netdev - Re: [PATCH] net: Fix device name resolving crash in default_device

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Date:   Fri, 22 Jun 2018 11:36:38 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     David Ahern <dsahern@...il.com>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, daniel@...earbox.net,
        jakub.kicinski@...ronome.com, ast@...nel.org,
        linux@...musvillemoes.dk, john.fastabend@...il.com,
        brouer@...hat.com
Subject: Re: [PATCH] net: Fix device name resolving crash in
 default_device_exit()

On 21.06.2018 18:28, David Ahern wrote:
> On 6/21/18 4:03 AM, Kirill Tkhai wrote:
>>> This patch does not remove the BUG, so does not really solve the
>>> problem. ie., it is fairly trivial to write a script (32k dev%d named
>>> devices in init_net) that triggers it again, so your commit subject and
>>> commit log are not correct with the references to 'fixing the problem'.
>>
>> 1)I'm not agree with you and I don't think removing the BUG() is a good idea.
>> This function is called from the place, where it must not fail. But it can
>> fail, and the problem with name is not the only reason of this happens.
>> We can't continue further pernet_operations in case of a problem happened
>> in default_device_exit(), and we can't remove the BUG() before this function
>> becomes of void type. But we are not going to make it of void type. So
>> we can't remove the BUG().
> 
> You missed my point: that the function can still fail means you are not
> "fixing" the problem, only delaying it.

Till the function is of int type and it can fail, we can't remove the BUG().
And this does not connected with name resolution.

>>
>> 2)In case of the script is trivial, can't you just post it here to show
>> what type of devices you mean? Is there real problem or this is
>> a theoretical thinking?
> 
> Current code:
> 
> # ip li sh dev eth2
> 4: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP
> mode DEFAULT group default qlen 1000
>     link/ether 02:e0:f9:46:64:80 brd ff:ff:ff:ff:ff:ff
> # ip netns add fubar
> # ip li set eth2 netns fubar
> # ip li add eth2 type dummy
> # ip li add dev4 type dummy
> # ip netns del fubar
> --> BUG
> kernel:[78079.127748] default_device_exit: failed to move eth2 to
> init_net: -17
> 
> 
> With your patch:
> 
> # ip li sh dev eth2
> 4: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP
> mode DEFAULT group default qlen 1000
>     link/ether 02:e0:f9:46:64:80 brd ff:ff:ff:ff:ff:ff
> # ip netns add fubar
> # ip li set eth2 netns fubar
> # ip li add eth2 type dummy
> # for n in $(seq 0 $((32*1024))); do
>   echo "li add dev${n} type dummy"
>   done > ip.batch
> # ip -batch ip.batch
> # ip netns del fubar
> --> BUG
> kernel:[   25.800024] default_device_exit: failed to move eth2 to
> init_net: -17

Yeah, this has a sense.

>>
>> All virtual devices I see have rtnl_link_ops, so that they just destroyed
>> in default_device_exit_batch(). According to physical devices, it's difficult
>> to imagine a node with 32k physical devices, and if someone tried to deploy
>> them it may meet problems not only in this place.
> 
> Nothing says it has to be a physical device. It is only checking for a name.
> 
>>
>>> The change does provide more variability in naming and reduces the
>>> likelihood of not being able to push a device back to init_net.
>>
>> No, it provides. With the patch one may move real device to a container,
>> and allow to do with the device anything including changing of device
>> index. Then, the destruction of the container does not resilt a kernel
>> panic just because of two devices have the same index.