From: Noah Goldstein <goldstein.w.n@gmail.com>
To: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Cc: GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [PATCH v1 1/2] random-bits: Factor out entropy generating function
Date: Wed, 30 Mar 2022 11:30:19 -0500 [thread overview]
Message-ID: <CAFUsyfKiE1c0uoYCd7ktJnhmt0P0XcrmtM59zSsL-puE_AZafw@mail.gmail.com> (raw)
In-Reply-To: <72332228-093c-5186-789f-8616cfb93793@linaro.org>
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.
next prev parent reply other threads:[~2022-03-30 16:30 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-28 22:09 Noah Goldstein
2022-03-28 22:09 ` [PATCH v1 2/2] x86: Use rdtsc for generating entropy for random_bits Noah Goldstein
2022-03-29 19:51 ` [PATCH v1 1/2] random-bits: Factor out entropy generating function Adhemerval Zanella
2022-03-29 19:56 ` Noah Goldstein
2022-03-29 20:04 ` Noah Goldstein
2022-03-29 20:14 ` H.J. Lu
2022-03-29 20:44 ` Adhemerval Zanella
2022-03-29 20:52 ` Noah Goldstein
2022-03-29 20:37 ` Adhemerval Zanella
2022-03-29 20:44 ` Noah Goldstein
2022-03-30 15:37 ` Adhemerval Zanella
2022-03-30 16:30 ` Noah Goldstein [this message]
2022-03-30 19:38 ` Cristian Rodríguez
2022-03-31 4:45 ` Jason A. Donenfeld
2022-03-31 10:08 ` Cristian Rodríguez
2022-03-31 11:17 ` Adhemerval Zanella
2022-03-31 11:25 ` Cristian Rodríguez
2022-03-31 11:48 ` Adhemerval Zanella
2022-03-31 12:14 ` Cristian Rodríguez
2022-03-31 13:12 ` Yann Droneaud
2022-03-31 15:31 ` Jason A. Donenfeld
2022-03-31 18:16 ` Noah Goldstein
2022-03-31 21:57 ` Cristian Rodríguez
2022-03-31 22:33 ` Noah Goldstein
2022-03-31 22:51 ` Jason A. Donenfeld
2022-03-31 23:05 ` Noah Goldstein
2022-03-31 23:25 ` Jason A. Donenfeld
2022-04-01 18:01 ` Cristian Rodríguez
2022-04-04 17:42 ` Adhemerval Zanella
2022-04-04 18:23 ` Noah Goldstein
2022-04-04 18:38 ` Adhemerval Zanella
2022-04-04 18:52 ` Noah Goldstein
2022-04-04 19:20 ` Adhemerval Zanella
2022-04-04 19:48 ` Noah Goldstein
2022-04-04 19:57 ` Adhemerval Zanella
2022-04-04 14:51 ` Florian Weimer
2022-04-04 14:54 ` Jason A. Donenfeld
2022-04-04 15:00 ` Florian Weimer
2022-04-04 16:51 ` Noah Goldstein
2022-04-04 17:22 ` Adhemerval Zanella
2022-04-04 18:32 ` Jason A. Donenfeld
2022-04-04 19:16 ` Noah Goldstein
2022-04-05 0:10 ` Cristian Rodríguez
2022-04-05 0:18 ` Jason A. Donenfeld
2022-04-05 13:45 ` Cristian Rodríguez
2022-04-05 9:22 ` Florian Weimer
2022-04-04 18:28 ` Jason A. Donenfeld
2022-04-05 9:20 ` Florian Weimer
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