---
> >>>>>  include/random-bits.h                 | 16 ++++++--------
> >>>>>  sysdeps/generic/random-bits-entropy.h | 31 +++++++++++++++++++++++++++
> >>>>>  2 files changed, 37 insertions(+), 10 deletions(-)
> >>>>>  create mode 100644 sysdeps/generic/random-bits-entropy.h
> >>>>>
> >>>>> diff --git a/include/random-bits.h b/include/random-bits.h
> >>>>> index 17665b479a..016b87576c 100644
> >>>>> --- a/include/random-bits.h
> >>>>> +++ b/include/random-bits.h
> >>>>> @@ -19,21 +19,17 @@
> >>>>>  #ifndef _RANDOM_BITS_H
> >>>>>  # define _RANDOM_BITS_H
> >>>>>
> >>>>> -#include <time.h>
> >>>>> -#include <stdint.h>
> >>>>> +# include <random-bits-entropy.h>
> >>>>> +# include <stdint.h>
> >>>>>
> >>>>> -/* Provides fast pseudo-random bits through clock_gettime.  It has unspecified
> >>>>> -   starting time, nano-second accuracy, its randomness is significantly better
> >>>>> -   than gettimeofday, and for mostly architectures it is implemented through
> >>>>> -   vDSO instead of a syscall.  Since the source is a system clock, the upper
> >>>>> -   bits will have less entropy. */
> >>>>> +/* Provides fast pseudo-random bits through architecture specific
> >>>>> +   random_bits_entropy.  Expectation is source is some timing function so
> >>>>> +   the upper bits have less entropy.  */
> >>>>>  static inline uint32_t
> >>>>>  random_bits (void)
> >>>>>  {
> >>>>> -  struct __timespec64 tv;
> >>>>> -  __clock_gettime64 (CLOCK_MONOTONIC, &tv);
> >>>>> +  uint32_t ret = random_bits_entropy ();
> >>>>>    /* Shuffle the lower bits to minimize the clock bias.  */
> >>>>> -  uint32_t ret = tv.tv_nsec ^ tv.tv_sec;
> >>>>>    ret ^= (ret << 24) | (ret >> 8);
> >>>>>    return ret;
> >>>>>  }
> >>>>
> >>>> We already provide hp-timing.h, which uses rdtsc on x86 and clock_gettime on
> >>>> generic interface (and other high precision timing on other architectures).
> >>>> So I think a better way would be to:
> >>>
> >>> For x86/generic that works but other architectures also have hp-timing
> >>> implementations that might not be suitable for this (i.e there might be
> >>> an entropy regression).
> >>
> >> I would expect that the entropy of the hp-timing.h instruction would be similar
> >> to the ones from system clock (which exception of legacy architecture like alpha),
> >> but I haven't checked yet.
> >
> > Would expect the same, but think it will probably take a test on a
> > per-arch basis.
> >
> > Also there are optimizations we can make since we only need the lower
> > 32-bits and
> > not a true timestamp.
> >
> > I.e no multiply for generic. Also on x86 we can skip combining the
> > results of rdtsc.
>
> I tested the entropy on some different architectures:
>
> aarch64:
> $ ent gettime-random.txt
> Entropy = 7.293634 bits per byte.
> $ ent hptiming-random.txt
> Entropy = 6.451314 bits per byte.
>
> ia64:
> $ ent gettime-random.txt
> Entropy = 7.613066 bits per byte.
> $ ent hptiming-random.txt
> Entropy = 7.458615 bits per byte.
>
> powerpc64le:
> $ ent gettime-random.txt
> Entropy = 7.413584 bits per byte.
> $ ent hptiming-random.txt
> Entropy = 7.243894 bits per byte.
>
> sparc64
> $ ent gettime-random.txt
> Entropy = 7.388590 bits per byte.
> $ ent hptiming-random.txt
> Entropy = 7.602368 bits per byte.
>
>
> So it seems that only aarch64 is really losing some entropy when using
> hp-timing.h (not sure why).

Thanks, I can add patches for sparc/powerpc64le/ia64. Still feel that
since it takes testing and there are optimization cases that we may
want to make it's still best to have a seperate file.