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

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Date:   Thu, 21 Jun 2018 09:28:21 -0600
From:   David Ahern <dsahern@...il.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.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 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.

> 
> 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


> 
> 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.
> 
> Kirill
>