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Bug#931130: Document that local configuration for dummy and bonding modules are getting overwritten by systemd



Hi all,

On 6/26/19 9:23 PM, Justin B Rye wrote:
> Baptiste BEAUPLAT wrote:
>> Systemd, in buster, will ship the file /lib/modprobe.d/systemd.conf,
>> which contains the following:
>>
>>     options bonding max_bonds=0
>>
>>     # Do the same for dummy0.
>>
>>     options dummy numdummies=0
>>
>> This breaks any configuration that an administrator could have added to
>> /etc/modprobe.d regarding the dummy and bonding modules.
> 
> We need more information about why an administrator might have done
> this, since otherwise for a start it's impossible to guess what would
> go wrong as a result.  VMs with sabotaged networking, or what?  Is
> there some other bugreport where we could read about these symptoms?

The dummy module can be used to create virtual interfaces, that can then
be configured for a particular use.

I've seen it a couple of times, usually for router or vpn box. It would
allow to bind some services especially for that virtual interface.

I stumble across this bug just as of today, while testing the upgrade to
buster. I don't think there is yet other bug report on it.

If you think that release-notes is not the place and that I should
report it to another package, I don't mind at all reassigning it.

>  
>> For instance, a file in /etc/modprobe.d/dummy.conf containing:
>>
>>     options dummy numdummies=1
>>
>> Will result in the following being executed by modprobe:
>>
>>     insmod /lib/modules/4.19.0-5-amd64/kernel/drivers/net/dummy.ko
>> numdummies=1 numdummies=0
>>
>> And the original configuration will be overridden.
>>
>> The only way to force modprobe to use local configuration is to rename
>> the /etc/modprobe.d/dummy.conf file to /etc/modprobe.d/systemd.conf.
> 
> This overrides the /lib/modprobe.d/systemd.conf entirely, doesn't it?
> In which case you'd lose any other things in that file that systemd
> might be depending on.  Checking on a Buster box I see it also defines
> "options bonding max_bonds=0" - if I want to avoid overriding that,
> would it be better to use a name like /etc/modprobe.d/zz-local.conf?

The current problem is the actual processing order of modprobe.

It should be /lib/modprobe.d then /etc/modprobe.d.

Currently, it merges both directories and then do the sorting.
The 'ill' effect is that it processes /lib/modprobe.d/systemd.conf
_after_ /etc/aa-systemd.conf, overwritting its content.

I now realize that I should have reported the issue directly against
modprobe. I don't think it will be fixed in time for buster release, so
it might be still useful to have the info on the release notes.

>> I thinks this should be documented in the release notes as admins would
>> need to be aware of that.
> 
> And/or quite possibly about max_bonds=0?  I'm afraid I know even less
> about what symptoms that might have.
> 

I'll open a new bug against modprobe with a better explaining of the
problem. I'll wait for maintainers' reply and see then.

Thanks for your quick replies,

Best,

-- 
Baptiste BEAUPLAT - lyknode

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