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Re: [Qemu-devel] [PATCH] qemu: json: Fix parsing of integers >= 0x800000

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [PATCH] qemu: json: Fix parsing of integers >= 0x8000000000000000
Date: Mon, 23 May 2011 16:41:16 +0100
User-agent: Mutt/1.5.21 (2010-09-15) 
On Mon, May 23, 2011 at 10:03:18AM -0500, Anthony Liguori wrote:
> On 05/23/2011 09:14 AM, Daniel P. Berrange wrote:
> >On Mon, May 23, 2011 at 08:45:54AM -0500, Anthony Liguori wrote:
> >>On 05/23/2011 08:40 AM, Daniel P. Berrange wrote:
> >>>On Mon, May 23, 2011 at 08:33:03AM -0500, Anthony Liguori wrote:
> >>>>On 05/23/2011 08:04 AM, Daniel P. Berrange wrote:
> >>>>>On Fri, May 20, 2011 at 01:11:05PM -0500, Anthony Liguori wrote:
> >>>>>>On 05/20/2011 01:03 PM, Richard W.M. Jones wrote:
> >>>>>>>
> >>>>>>>There seem to be a few unsafe uses of strto* functions.  This patch
> >>>>>>>just fixes the one that affects me :-)
> >>>>>>
> >>>>>>Sending an integer of this size is not valid JSON.
> >>>>>>
> >>>>>>Your patch won't accept negative numbers, correct?
> >>>>>>
> >>>>>>JSON only supports int64_t.
> >>>>>
> >>>>>That's not really true. JSON supports arbitrarily large numbers
> >>>>>&    integers.
> >>>>
> >>>>Try the following snippet in your browser:
> >>>>
> >>>><html>
> >>>><head>
> >>>><script type="text/javascript">
> >>>>alert(9223372036854775807);
> >>>></script>
> >>>></head>
> >>>></html>
> >>>>
> >>>>The actual value of the alert will surprise you :-)
> >>>>
> >>>>Integers in Javascript are actually represented as doubles
> >>>>internally which means that integer constants are only accurate up
> >>>>to 52 bits.
> >>>>
> >>>>So really, we should cap integers at 32-bit :-/
> >>>>
> >>>>Have I mentioned recently that I really dislike JSON...
> >>>
> >>>NB, I am distinguishing between JSON the generic specification and
> >>>JSON as implemented in web browsers. JSON the specification has *no*
> >>>limitation on integers. Any limitation, like the one you demonstrate,
> >>>is inherantly just specific to the implementation.
> >>
> >>No, EMCA is very specific in how integers are handled in JavaScript.
> >>Every implementation of JavaScript is going to exhibit this
> >>behavior.
> >>
> >>The JSON specification lack of specificity here I think has to be
> >>interpreted as a deferral to the EMCA specification.
> >
> >The EMCA spec declares that integers upto 52-bits can be stored
> >without loosing precision. This doesn't forbid sending of 64-bit
> >integers via JSON. It merely implies that when parsed into a
> >EMCA-Script object you'll loose precision. So this doesn't mean that
> >QEMU has to throw away the extra precision when parsing JSON, nor
> >do client apps have to throw away precision when generating JSON
> >for QEMU. Both client&  QEMU can use a full uint64 if desired.
> 
> Thinking more carefully about this, I think the following rule is important:
> 
> 1) Integers that would cause overflow should be treated as double
> precision floating point numbers.
> 
> 2) A conforming implementation must support integer precision up to
> 52-bit signed integers.
> 
> I think this is valid because the string:
> 
> 9223372036854775808
> 
> Is a representation of:
> 
> 9223372036854776e3
> 
> Both are equivalent representations of the same number.  So we can
> send and accept arbitrarily large integers provided that we always
> fallback to representing integers as double precision floating
> points if the integer would otherwise truncate.
> 
> I think this means we need to drop QFloat and QInt, add a QNumber,
> and then add _from_uint64/to_uint64 and _from_double/to_double.

As long as QNumber is using the string as its internal representation,
and only converting to a more limited integer/float format at time of
use, this sounds workable.

Daniel
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