full-disclosure - Re: Firefox 2.0.0.7 has a very serious calculation bug

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Message-ID: <46FD4365.9000003@susam.in>
Date: Fri, 28 Sep 2007 23:39:41 +0530
From: Susam Pal <susam@...am.in>
To: carl hardwick <hardwick.carl@...il.com>
Cc: full-disclosure@...ts.grok.org.uk
Subject: Re: Firefox 2.0.0.7 has a very serious
 calculation bug

Let's take this C code.

#include <stdio.h>

int main(int argc, char **argv) {
  float a = 0.7;
  if(a == 0.7) {
    printf("%f is equal to %f\n", a, 0.7);
  } else {
    printf("%f is not equal to %f\n", a, 0.7);
  }
}

On many implementations (not necessarily all implementations) we will
get the output as:-

0.700000 is not equal to 0.700000

For example, on my Debian Etch with gcc 4.1.2, the output is as shown
above. This doesn't mean it is a bug in GCC. We can't call this a bug in 
GCC because it's just a limitation of floating point math. The 
programmer should be careful of these floating point issues while 
programming.

Similarly, if someone doesn't take care of the floating point behavior 
while writing code in JavaScript, we should say that the JavaScript code 
has the bug instead of saying that the bug is in Firefox.

Regards,
Susam Pal
http://susam.in/

carl hardwick wrote, On Friday 28 September 2007 09:46 PM:
> There's a flaw in Firefox 2.0.0.7 allows javascript to execute wrong
> subtractions.
> 
> PoC concept here:
> javascript:5.2-0.1
> (copy this code into address bar)
> 
> Firefox 2.0.0.7 result: 5.1000000000000005 (WRONG!)
> Internet Explorer 7 result: 5.1 (OK)

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