[libvirt] [PATCH 07/10] libxl: prevent attaching multiple netdevs with the same MAC

[Jim Fehlig]

Marek Marczykowski-Górecki wrote:
> On Thu, Feb 19, 2015 at 03:58:30PM -0700, Jim Fehlig wrote:
>   
>> Marek Marczykowski-Górecki wrote:
>>     
>>> On Thu, Feb 19, 2015 at 03:10:13PM -0700, Jim Fehlig wrote:
>>>       
>>>> Jim Fehlig wrote:
>>>>         
>>>>> Marek Marczykowski-Górecki wrote:
>>>>>           
>>>>>> On Thu, Feb 19, 2015 at 01:58:02PM -0700, Jim Fehlig wrote:
>>>>>>             
>>>>>>> Marek Marczykowski-Górecki wrote:
>>>>>>>               
>>>>>>>> On Thu, Feb 19, 2015 at 11:43:15AM -0700, Jim Fehlig wrote:
>>>>>>>>                 
>>>>>>>>> Marek Marczykowski-Górecki wrote:
>>>>>>>>>                   
>>>>>>>>>> It will not be possible to detach such device later. Also improve
>>>>>>>>>> logging in such cases.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Marek Marczykowski-Górecki <marmarek at invisiblethingslab.com>
>>>>>>>>>> ---
>>>>>>>>>>  src/libxl/libxl_driver.c | 41 +++++++++++++++++++++++++++++++++++++++--
>>>>>>>>>>  1 file changed, 39 insertions(+), 2 deletions(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/src/libxl/libxl_driver.c b/src/libxl/libxl_driver.c
>>>>>>>>>> index ce3a99b..005cc96 100644
>>>>>>>>>> --- a/src/libxl/libxl_driver.c
>>>>>>>>>> +++ b/src/libxl/libxl_driver.c
>>>>>>>>>> @@ -2787,6 +2787,7 @@ libxlDomainAttachNetDevice(libxlDriverPrivatePtr driver,
>>>>>>>>>>      int actualType;
>>>>>>>>>>      libxl_device_nic nic;
>>>>>>>>>>      int ret = -1;
>>>>>>>>>> +    char mac[VIR_MAC_STRING_BUFLEN];
>>>>>>>>>>  
>>>>>>>>>>      /* preallocate new slot for device */
>>>>>>>>>>      if (VIR_REALLOC_N(vm->def->nets, vm->def->nnets + 1) < 0)
>>>>>>>>>> @@ -2801,6 +2802,14 @@ libxlDomainAttachNetDevice(libxlDriverPrivatePtr driver,
>>>>>>>>>>  
>>>>>>>>>>      actualType = virDomainNetGetActualType(net);
>>>>>>>>>>  
>>>>>>>>>> +    /* -2 means "multiple matches" so then fail also */
>>>>>>>>>>   
>>>>>>>>>>       
>>>>>>>>>>           
>>>>>>>>>>             
>>>>>>>>>>                 
>>>>>>>>>>                     
>>>>>>>>> No longer true after commit 2fbae1b2.  I think you just want to check if
>>>>>>>>> virDomainNetFindIdx() >= 0, meaning the def already contains a net
>>>>>>>>> device with the same mac address.
>>>>>>>>>     
>>>>>>>>>         
>>>>>>>>>           
>>>>>>>>>               
>>>>>>>>>                   
>>>>>>>> But here the error is when the device *is* found, so the opposite case
>>>>>>>> than already reported as an error by virDomainNetFindIdx.
>>>>>>>>       
>>>>>>>>         
>>>>>>>>             
>>>>>>>>                 
>>>>>>> If you find an idx >= 0, then the domain def already contains a net
>>>>>>> device with the same mac address, right?  In that case, you report an
>>>>>>> error and return failure from libxlDomainAttachNetDevice().
>>>>>>>     
>>>>>>>       
>>>>>>>           
>>>>>>>               
>>>>>> Right, but if I do not find one (idx == -1), I will proceed with
>>>>>> (possibly successful) adding the device, while the error was already
>>>>>> reported by virDomainNetFindIdx.
>>>>>>   
>>>>>>     
>>>>>>         
>>>>>>             
>>>>> Ah, right :-/.
>>>>>
>>>>> Another option: introduce virDomainHasNet() to detect if the domain def
>>>>> already contains the net device.
>>>>>   
>>>>>       
>>>>>           
>>>> A better option would be to fix this in libxl, for the benefit of other
>>>> libxl apps.
>>>>     
>>>>         
>>> Actually libxl has no problem with duplicated mac addresses, its libvirt
>>> that makes problem.
>>>       
>> Yeah, it appears duplicate mac addresses are only valid if on different
>> PCI devices. 
>>     
>
> Is that true for libvirt generally (all drivers)?
>
>   
>> Back to virDomainHasNet? :-)  Or checking for the
>> duplicate directly in libxlDomainAttachNetDevice()?
>>     
>
> What do you mean by "directly"? This is exactly what my patch did (until
> virDomainNetFindIdx stopped reporting duplicates).
>   

I mean coding up the search for an existing mac directly in
libxlDomainAttachNetDevice().  E.g.

static int
libxlDomainAttachNetDevice(libxlDriverPrivatePtr driver,
                           libxlDomainObjPrivatePtr priv,
                           virDomainObjPtr vm,
                           virDomainNetDefPtr net)
{
    int actualType;
    libxl_device_nic nic;
    int ret = -1;
    size_t i;
    virDomainDefPtr def = vm->def;

    for (i = 0; i < def->nnets; i++) {
        if (virMacAddrCmp(&def->nets[i]->mac, &net->mac) == 0)
            error;
     }
    ...
}

Regards,
Jim