Re: [Qemu-devel] [PATCH] qemu: json: Fix parsing of integers >= 0x8000000000000000

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, May 23, 2011 at 04:29:55PM +0200, Markus Armbruster wrote:
> Anthony Liguori <anthony@codemonkey.ws> writes:
> 
> >>> 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.
> >
> > The spec has no notion of integers at all.  Here's the relevant
> 
> It doesn't differentiate between integers and floating-point numbers
> *syntactically*.  There are just numbers.  Some of them happen to be
> integers.
> 
> > text. Note that the BNF only has a single entry point for numbers.  It
> > does not distinguish between integers and floating point numbers.
> > Also, the only discussion of valid numbers is about whether the number
> > can be represented as a rational number.  I think the only way to read
> > the spec here is that *all* numbers are meant to be represented as
> > floating point numbers.
> >
> > Regards,
> >
> > Anthony Liguori
> >
> > 2.4.  Numbers
> >
> >    The representation of numbers is similar to that used in most
> >    programming languages.  A number contains an integer component that
> >    may be prefixed with an optional minus sign, which may be followed by
> >    a fraction part and/or an exponent part.
> >
> >    Octal and hex forms are not allowed.  Leading zeros are not allowed.
> >
> >    A fraction part is a decimal point followed by one or more digits.
> >
> >    An exponent part begins with the letter E in upper or lowercase,
> >    which may be followed by a plus or minus sign.  The E and optional
> >    sign are followed by one or more digits.
> >
> >    Numeric values that cannot be represented as sequences of digits
> >    (such as Infinity and NaN) are not permitted.
> 
> Therefore, the number 1234567890123456789 is permitted.
> 
> >
> >
> >          number = [ minus ] int [ frac ] [ exp ]
> >
> >          decimal-point = %x2E       ; .
> >
> >          digit1-9 = %x31-39         ; 1-9
> >
> >          e = %x65 / %x45            ; e E
> >
> >          exp = e [ minus / plus ] 1*DIGIT
> >
> >          frac = decimal-point 1*DIGIT
> >
> >          int = zero / ( digit1-9 *DIGIT )
> >
> >          minus = %x2D               ; -
> >
> >          plus = %x2B                ; +
> >
> >          zero = %x30                ; 0
> 
> There's more:
> 
> 4.  Parsers
> 
>    A JSON parser transforms a JSON text into another representation.  A
>    JSON parser MUST accept all texts that conform to the JSON grammar.
>    A JSON parser MAY accept non-JSON forms or extensions.

Incidentally, we should likely use the QMP capabilities handshake to
optionally enable parsing support for Infinity/-Infinity/NaN in QEMU
since that seems to be a common extension used[1].

Regards,
Daniel

[1] http://deron.meranda.us/python/comparing_json_modules/numbers
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