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

[H.J. Lu]

On Tue, Mar 29, 2022 at 12:56 PM Noah Goldstein via Libc-alpha
<libc-alpha@sourceware.org> 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).

The default hp-timing.h has

# define HP_TIMING_NOW(var) \
({ \
  struct __timespec64 tv; \
  __clock_gettime64 (CLOCK_MONOTONIC, &tv); \
  (var) = (tv.tv_nsec + UINT64_C(1000000000) * tv.tv_sec); \
})

It isn't the same as the current include/random-bits.h.

> >
> >   static inline uint32_t
> >   random_bits (void)
> >   {
> >     hp_timing_t hp;
> >     HP_TIMING_NOW (hp);
> >     /* Shuffle the lower bits to minimize the clock bias.  */
> >     uint32_t ret = hp >> 32 ^ (uint32_t) hp;
> >     ret ^= (ret << 24) | (ret >> 8);
> >     return ret;
> >   }
> >
> > And keep the XOR on with higher bits to keep the clock bias.
> >
> > > diff --git a/sysdeps/generic/random-bits-entropy.h b/sysdeps/generic/random-bits-entropy.h
> > > new file mode 100644
> > > index 0000000000..53290c7f7a
> > > --- /dev/null
> > > +++ b/sysdeps/generic/random-bits-entropy.h
> > > @@ -0,0 +1,31 @@
> > > +/* Fast function for generating entropy of random_bits.
> > > +   Copyright (C) 2022 Free Software Foundation, Inc.
> > > +   This file is part of the GNU C Library.
> > > +
> > > +   The GNU C Library is free software; you can redistribute it and/or
> > > +   modify it under the terms of the GNU Lesser General Public
> > > +   License as published by the Free Software Foundation; either
> > > +   version 2.1 of the License, or (at your option) any later version.
> > > +
> > > +   The GNU C Library is distributed in the hope that it will be useful,
> > > +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> > > +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> > > +   Lesser General Public License for more details.
> > > +
> > > +   You should have received a copy of the GNU Lesser General Public
> > > +   License along with the GNU C Library; if not, see
> > > +   <https://www.gnu.org/licenses/>.  */
> > > +
> > > +#include <stdint.h>
> > > +#include <time.h>
> > > +
> > > +/* Generically use 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.  */
> > > +static inline uint32_t
> > > +random_bits_entropy (void)
> > > +{
> > > +  struct __timespec64 tv;
> > > +  __clock_gettime64 (CLOCK_MONOTONIC, &tv);
> > > +  return tv.tv_nsec ^ tv.tv_sec;
> > > +}



-- 
H.J.