[PATCH] Give a useful meaning to arc4random_uniform(0);

[Alejandro Colomar]

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.

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);
-- 
2.39.0