[libvirt PATCH 2/2] conf: log error on attempts to modify ACPI index of active device

[Michal Prívozník]

On 9/10/21 9:30 AM, Peter Krempa wrote:
> On Thu, Sep 09, 2021 at 13:46:41 -0400, Laine Stump wrote:
>> The ACPI index of a device in a running guest can't be modified, and
>> libvirt doesn't actually attempt to modify it, but it was possible for
>> a user to request such a modification, and libvirt wouldn't complain,
>> thus misleading the user into thinking that it had actually been changed.
>>
>> Resolves: https://bugzilla.redhat.com/1998920
>>
>> Signed-off-by: Laine Stump <laine at redhat.com>
>> ---
>>  src/conf/domain_conf.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
>> index fefc527901..7ff890d8b7 100644
>> --- a/src/conf/domain_conf.c
>> +++ b/src/conf/domain_conf.c
>> @@ -28485,6 +28485,12 @@ virDomainDefCompatibleDevice(virDomainDef *def,
>>                             _("changing device alias is not allowed"));
>>              return -1;
>>          }
>> +
>> +        if (data.newInfo->acpiIndex != data.oldInfo->acpiIndex) {
>> +            virReportError(VIR_ERR_OPERATION_DENIED, "%s",
>> +                           _("changing device 'acpi index' is not allowed"));
>> +            return -1;
>> +        }
> 
> I don't remember any more what the intended semantics of the 'update'
> API are in regards to asking the user to provide a complete definition
> of the device, but according to the 'alias' check above we seem to be
> specifically ignoring the situation when the new definition didn't
> provide an alias. This seems to be hinting that we ignore stuff that the
> used didn't provide.
> 
> If that's the case you'll need to specifically exclude index '0' from
> the above check.
> 

While I agree that it would be user friendly I think we mandate users to
provide full device XML and looking into the doc we really do:

 * virDomainUpdateDeviceFlags:
 * @domain: pointer to domain object
 * @xml: pointer to XML description of one device
 * @flags: bitwise-OR of virDomainDeviceModifyFlags
 *

 * The supplied XML description of the device should contain all
 * the information that is found in the corresponding domain XML.
 * Leaving out any piece of information may be treated as a
 * request for its removal, which may be denied.

We can special case acpiIndex as you suggest but I think that's the
farthest we should go. Otherwise it'll be hard to distinguish user's
laziness to provide full XML from genuine request for removal.

Michal