From: Vince Weaver <vincent.weaver@maine.edu>
To: Dave Jones <davej@redhat.com>
Cc: Andrew Honig <ahonig@google.com>, trinity@vger.kernel.org
Subject: Re: [patch] fix divide by zero in random/taviso()
Date: Tue, 21 Jan 2014 14:47:57 -0500 (EST) [thread overview]
Message-ID: <alpine.DEB.2.10.1401211441570.5901@vincent-weaver-1.um.maine.edu> (raw)
In-Reply-To: <20140121190703.GA27774@redhat.com>
On Tue, 21 Jan 2014, Dave Jones wrote:
> random.c:57:16: warning: Division by zero
> if (!temp) r %= temp;
> ~~^~~~~~~
> random.c:62:26: warning: Division by zero
> if (!temp) r |= rand() % temp;
> ~~~~~~~^~~~~~
>
> Vince, want to send a follow-up fix ?
Yes, brown-paper bag time. Never trust "it compiles and runs" as
testing for a bug that only shows up 1 in 4 billion times anyway.
I've copied the code into a test harness and actually ran it with code
that returns zero more often to verify it works (after hacking things
so gcc didn't "helpfully" optimize the zero generating code away).
---
The previous fix to the divide-by-zero bug in random/taviso()
(fea56c28830f "fix divide by zero in random/taviso()")
had the zero-checks inverted. This meant the old div-by-zero bug
was still there, and worse, it meant that the number being generated
always had 32-bits of zeros in the bottom half on a 64-bit machine.
Signed-off-by: Vince Weaver <vincent.weaver@maine.edu>
diff --git a/random.c b/random.c
index 75fa868..38ed22f 100644
--- a/random.c
+++ b/random.c
@@ -54,12 +54,12 @@ static unsigned long taviso(void)
case 1: temp = rand();
r = rand();
- if (!temp) r %= temp;
+ if (temp) r %= temp;
#if __WORDSIZE == 64
r <<= 32;
temp = rand();
- if (!temp) r |= rand() % temp;
+ if (temp) r |= rand() % temp;
#endif
break;
prev parent reply other threads:[~2014-01-21 19:47 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-01-17 20:21 divide by zero in random/taviso() Vince Weaver
2014-01-17 20:41 ` [patch] fix " Vince Weaver
2014-01-17 21:03 ` Dave Jones
[not found] ` <CAKB9nXvQmAU-PXjqbezf7G5bWfrjMTD5LjoFnj98K8Tyy5eQ2Q@mail.gmail.com>
2014-01-21 19:07 ` Dave Jones
2014-01-21 19:47 ` Vince Weaver [this message]
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