From: Florian Weimer <fweimer@redhat.com>
To: Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org>
Subject: Re: [PATCH v6 10/10] manual: Add documentation for arc4random functions
Date: Tue, 28 Jun 2022 14:09:40 +0200 [thread overview]
Message-ID: <87czetf0qj.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <20220518191424.3630729-11-adhemerval.zanella@linaro.org> (Adhemerval Zanella via Libc-alpha's message of "Wed, 18 May 2022 16:14:24 -0300")
* Adhemerval Zanella via Libc-alpha:
> +@node High Quality Random
> +@subsection High Quality Random Number Functions
> +
> +This section describes the random number functions provided as a GNU
> +extension, based on OpenBSD interfaces.
> +
> +@Theglibc{} uses kernel entropy obtained either through @code{getrandom}
> +or by reading @file{/dev/urandom} to seed and periodically re-seed the
> +internal state. A per-thread data pool is used, which allows fast output
> +generation.
> +
> +Although these functions provide higher random quality than ISO, BSD, and
> +SVID functions, these still use a Pseudo-Random generator and should not
> +be used in cryptographic contexts.
> +
> +The internal state is cleared and reseed with kernel entropy on @code{fork}
> +and @code{_Fork} (it is not cleared for either direct @code{clone} syscall
> +or through glibc wrapper).
“or when using @theglibc{} @code{syscall} funcition”?
> +The prototypes for these functions are in @file{stdlib.h}.
> +@pindex stdlib.h
> +
> +@deftypefun int32_t arc4random (void)
> +@standards{GNU, stdlib.h}
Should be BSD, I think. Likewise below.
> +@safety{@mtsafe{}@asunsafe{@asucorrupt{}}@acsafe{}}
> +This function returns a single 32-bit value in the range of 0 to 2^32−1,
> +which is twice the range of @code{rand} and @code{random}.
> +@end deftypefun
Can we use @math for the exponent? And please say that the range is
inclusive.
> +@deftypefun uint32_t arc4random_uniform (uint32_t @var{upper_bound})
> +@standards{GNU, stdlib.h}
> +@safety{@mtsafe{}@asunsafe{@asucorrupt{}}@acsafe{}}
> +This function returns a single 32-bit value, uniformly distributed but
> +less than the @var{upper_bound}. It avoids the @w{modulo bias} when the
> +upper bound is not a power of two.
> +
> +The algorithm obtains the exact sampling of a discrete uniform variable
> +using an optimal number of random bits for any range @var{upper_bounds},
> +allowing to consume as less as possible data from the per-thread entropy
> +pool.
I think this overspecifies the implementation. And we obtain randomness
in blocks of 8 bits, IIRC, so it's not optimal in our implementation.
Thanks,
Florian
next prev parent reply other threads:[~2022-06-28 12:09 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-18 19:14 [PATCH v6 00/10] Add arc4random support Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 01/10] stdlib: Add arc4random, arc4random_buf, and arc4random_uniform (BZ #4417) Adhemerval Zanella
2022-06-02 9:44 ` Yann Droneaud
2022-06-10 17:26 ` Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 02/10] stdlib: Add arc4random tests Adhemerval Zanella
2022-06-28 11:59 ` Florian Weimer
2022-06-28 16:58 ` Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 03/10] benchtests: Add arc4random benchtest Adhemerval Zanella
2022-06-28 12:01 ` Florian Weimer
2022-06-28 17:06 ` Adhemerval Zanella
2022-06-28 17:13 ` Noah Goldstein
2022-06-28 17:14 ` Noah Goldstein
2022-06-28 17:22 ` Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 04/10] aarch64: Add optimized chacha20 Adhemerval Zanella
2022-06-28 12:03 ` Florian Weimer
2022-06-28 17:39 ` Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 05/10] x86: Add SSE2 " Adhemerval Zanella
2022-05-18 20:44 ` Noah Goldstein
2022-05-18 19:14 ` [PATCH v6 06/10] x86: Add AVX2 " Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 07/10] powerpc64: Add " Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 08/10] s390x: " Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 09/10] stdlib: Add TLS optimization to arc4random Adhemerval Zanella
2022-06-28 12:19 ` Florian Weimer
2022-06-28 19:45 ` Adhemerval Zanella
2022-05-18 19:14 ` [PATCH v6 10/10] manual: Add documentation for arc4random functions Adhemerval Zanella
2022-06-28 12:09 ` Florian Weimer [this message]
2022-06-28 19:15 ` Adhemerval Zanella
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87czetf0qj.fsf@oldenburg.str.redhat.com \
--to=fweimer@redhat.com \
--cc=libc-alpha@sourceware.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).