From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
To: Noah Goldstein <goldstein.w.n@gmail.com>, libc-alpha@sourceware.org
Subject: Re: [PATCH v1 1/2] random-bits: Factor out entropy generating function
Date: Tue, 29 Mar 2022 16:51:11 -0300 [thread overview]
Message-ID: <c6837918-aa51-4e71-bbc8-74e522f1c0c9@linaro.org> (raw)
In-Reply-To: <20220328220936.2724834-1-goldstein.w.n@gmail.com>
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:
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;
> +}
next prev parent reply other threads:[~2022-03-29 19:51 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 ` Adhemerval Zanella [this message]
2022-03-29 19:56 ` [PATCH v1 1/2] random-bits: Factor out entropy generating function 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
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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