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Re: [Qemu-devel] [PATCH v2] virtio-rng: Add human-readable error message


From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH v2] virtio-rng: Add human-readable error message for negative max-bytes parameter
Date: Mon, 21 Jul 2014 21:15:34 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

John Snow <address@hidden> writes:

> On 07/21/2014 01:33 PM, Markus Armbruster wrote:
>> John Snow <address@hidden> 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.

That's a pretty horrid way to let people say "maximum, please".

But I accept a backward compatibility argument.

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



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