qemu-devel
[Top][All Lists]
Advanced
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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 15:14:08 +0100
User-agent: Mutt/1.5.21 (2010-09-15) 
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.

> But to the point, I don't see what the point of using JSON is if our
> interpretation doesn't actually work with JavaScript.

This simply means JavaScript is a useless language for talking to the
QEMU monitor, because it'll loose precision for integers > 52bits.

> >We have no need to
> >limit ourselves to what web browsers currently support for integers in
> >JSON.
> 
> It's not web browsers.  This behavior is well specified in the EMCA
> specification.
> 
> >Indeed, limiting ourselves to what browsers support will make the
> >JSON monitor mode essentially useless, requiring yet another new mode
> >with a format which can actually represent the data we need to use.
> >
> >What I suggested is in compliance with the JSON specification and allows
> >us to support uint64 which we need for commands which take disk or memory
> >offsets.
> 
> At the end of the day, we need to worry about supporting clients.  I
> think clients are going to refer to the behavior of JavaScript for
> guidance.  So if we expect a client to not round integers, we can't
> send ones that are greater than 52-bit.
> 
> This is an extremely nasty silent failure mode.
> 
> Or, we need to just say that we're not JSON compatible.

I don't see this as a JSON compatiblity problem. JSON allows arbitrary
numbers, the only restriction is wrt to the precision of the parsers
when using JavaScript. A C app can encode+decode a value of MAX_UINT64
in JSON precisely and remain JSON compatible. A JavaScript app will still
be able to decode the values without any trouble, it will simply loose
some precision at time of parsing.

Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|
reply via email to
[Prev in Thread] Current Thread [Next in Thread]