netdev - Re: [RFC PATCH 2/3] netdev: kernel-only IFF

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Date:   Tue, 3 Apr 2018 08:57:58 -0600
From:   David Ahern <dsahern@...il.com>
To:     Siwei Liu <loseweigh@...il.com>
Cc:     Si-Wei Liu <si-wei.liu@...cle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        David Miller <davem@...emloft.net>,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        Jakub Kicinski <kubakici@...pl>,
        Jason Wang <jasowang@...hat.com>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Netdev <netdev@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        virtio-dev@...ts.oasis-open.org
Subject: Re: [RFC PATCH 2/3] netdev: kernel-only IFF_HIDDEN netdevice

On 4/3/18 1:40 AM, Siwei Liu wrote:
>> There are other use cases that want to hide a device from userspace.
> 
> Can you elaborate your case in more details? Looking at the links
> below I realize that the purpose of hiding devices in your case is
> quite different from the our migration case. Particularly, I don't

some kernel drivers create "control" netdev's. They are not intended for
users to manipulate and doing so may actually break networking.

> like the part of elaborately allowing user to manipulate the link's
> visibility - things fall apart easily while live migration is on
> going. And, why doing additional check for invisible links in every
> for_each_netdev() and its friends. This is effectively changing
> semantics of internal APIs that exist for decades.

Read the patch again: there are 40 references to for_each_netdev and
that patch touches 2 of them -- link dumps via rtnetlink and link dumps
via ioctl.

>> one that includes an API for users to list all devices -- even ones
> 
> What kind of API you would like to query for hidden devices?
> rtnetlink? a private socket API? or something else?

There are existing, established APIs for dumping links. No new API is
needed. As suggested in the 2 patches I referenced the hidden /
invisibility cloak is an attribute of the device. When a link dump is
requested if the attribute is set, the device is skipped and not
included in the dump. However, if the user knows the device name the
GETLINK / SETLINK / DELLINK apis all work as normal. This allows the
device to be hidden from apps like snmpd, lldpd, etc, yet still usable.

> 
> For our case, the sysfs interface is what we need and is sufficient,
> since udev is the main target we'd like to support to make the naming
> of virtio_bypass consistent and compatible.

You are not hiding a device if it is visible in 1 API (/sysfs) and not
visible by another API (rtnetlink). That only creates confusion.

> 
>> hidden by default.
>>
>> https://github.com/dsahern/linux/commit/48a80a00eac284e58bae04af10a5a932dd7aee00
>>
>> https://github.com/dsahern/iproute2/commit/7563f5b26f5539960e99066e34a995d22ea908ed
>>
>> Also, why are you suggesting that the device should still be visible via
>> /sysfs? That leads to inconsistent views of networking state - /sys
>> shows a device but a link dump does not.
> 
> See my clarifications above. I don't mind kernel-only netdevs being
> visible via sysfs, as that way we get a good trade-off between
> backwards compatibility and visibility. There's still kobject created
> there right. Bottom line is that all kernel devices and its life-cycle
> uevents are made invisible to userpace network utilities, and I think
> it simply gets to the goal of not breaking existing apps while being
> able to add new features.