[PATCH v2] libebl: recognize FDO Packaging Metadata ELF note

[Florian Weimer]

* Frank Ch. Eigler via Elfutils-devel:

> Hi -
>
> On Tue, Nov 30, 2021 at 12:25:41PM +0100, Mark Wielaard wrote:
>> [...]
>> The spec does explain the requirements for JSON numbers, but doesn't
>> mention any for JSON strings or JSON objects. It would be good to also
>> explain how to make the strings and objects unambiguous. [...]
>> For Objects it should require that all names are unique. [...]
>> For Strings it should require that \uXXXX escaping isn't used [...]
>> 
>> That should get rid of various corner cases that different parsers are
>> known to get wrong. 
>
> Are such buggy parsers seen in the wild, now, after all this time with
> JSON?  It seems to me it's not really elfutils' or systemd's place to
> impose -additional- constraints on customary JSON.

JSON has been targeted at the Windows/Java UTF-16 world, there is always
going to be a mismatch if you try to represent it in UTF-8 or anything
that doesn't have surrogate pairs.

>> Especially \uXXXX escaping is really confusing when using the UTF-8
>> encoding (and it is totally necessary since UTF-8 can express any
>> valid UTF character already).
>
> Yes, and yet we have had the bidi situation recently where UTF-8 raw
> codes could visually confuse a human reader whereas escaped \uXXXX
> wouldn't.  If we forbid \uXXXX unilaterally, we literally become
> incompatible with JSON (RFC8259 7. String. "Any character may be
> escaped."), and for what?

RFC 8259 says this:

   However, the ABNF in this specification allows member names and
   string values to contain bit sequences that cannot encode Unicode
   characters; for example, "\uDEAD" (a single unpaired UTF-16
   surrogate).  Instances of this have been observed, for example, when
   a library truncates a UTF-16 string without checking whether the
   truncation split a surrogate pair.  The behavior of software that
   receives JSON texts containing such values is unpredictable; for
   example, implementations might return different values for the length
   of a string value or even suffer fatal runtime exceptions.

A UTF-8 environment has to enforce *some* additional constraints
compared to the official JSON syntax.

Thanks,
Florian