From 9f93f6d119387f915b8a26f8ab800abee2482373 Mon Sep 17 00:00:00 2001
From: Rich Felker
Date: Mon, 22 Sep 2014 17:55:08 -0400
Subject: fix incorrect sequence generation in *rand48 prng functions
patch by Jens Gustedt. this fixes a bug reported by Nadav Har'El. the
underlying issue was that a left-shift by 16 bits after promotion of
unsigned short to int caused integer overflow. while some compilers
define this overflow case as "shifting into the sign bit", doing so
doesn't help; the sign bit then gets extended through the upper bits
in subsequent arithmetic as unsigned long long. this patch imposes a
promotion to unsigned prior to the shift, so that the result is
well-defined and matches the specified behavior.
(cherry picked from commit 05cef96d9e63a00b319f88343cf9869c8e612843)
---
src/prng/__rand48_step.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/src/prng/__rand48_step.c b/src/prng/__rand48_step.c
index ccaffc37..961d30fc 100644
--- a/src/prng/__rand48_step.c
+++ b/src/prng/__rand48_step.c
@@ -3,8 +3,8 @@
uint64_t __rand48_step(unsigned short *xi, unsigned short *lc)
{
uint64_t a, x;
- x = xi[0] | xi[1]<<16 | xi[2]+0ULL<<32;
- a = lc[0] | lc[1]<<16 | lc[2]+0ULL<<32;
+ x = xi[0] | xi[1]+0U<<16 | xi[2]+0ULL<<32;
+ a = lc[0] | lc[1]+0U<<16 | lc[2]+0ULL<<32;
x = a*x + lc[3];
xi[0] = x;
xi[1] = x>>16;
--
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