From: tb <tb@openbsd.org>
Date: Mon, 26 Jun 2017 17:00:49 +0000 (+0000)
Subject: RANDOM1 was subject to rather bad modulus bias. Improve the situation a bit
X-Git-Url: http://artulab.com/gitweb/?a=commitdiff_plain;h=f1a3b71a2102e9f7987b4dc589a4639c09526c93;p=openbsd

RANDOM1 was subject to rather bad modulus bias. Improve the situation a bit
by implementing an arc4random_uniform(3) clone with beloved jot(1) and
using it.

discussed with and ok deraadt
---

diff --git a/sys/conf/makegap.sh b/sys/conf/makegap.sh
index ddf16ffe7e6..e104163fb48 100644
--- a/sys/conf/makegap.sh
+++ b/sys/conf/makegap.sh
@@ -1,16 +1,28 @@
 #!/bin/sh -
 
+random_uniform() {
+	local	_upper_bound
+
+	if [[ $1 -gt 0 ]]; then
+		_upper_bound=$(($1 - 1))
+	else
+		_upper_bound=0
+	fi
+
+	echo `jot -r 1 0 $_upper_bound 2>/dev/null`
+}
+
 umask 007
 
 PAGE_SIZE=`sysctl -n hw.pagesize`
 PAD=$1
 GAPDUMMY=$2
 
-RANDOM1=$((RANDOM % (3 * PAGE_SIZE)))
-RANDOM2=$((RANDOM % PAGE_SIZE))
-RANDOM3=$((RANDOM % PAGE_SIZE))
-RANDOM4=$((RANDOM % PAGE_SIZE))
-RANDOM5=$((RANDOM % PAGE_SIZE))
+RANDOM1=`random_uniform $((3 * PAGE_SIZE))`
+RANDOM2=`random_uniform $PAGE_SIZE`
+RANDOM3=`random_uniform $PAGE_SIZE`
+RANDOM4=`random_uniform $PAGE_SIZE`
+RANDOM5=`random_uniform $PAGE_SIZE`
 
 cat > gap.link << __EOF__