|
From: | Anthony Liguori |
Subject: | Re: [Qemu-devel] [PATCH] qemu: json: Fix parsing of integers >= 0x8000000000000000 |
Date: | Mon, 23 May 2011 10:03:18 -0500 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110424 Lightning/1.0b2 Thunderbird/3.1.10 |
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: 9223372036854776e3Both 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.
Regards, Anthony Liguori
[Prev in Thread] | Current Thread | [Next in Thread] |