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

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Date:   Fri, 15 Jun 2018 12:44:32 +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 14.06.2018 20:11, David Ahern wrote:
> On 6/14/18 6:38 AM, Kirill Tkhai wrote:
>> The following script makes kernel to crash since it can't obtain
>> a name for a device, when the name is occupied by another device:
>>
>> #!/bin/bash
>> ifconfig eth0 down
>> ifconfig eth1 down
>> index=`cat /sys/class/net/eth1/ifindex`
>> ip link set eth1 name dev$index
>> unshare -n sleep 1h &
>> pid=$!
>> while [[ "`readlink /proc/self/ns/net`" == "`readlink /proc/$pid/ns/net`" ]]; do continue; done
>> ip link set dev$index netns $pid
>> ip link set eth0 name dev$index
>> kill -9 $pid
>>
>> Kernel messages:
>>
>> virtio_net virtio1 dev3: renamed from eth1
>> virtio_net virtio0 dev3: renamed from eth0
>> default_device_exit: failed to move dev3 to init_net: -17
>> ------------[ cut here ]------------
>> kernel BUG at net/core/dev.c:8978!
>> invalid opcode: 0000 [#1] PREEMPT SMP
>> CPU: 1 PID: 276 Comm: kworker/u8:3 Not tainted 4.17.0+ #292
>> Workqueue: netns cleanup_net
>> RIP: 0010:default_device_exit+0x9c/0xb0
>> [stack trace snipped]
>>
>> This patch gives more variability during choosing new name
>> of device and fixes the problem.
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
>> ---
>>  net/core/dev.c |    4 +---
>>  1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index 6e18242a1cae..6c9b9303ded6 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -8959,7 +8959,6 @@ static void __net_exit default_device_exit(struct net *net)
>>  	rtnl_lock();
>>  	for_each_netdev_safe(net, dev, aux) {
>>  		int err;
>> -		char fb_name[IFNAMSIZ];
>>  
>>  		/* Ignore unmoveable devices (i.e. loopback) */
>>  		if (dev->features & NETIF_F_NETNS_LOCAL)
>> @@ -8970,8 +8969,7 @@ static void __net_exit default_device_exit(struct net *net)
>>  			continue;
>>  
>>  		/* Push remaining network devices to init_net */
>> -		snprintf(fb_name, IFNAMSIZ, "dev%d", dev->ifindex);
>> -		err = dev_change_net_namespace(dev, &init_net, fb_name);
>> +		err = dev_change_net_namespace(dev, &init_net, "dev%d");
>>  		if (err) {
>>  			pr_emerg("%s: failed to move %s to init_net: %d\n",
>>  				 __func__, dev->name, err);
>>
> 
> This could cause repeated looping over __dev_alloc_name. If init_net has
> a large number of devices, it is going to be a performance bottleneck.

Hm, but is this a likely case, when real device is moved to net ns, so it
requires moving to init_net back? It seems the most devices moved to !init_net
are virtual and they just destroyed in default_device_exit_batch(). Or we have
more devices to care here?

I don't much want to insert here something like below:

	if (__dev_get_by_name(&init_net, dev->name))
		snprintf(fb_name, IFNAMSIZ, "dev%d", dev->ifindex);
	err = dev_change_net_namespace(dev, &init_net, "dev%d");

because dev_change_net_namespace() is generic interface and it's used not only here,
and this will crumble the code in corner cases.

Maybe you have better ideas about this?

Kirill