Re: [Qemu-devel] [PATCH v2] virtio-rng: Add human-readable error message for negative max-bytes parameter

[John Snow <jsnow@redhat.com>]

On 07/21/2014 01:33 PM, Markus Armbruster wrote:
> John Snow <jsnow@redhat.com> writes:
>
>> On 07/21/2014 03:48 AM, Markus Armbruster wrote:
>>> It certainly could check whether the value fits into uint64_t.
>>>
>>> A quick peek at how string-input-visitor.c uses strtoll() makes me
>>> cringe.
>>>
>>> [...]
>> What I meant by that was to say that by the time a value was returned
>> to visit_type_uint64, the value has already been possibly converted
>> implicitly from a negative value, and we can't tell at this level if
>> that happened without re-inspecting the string we were passed. At that
>> point, why not just fix the string parsing mechanics one more layer
>> down in parse_type_int() -- or by creating another routine primitive;
>> i.e parse_type_uint.
>>
>> As Eric Blake noted elsewhere in the thread, it would be nice to have
>> the ability to have three behaviors at the lowest level -- signed,
>> unsigned with wraparound, and unsigned strict. The biggest question in
>> my mind is how to add the property flag to allow authors to opt-in to
>> the unsigned with wraparound option, where the unsigned strict option
>> makes the most sense to me as a default.
> Do we have a use case for silently mapping negative numbers to positive
> ones?

Via Eric Blake, for cases where "-1" is a convenient shorthand for "MAX" 
in lieu of writing out gibberish values like 4 billion or 18 
quintillion. I don't know if anyone actually relies on this behavior, 
but I don't know that they're not. I can easily imagine something like 
--max-log-messages=-1, for instance.

eblake had said that libvirt recently made a similar parsing change and 
they wound up with those three backing cases.