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

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Message-ID: <Pine.LNX.4.58.0709281914500.16489@dione.cc>
Date: Fri, 28 Sep 2007 19:22:35 +0200 (CEST)
From: Michal Zalewski <lcamtuf@...ne.cc>
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

On Fri, 28 Sep 2007, carl hardwick wrote:

> javascript:5.2-0.1
> Firefox 2.0.0.7 result: 5.1000000000000005 (WRONG!)

This is a proper behavior of IEEE 754 64-bit double float, which, IIRC, is
precisely what ECMA standard mandates.

You will get the same from any C-style 'double' arithmetics.

> Internet Explorer 7 result: 5.1 (OK)

They use a marginally higher precision. Now try 5.002-.001 - chances are,
you will get 5.00999...

Neither is a "very serious calculation bug". Javascript does not guarantee
- and nowhere actually delivers - arbitrary GMP-style precision.

/mz

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