ipptool: Add support for arbitrary group tags and attribute syntaxes (value tags)

[michaelrsweet]

Currently libcups (and thus ipptool and ippserver) only supports the registered attribute syntaxes. However, section 3.5.2 of RFC 8010 allocates ranges of value tags for out of band (0x10 to 0x1f), integer (0x20 to 0x2f), octetString (0x30 to 0x3f), character-string (0x40 to 0x5f), and vendor extension values (0x40000000 to 0x7fffffff).

libcups should be extended to support these values, probably using a new ippAddOctetString2 function (with a value_tag argument, like we do for character strings and integers) for the octetString and vendor extension ranges. The _ippFile parsing code can then be extended to support these value tags accordingly.

Similarly, the tags from 0x00 to 0x0f are reserved for attribute group tags (in section 3.5.1 of RFC 8010) but should be supported as group_tag values.

[gmuth]

What is the current behavior? Are attributes with out-of-band values just ignored?
From my experience it's quite common for IPP server/printer implementations to use out-of-band tags in situations where something goes wrong.

[michaelrsweet]

@gmuth

What is the current behavior? Are attributes with out-of-band values just ignored?

Not at all. The current behavior is to support the registered group and out-of-band tag values. This bug tracks adding support for using non-registered values, both for experimentation and fuzzing/testing.