From: Alejandro Colomar <alx.manpages@gmail.com>
To: libc-alpha@sourceware.org
Cc: "Alejandro Colomar" <alx@kernel.org>,
"Theo de Raadt" <deraadt@theos.com>,
"Todd C . Miller" <Todd.Miller@sudo.ws>,
"Jason A. Donenfeld" <Jason@zx2c4.com>,
"Cristian Rodríguez" <crrodriguez@opensuse.org>,
"Adhemerval Zanella" <adhemerval.zanella@linaro.org>,
"Yann Droneaud" <ydroneaud@opteya.com>,
"Joseph Myers" <joseph@codesourcery.com>
Subject: Re: [PATCH] Give a useful meaning to arc4random_uniform(0);
Date: Sat, 31 Dec 2022 03:48:07 +0100 [thread overview]
Message-ID: <caa400e0-76ff-d0b1-d105-8217a742014b@gmail.com> (raw)
In-Reply-To: <20221231023653.41877-1-alx@kernel.org>
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On 12/31/22 03:36, Alejandro Colomar wrote:
> Special-casing it in the implementation to return 0 was useless.
> Instead, considering 0 as the value after UINT32_MAX has the property
> that it allows implementing the following function without any
> special cases:
>
> uint32_t
> arc4random_range(uint32_t min, uint32_t max)
> {
> return arc4random_uniform(max - min + 1) + min;
> }
>
> This works for any values of min and max (as long as min <= max, of
> course), even for (0, UINT32_MAX).
>
> Oh, and the implementation of arc4random_uniform(3) is now 2 lines
> simpler. :)
>
> This will work with the current implementation because powerof2(3) will
> also consider 0 as a power of 2. See powerof2(3):
>
> SYNOPSIS
> #include <sys/param.h>
>
> int powerof2(x);
>
> DESCRIPTION
> This macro returns true if x is a power of 2, and false
> otherwise.
>
> 0 is considered a power of 2. This can make sense con‐
> sidering wrapping of unsigned integers, and has interest‐
> ing properties.
>
> Cc: Theo de Raadt <deraadt@theos.com>
> Cc: Todd C. Miller <Todd.Miller@sudo.ws>
> Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
> Cc: Cristian Rodríguez <crrodriguez@opensuse.org>
> Cc: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> Cc: Yann Droneaud <ydroneaud@opteya.com>
> Cc: Joseph Myers <joseph@codesourcery.com>
> Signed-off-by: Alejandro Colomar <alx@kernel.org>
> ---
>
> Hi,
>
> I CCd Theo and Todd, because theirs is the original implementation, and
> while this is a useful feature (IMO), it wouldn't make sense to do it
> without consensus with other implementations, and especially with the
> original implementation.
>
> I found this useful for shadow, where the existing code had a function
> that produced a "random" value within a range, but due to the bogus
> implementation, it had bias for higher values. Implementing a *_range()
> variant in terms of *_uniform() made it really simple, but the
> *_uniform() function needed to do something useful for 0 for that to
> work.
I forgot to link to the situation mentioned above:
<https://github.com/shadow-maint/shadow/pull/624>
>
> Cheers,
>
> Alex
>
> stdlib/arc4random_uniform.c | 10 ++++------
> 1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/stdlib/arc4random_uniform.c b/stdlib/arc4random_uniform.c
> index 5aa98d1c13..1cd52c0d1c 100644
> --- a/stdlib/arc4random_uniform.c
> +++ b/stdlib/arc4random_uniform.c
> @@ -29,15 +29,13 @@
> the asked range after range adjustment.
>
> The algorithm avoids modulo and divide operations, which might be costly
> - depending on the architecture. */
> + depending on the architecture.
> +
> + 0 is treated as if it were UINT32_MAX + 1, and so arc4random_uniform(0)
> + is equivalent to arc4random(). */
> uint32_t
> __arc4random_uniform (uint32_t n)
> {
> - if (n <= 1)
> - /* There is no valid return value for a zero limit, and 0 is the
> - only possible result for limit 1. */
> - return 0;
> -
> /* Powers of two are easy. */
> if (powerof2 (n))
> return __arc4random () & (n - 1);
--
<http://www.alejandro-colomar.es/>
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next prev parent reply other threads:[~2022-12-31 2:48 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-31 2:36 Alejandro Colomar
2022-12-31 2:48 ` Alejandro Colomar [this message]
2022-12-31 2:57 ` Jason A. Donenfeld
2022-12-31 13:39 ` Alejandro Colomar
2022-12-31 8:50 ` Theo de Raadt
2022-12-31 8:51 ` Theo de Raadt
2022-12-31 14:56 ` Alejandro Colomar
2022-12-31 15:13 ` Alejandro Colomar
2022-12-31 15:17 ` Alejandro Colomar
2022-12-31 15:59 ` Alejandro Colomar
2022-12-31 16:03 ` Alejandro Colomar
2023-01-01 8:41 ` Theo de Raadt
2022-12-31 23:07 ` Damien Miller
2022-12-31 23:58 ` Alejandro Colomar
2023-01-01 7:48 ` Ariadne Conill
2023-01-01 9:21 ` Otto Moerbeek
2023-01-01 14:05 ` Alejandro Colomar
2023-01-01 8:34 ` Theo de Raadt
2023-01-01 21:37 ` Arsen Arsenović
2023-01-01 23:50 ` Alejandro Colomar
2023-01-02 0:02 ` Arsen Arsenović
2023-01-02 11:24 ` Alejandro Colomar
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