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

[John Snow <jsnow@redhat.com>]

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On 07/18/2014 09:16 AM, Markus Armbruster wrote:
> Amit Shah <amit.shah@redhat.com> writes:
>
>> On (Fri) 18 Jul 2014 [13:54:01], Markus Armbruster wrote:
>>> Amit Shah <amit.shah@redhat.com> writes:
>>>
>>>> On (Fri) 18 Jul 2014 [13:15:18], Markus Armbruster wrote:
>>>>> Amit Shah <amit.shah@redhat.com> writes:
>>>>>
>>>>>> On (Fri) 18 Jul 2014 [08:27:59], Markus Armbruster wrote:
>>>>>>> John Snow <jsnow@redhat.com> writes:
>>>>>>>
>>>>>>>> If a negative integer is used for the max_bytes parameter, QEMU currently
>>>>>>>> calls abort() and leaves behind a core dump. This patch adds a simple
>>>>>>>> error message to make the reason for the termination clearer.
>>>>>>>>
>>>>>>>> Signed-off-by: John Snow <jsnow@redhat.com>
>>>>>>>> ---
>>>>>>>> v2: Changed 0L constant to (uint64_t)0 constant to match PRId64 format code
>>>>>>>>      on both 32bit and 64bit systems. Tested via -m32 flag.
>>>>>>>>
>>>>>>>>   hw/virtio/virtio-rng.c | 6 +++++-
>>>>>>>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>>>>>>>
>>>>>>>> diff --git a/hw/virtio/virtio-rng.c b/hw/virtio/virtio-rng.c
>>>>>>>> index 1356aca..64c7d23 100644
>>>>>>>> --- a/hw/virtio/virtio-rng.c
>>>>>>>> +++ b/hw/virtio/virtio-rng.c
>>>>>>>> @@ -181,7 +181,11 @@ static void virtio_rng_device_realize(DeviceState *dev, Error **errp)
>>>>>>>>   
>>>>>>>>       vrng->vq = virtio_add_queue(vdev, 8, handle_input);
>>>>>>>>   
>>>>>>>> -    assert(vrng->conf.max_bytes <= INT64_MAX);
>>>>>>>> +    if (vrng->conf.max_bytes > INT64_MAX) {
>>>>>>>> +        error_set(errp, QERR_PROPERTY_VALUE_OUT_OF_RANGE, "virtio-rng",
>>>>>>>> +                  "max_bytes", vrng->conf.max_bytes, (uint64_t)0, INT64_MAX);
>>> Missed this initially: the property name is "max-bytes", not
>>> "max_bytes".  Please fix.
>>>
>>>>>>>> +        return;
>>>>>>>> +    }
>>>>>>>>       vrng->quota_remaining = vrng->conf.max_bytes;
>>>>>>>>   
>>>>>>>>       vrng->rate_limit_timer = timer_new_ms(QEMU_CLOCK_VIRTUAL,
>>>>>>> Elsewhere in this function, we use
>>>>>>>
>>>>>>>          error_set(errp, QERR_INVALID_PARAMETER_VALUE, "period",
>>>>>>>                    "a positive number");
>>>>>>>
>>>>>>> Existing uses of QERR_PROPERTY_VALUE_OUT_OF_RANGE are all for intervals
>>>>>>> with small bounds.
>>>>>> That's suggestion for a 2.2 patch, right?
>>>>> This *is* a 2.2 patch, isn't it?
>>>> This one I proposed for 2.1 (because a device hotplug could cause qemu
>>>> to abort).
>>> Okay.
>>>
>>>>>> Do you think the usage as in this patch is fine?
>>>>> It's not wrong, just inconsistent with existing usage.  I'd prefer
>>>>> consistency.
>>>> Right.  Which one do you prefer -- both using
>>>> QERR_INVALID_PARAMETER_VALUE, or QERR_PROPERTY_VALUE_OUT_OF_RANGE?  I
>>>> prefer the latter.
>>> I prefer
>>>
>>>      qemu: -device virtio-rng-pci,period=0: Parameter 'period'
>>> expects a positive number
>>>
>>> over
>>>
>>>      qemu: -device virtio-rng-pci,max-bytes=-1: Property
>>> virtio-rng.max_bytes doesn't take value -1 (minimum: 0, maximum:
>>> 9223372036854775807)
>>>
>>> because frankly, "maximum: 9223372036854775807", while precise, borders
>>> on gibberish.  Precise gibberish, I guess :)
>>>
>>> Taking a step back: the property is uint64_t.  Why isn't the upper bound
>>> simply UINT64_MAX?
>> OK - let's take this for 2.2 and get this answered.  The risk isn't
>> too great for the abort, since the user cannot innocently provide
>> negative values.
> Mekse sense.
>
>> John, can you look at Markus's comments and address them in a series?
> Yes, please.
>
> Understand that my opinion on the error messages is just an opinion :)

1) /should/ the property be uint64_t? Do we really want to allow people 
to specify 16EiB? Logically the property should be unsigned of some 
sort, but the range we wish to restrict it to is probably much smaller. 
Is there an existing codepath that enforces signed/unsigned integers 
earlier, at parsing?

2) If we don't constrict the range further, then I agree: the "positive" 
error message is more user-friendly. It was a semantic choice on my 
part, but I think the less technical error is better.

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