From: Stuart Henderson <stu@spacehopper.org>
Subject: Re: ksh(1), use arc4random_uniform(3) to calculate $RANDOM
To: Biarder <mss091204.1@gmail.com>, <tech@openbsd.org>
Date: Fri, 22 May 2026 12:44:30 +0100

read about RANDOM in ksh(1), it is required that this give deterministic 
output in some cases

-- 
  Sent from a phone, apologies for poor formatting.

On 22 May 2026 10:54:11 Biarder <mss091204.1@gmail.com> wrote:

> Currently, ksh(1) uses rand(3) and modular arithmetic to calculate
> $RANDOM.  However, OpenBSD recommend using arc4random_uniform(3)
> to calculate a uniform random integer because rand function is
> predictable and also has modular bias when modulo operation is
> performed.
>
> I passed all regress tests on OpenBSD 7.9 GENERIC.MP#495 amd64.
>
> Index: jobs.c
> ===================================================================
> RCS file: /cvs/src/bin/ksh/jobs.c,v
> diff -u -p -r1.62 jobs.c
> --- jobs.c	7 Jul 2020 10:33:58 -0000	1.62
> +++ jobs.c	22 May 2026 09:50:35 -0000
> @@ -528,8 +528,6 @@ exchild(struct op *t, int flags, volatil
> 	}
>
> 	/* shell (parent) stuff */
> -	/* Ensure next child gets a (slightly) different $RANDOM sequence */
> -	change_random();
> 	if (!(flags & XPIPEO)) {	/* last process in a job */
> 		/* YYY: Is this needed? (see also YYY above)
> 		   if (Flag(FMONITOR) && !(flags&(XXCOM|XBGND)))
> Index: sh.h
> ===================================================================
> RCS file: /cvs/src/bin/ksh/sh.h,v
> diff -u -p -r1.78 sh.h
> --- sh.h	5 Mar 2026 05:38:58 -0000	1.78
> +++ sh.h	22 May 2026 09:50:35 -0000
> @@ -598,7 +598,6 @@ char	*skip_wdvarname(const char *, int);
> int	is_wdvarname(const char *, int);
> int	is_wdvarassign(const char *);
> char **	makenv(void);
> -void	change_random(void);
> int	array_ref_len(const char *);
> char *	arrayname(const char *);
> void    set_array(const char *, int, char **);
> Index: var.c
> ===================================================================
> RCS file: /cvs/src/bin/ksh/var.c,v
> diff -u -p -r1.76 var.c
> --- var.c	5 Mar 2026 05:40:37 -0000	1.76
> +++ var.c	22 May 2026 09:50:35 -0000
> @@ -880,17 +880,6 @@ makenv(void)
> }
>
> /*
> - * Called after a fork in parent to bump the random number generator.
> - * Done to ensure children will not get the same random number sequence
> - * if the parent doesn't use $RANDOM.
> - */
> -void
> -change_random(void)
> -{
> -	rand();
> -}
> -
> -/*
>  * handle special variables with side effects - PATH, SECONDS.
>  */
>
> @@ -939,7 +928,7 @@ getspec(struct tbl *vp)
> 		break;
> 	case V_RANDOM:
> 		vp->flag &= ~SPECIAL;
> -		setint(vp, (int64_t) (rand() & 0x7fff));
> +		setint(vp, (int64_t) arc4random_uniform(0x8000));
> 		vp->flag |= SPECIAL;
> 		break;
> 	case V_HISTSIZE: