Re: [patch] fix divide by zero in random/taviso()

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 <[email protected]>

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;