[PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Vitaly Kuznetsov]

Recent 'transparenf VF' changes to netvsc driver made VF interfaces
register as netvsc interface slaves upon appearance. This led to udev
not being able to rename the interface according to the 'predictable
interface names' scheme:

 kernel: mlx4_core 0002:00:02.0 eth2: joined to eth1
 kernel: hv_netvsc 33b7a6f9-6736-451f-8fce-b382eaa50bee eth1: VF
  registering: eth2
 kernel: mlx4_en: eth2: Link Up
 kernel: hv_netvsc 33b7a6f9-6736-451f-8fce-b382eaa50bee eth1: Data path
  switched to VF: eth2
 systemd-udevd[1785]: Error changing net interface name 'eth2' to
  'enP2p0s2': Device or resource busy
 systemd-udevd[1785]: could not rename interface '5' from 'eth2' to
  'enP2p0s2': Device or resource busy

What happens is: __netvsc_vf_setup() does dev_open() for the VF device and
the consecutive dev_change_name() fails with -EBUSY because of the
(dev->flags & IFF_UP) check. The history of this code predates git so I
wasn't able to figure out when and why the check was added, everything
seems to work fine without it. dev_change_name() has only two call sites,
both hold rtnl_lock.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 RFC: I'm probably miossing something obvious and the check can't be just
 dropped. Stephen suggested a different solution to the isuue:
 https://www.spinics.net/lists/netdev/msg448243.html but it has its own
 drawbacks.
---
 net/core/dev.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 1d75499add72..c608e233a78a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1186,8 +1186,6 @@ int dev_change_name(struct net_device *dev, const char *newname)
 	BUG_ON(!dev_net(dev));
 
 	net = dev_net(dev);
-	if (dev->flags & IFF_UP)
-		return -EBUSY;
 
 	write_seqcount_begin(&devnet_rename_seq);
 
-- 
2.13.4

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[吉藤英明]

2017-08-09 19:42 GMT+09:00 Vitaly Kuznetsov <vkuznets@redhat.com>:
> What happens is: __netvsc_vf_setup() does dev_open() for the VF device and
> the consecutive dev_change_name() fails with -EBUSY because of the
> (dev->flags & IFF_UP) check. The history of this code predates git so I
> wasn't able to figure out when and why the check was added, everything
> seems to work fine without it. dev_change_name() has only two call sites,
> both hold rtnl_lock.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  RFC: I'm probably miossing something obvious and the check can't be just
>  dropped. Stephen suggested a different solution to the isuue:
>  https://www.spinics.net/lists/netdev/msg448243.html but it has its own
>  drawbacks.
> ---
>  net/core/dev.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 1d75499add72..c608e233a78a 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -1186,8 +1186,6 @@ int dev_change_name(struct net_device *dev, const char *newname)
>         BUG_ON(!dev_net(dev));
>
>         net = dev_net(dev);
> -       if (dev->flags & IFF_UP)
> -               return -EBUSY;
>
>         write_seqcount_begin(&devnet_rename_seq);

I think people expect the name won't change while up
and I don't think it is a good idea to allow changing the
name while the interface is up.

--yoshfuji


>
> --
> 2.13.4
>

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Vitaly Kuznetsov]

吉藤英明 <hideaki.yoshifuji@miraclelinux.com> writes:

> 2017-08-09 19:42 GMT+09:00 Vitaly Kuznetsov <vkuznets@redhat.com>:
>> What happens is: __netvsc_vf_setup() does dev_open() for the VF device and
>> the consecutive dev_change_name() fails with -EBUSY because of the
>> (dev->flags & IFF_UP) check. The history of this code predates git so I
>> wasn't able to figure out when and why the check was added, everything
>> seems to work fine without it. dev_change_name() has only two call sites,
>> both hold rtnl_lock.
>>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>>  RFC: I'm probably miossing something obvious and the check can't be just
>>  dropped. Stephen suggested a different solution to the isuue:
>>  https://www.spinics.net/lists/netdev/msg448243.html but it has its own
>>  drawbacks.
>> ---
>>  net/core/dev.c | 2 --
>>  1 file changed, 2 deletions(-)
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index 1d75499add72..c608e233a78a 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -1186,8 +1186,6 @@ int dev_change_name(struct net_device *dev, const char *newname)
>>         BUG_ON(!dev_net(dev));
>>
>>         net = dev_net(dev);
>> -       if (dev->flags & IFF_UP)
>> -               return -EBUSY;
>>
>>         write_seqcount_begin(&devnet_rename_seq);
>
> I think people expect the name won't change while up
> and I don't think it is a good idea to allow changing the
> name while the interface is up.

I understand the 'legacy' concern but at the same time we don't want to
have aftificial limitations too. Name change, in particular, doesn't
happen 'under the hood' -- someone privileged enough needs to request
the change.

Can you think of any particular real world scenarios which are broken by
the change?

-- 
  Vitaly

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Andrew Lunn]

> I understand the 'legacy' concern but at the same time we don't want to
> have aftificial limitations too. Name change, in particular, doesn't
> happen 'under the hood' -- someone privileged enough needs to request
> the change.
> 
> Can you think of any particular real world scenarios which are broken by
> the change?

How about:

man 8 dhclient-script

The interface name is passed in $interface to the scripts. Do we get
the old name or the new name? I suspect scripts are going to break if
they are given the old name, which no longer exists.

     Andrew

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Vitaly Kuznetsov]

Andrew Lunn <andrew@lunn.ch> writes:

>> I understand the 'legacy' concern but at the same time we don't want to
>> have aftificial limitations too. Name change, in particular, doesn't
>> happen 'under the hood' -- someone privileged enough needs to request
>> the change.
>> 
>> Can you think of any particular real world scenarios which are broken by
>> the change?
>
> How about:
>
> man 8 dhclient-script
>
> The interface name is passed in $interface to the scripts. Do we get
> the old name or the new name? I suspect scripts are going to break if
> they are given the old name, which no longer exists.

Yes but why would anyone change interface name while dhclient-script is
running? Things will also go wrong if you try bringing interface down
during the run or do some other configuration, right? Running multiple
configuration tools at the same moment is a bad idea, you never know
what you're gonna end up with. 

As I see it, checks in kernel we have are meant to protect kernel
itself, not to disallow all user<->kernel interactions leading to
imperfect result.

(AFAIU) If we remove the check nothing is going to change: udev will
still be renaming interfaces before bringing them up. In netvsc case
users are not supposed to configure the VF interface at all, it just
becomes a slave of netvsc interface.

-- 
  Vitaly

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Eric Dumazet]

On Thu, 2017-08-10 at 10:41 +0200, Vitaly Kuznetsov wrote:
> Andrew Lunn <andrew@lunn.ch> writes:
> 
> >> I understand the 'legacy' concern but at the same time we don't want to
> >> have aftificial limitations too. Name change, in particular, doesn't
> >> happen 'under the hood' -- someone privileged enough needs to request
> >> the change.
> >> 
> >> Can you think of any particular real world scenarios which are broken by
> >> the change?
> >
> > How about:
> >
> > man 8 dhclient-script
> >
> > The interface name is passed in $interface to the scripts. Do we get
> > the old name or the new name? I suspect scripts are going to break if
> > they are given the old name, which no longer exists.
> 
> Yes but why would anyone change interface name while dhclient-script is
> running? Things will also go wrong if you try bringing interface down
> during the run or do some other configuration, right? Running multiple
> configuration tools at the same moment is a bad idea, you never know
> what you're gonna end up with. 
> 
> As I see it, checks in kernel we have are meant to protect kernel
> itself, not to disallow all user<->kernel interactions leading to
> imperfect result.
> 
> (AFAIU) If we remove the check nothing is going to change: udev will
> still be renaming interfaces before bringing them up. In netvsc case
> users are not supposed to configure the VF interface at all, it just
> becomes a slave of netvsc interface.

Are we sending an event if device name is changed ?

If yes, your patch is fine.

If not, daemons would not be aware the need to refresh their view of the
world.

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Vitaly Kuznetsov]

Eric Dumazet <eric.dumazet@gmail.com> writes:

> On Thu, 2017-08-10 at 10:41 +0200, Vitaly Kuznetsov wrote:
>> Andrew Lunn <andrew@lunn.ch> writes:
>> 
>> >> I understand the 'legacy' concern but at the same time we don't want to
>> >> have aftificial limitations too. Name change, in particular, doesn't
>> >> happen 'under the hood' -- someone privileged enough needs to request
>> >> the change.
>> >> 
>> >> Can you think of any particular real world scenarios which are broken by
>> >> the change?
>> >
>> > How about:
>> >
>> > man 8 dhclient-script
>> >
>> > The interface name is passed in $interface to the scripts. Do we get
>> > the old name or the new name? I suspect scripts are going to break if
>> > they are given the old name, which no longer exists.
>> 
>> Yes but why would anyone change interface name while dhclient-script is
>> running? Things will also go wrong if you try bringing interface down
>> during the run or do some other configuration, right? Running multiple
>> configuration tools at the same moment is a bad idea, you never know
>> what you're gonna end up with. 
>> 
>> As I see it, checks in kernel we have are meant to protect kernel
>> itself, not to disallow all user<->kernel interactions leading to
>> imperfect result.
>> 
>> (AFAIU) If we remove the check nothing is going to change: udev will
>> still be renaming interfaces before bringing them up. In netvsc case
>> users are not supposed to configure the VF interface at all, it just
>> becomes a slave of netvsc interface.
>
> Are we sending an event if device name is changed ?
>

We are - rtnetlink_event() does the job. We, however, don't have a
special IFLA_EVENT_* for name change and end up with IFLA_EVENT_NONE.

> If yes, your patch is fine.
>
> If not, daemons would not be aware the need to refresh their view of the
> world.

Yes but AFAIU daemons may need to do the same refresh when the interface
is down too (and, hopefully, they do it already).

-- 
  Vitaly

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Andrew Lunn]

> We are - rtnetlink_event() does the job. We, however, don't have a
> special IFLA_EVENT_* for name change and end up with IFLA_EVENT_NONE.

What is in this event? Old and new name? Just the new name?

     Andrew

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Vitaly Kuznetsov]

Andrew Lunn <andrew@lunn.ch> writes:

>> We are - rtnetlink_event() does the job. We, however, don't have a
>> special IFLA_EVENT_* for name change and end up with IFLA_EVENT_NONE.
>
> What is in this event? Old and new name? Just the new name?

Basically, it's everything we know about the interface - type, index,
name, mtu, qdisc, ... - see rtnl_fill_ifinfo(). Back to your question -
it's only the new name.

-- 
  Vitaly

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Andrew Lunn]

On Thu, Aug 10, 2017 at 05:24:55PM +0200, Vitaly Kuznetsov wrote:
> Andrew Lunn <andrew@lunn.ch> writes:
> 
> >> We are - rtnetlink_event() does the job. We, however, don't have a
> >> special IFLA_EVENT_* for name change and end up with IFLA_EVENT_NONE.
> >
> > What is in this event? Old and new name? Just the new name?
> 
> Basically, it's everything we know about the interface - type, index,
> name, mtu, qdisc, ... - see rtnl_fill_ifinfo(). Back to your question -
> it's only the new name.

So the program needs to keep track of ifindex to know which interface
has changed name. Doable.

I still expect this has the potential to break something. You probably
should be asking on linux-api for the API experts opinion.

       Andrew

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Vitaly Kuznetsov]

Andrew Lunn <andrew@lunn.ch> writes:

> On Thu, Aug 10, 2017 at 05:24:55PM +0200, Vitaly Kuznetsov wrote:
>> Andrew Lunn <andrew@lunn.ch> writes:
>> 
>> >> We are - rtnetlink_event() does the job. We, however, don't have a
>> >> special IFLA_EVENT_* for name change and end up with IFLA_EVENT_NONE.
>> >
>> > What is in this event? Old and new name? Just the new name?
>> 
>> Basically, it's everything we know about the interface - type, index,
>> name, mtu, qdisc, ... - see rtnl_fill_ifinfo(). Back to your question -
>> it's only the new name.
>
> So the program needs to keep track of ifindex to know which interface
> has changed name. Doable.
>

Yes, and I'd expect that's what these daemons do nowdays to track name
changes for down interfaces (if/when they care).

> I still expect this has the potential to break something. You probably
> should be asking on linux-api for the API experts opinion.
>

Good idea, I'll do RFCv2 submission.

-- 
  Vitaly

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[David Miller]

From: Andrew Lunn <andrew@lunn.ch>
Date: Thu, 10 Aug 2017 18:27:22 +0200

> On Thu, Aug 10, 2017 at 05:24:55PM +0200, Vitaly Kuznetsov wrote:
>> Andrew Lunn <andrew@lunn.ch> writes:
>> 
>> >> We are - rtnetlink_event() does the job. We, however, don't have a
>> >> special IFLA_EVENT_* for name change and end up with IFLA_EVENT_NONE.
>> >
>> > What is in this event? Old and new name? Just the new name?
>> 
>> Basically, it's everything we know about the interface - type, index,
>> name, mtu, qdisc, ... - see rtnl_fill_ifinfo(). Back to your question -
>> it's only the new name.
> 
> So the program needs to keep track of ifindex to know which interface
> has changed name. Doable.
> 
> I still expect this has the potential to break something. You probably
> should be asking on linux-api for the API experts opinion.

But a greater point is that nobody is monitoring device renames
explicitly right now.

It's hard to legitimize imposing new requirements like this on daemons
which didn't exist before.

If they could assume the name didn't change, they probably are doing
so.

You can't expect that just because they might already be listening to
this event for device up and down tracking that they are handling
device renames.

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[David Ahern]

On 8/10/17 10:48 AM, David Miller wrote:
> From: Andrew Lunn <andrew@lunn.ch>
> Date: Thu, 10 Aug 2017 18:27:22 +0200
> 
>> On Thu, Aug 10, 2017 at 05:24:55PM +0200, Vitaly Kuznetsov wrote:
>>> Andrew Lunn <andrew@lunn.ch> writes:
>>>
>>>>> We are - rtnetlink_event() does the job. We, however, don't have a
>>>>> special IFLA_EVENT_* for name change and end up with IFLA_EVENT_NONE.
>>>>
>>>> What is in this event? Old and new name? Just the new name?
>>>
>>> Basically, it's everything we know about the interface - type, index,
>>> name, mtu, qdisc, ... - see rtnl_fill_ifinfo(). Back to your question -
>>> it's only the new name.
>>
>> So the program needs to keep track of ifindex to know which interface
>> has changed name. Doable.
>>
>> I still expect this has the potential to break something. You probably
>> should be asking on linux-api for the API experts opinion.
> 
> But a greater point is that nobody is monitoring device renames
> explicitly right now.

Just to throw in an example:
  https://github.com/kobolabs/dhcpcd/blob/kobo/if-linux.c#L761

Learned of its use from a recent regression:
https://bugzilla.kernel.org/show_bug.cgi?id=196355

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Stephen Hemminger]

On Thu, 10 Aug 2017 10:55:01 -0600
David Ahern <dsahern@gmail.com> wrote:

> On 8/10/17 10:48 AM, David Miller wrote:
> > From: Andrew Lunn <andrew@lunn.ch>
> > Date: Thu, 10 Aug 2017 18:27:22 +0200
> >   
> >> On Thu, Aug 10, 2017 at 05:24:55PM +0200, Vitaly Kuznetsov wrote:  
> >>> Andrew Lunn <andrew@lunn.ch> writes:
> >>>  
> >>>>> We are - rtnetlink_event() does the job. We, however, don't have a
> >>>>> special IFLA_EVENT_* for name change and end up with IFLA_EVENT_NONE.  
> >>>>
> >>>> What is in this event? Old and new name? Just the new name?  
> >>>
> >>> Basically, it's everything we know about the interface - type, index,
> >>> name, mtu, qdisc, ... - see rtnl_fill_ifinfo(). Back to your question -
> >>> it's only the new name.  
> >>
> >> So the program needs to keep track of ifindex to know which interface
> >> has changed name. Doable.
> >>
> >> I still expect this has the potential to break something. You probably
> >> should be asking on linux-api for the API experts opinion.  
> > 
> > But a greater point is that nobody is monitoring device renames
> > explicitly right now.  
> 
> Just to throw in an example:
>   https://github.com/kobolabs/dhcpcd/blob/kobo/if-linux.c#L761
> 
> Learned of its use from a recent regression:
> https://bugzilla.kernel.org/show_bug.cgi?id=196355

Quagga is another example of what might break. Especially with all the new
forks..

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Vitaly Kuznetsov]

Stephen Hemminger <stephen@networkplumber.org> writes:

> On Thu, 10 Aug 2017 10:55:01 -0600
> David Ahern <dsahern@gmail.com> wrote:
>
>> On 8/10/17 10:48 AM, David Miller wrote:
>> > From: Andrew Lunn <andrew@lunn.ch>
>> > Date: Thu, 10 Aug 2017 18:27:22 +0200
>> >   
>> >> On Thu, Aug 10, 2017 at 05:24:55PM +0200, Vitaly Kuznetsov wrote:  
>> >>> Andrew Lunn <andrew@lunn.ch> writes:
>> >>>  
>> >>>>> We are - rtnetlink_event() does the job. We, however, don't have a
>> >>>>> special IFLA_EVENT_* for name change and end up with IFLA_EVENT_NONE.  
>> >>>>
>> >>>> What is in this event? Old and new name? Just the new name?  
>> >>>
>> >>> Basically, it's everything we know about the interface - type, index,
>> >>> name, mtu, qdisc, ... - see rtnl_fill_ifinfo(). Back to your question -
>> >>> it's only the new name.  
>> >>
>> >> So the program needs to keep track of ifindex to know which interface
>> >> has changed name. Doable.
>> >>
>> >> I still expect this has the potential to break something. You probably
>> >> should be asking on linux-api for the API experts opinion.  
>> > 
>> > But a greater point is that nobody is monitoring device renames
>> > explicitly right now.  
>> 
>> Just to throw in an example:
>>   https://github.com/kobolabs/dhcpcd/blob/kobo/if-linux.c#L761
>> 
>> Learned of its use from a recent regression:
>> https://bugzilla.kernel.org/show_bug.cgi?id=196355
>
> Quagga is another example of what might break. Especially with all the new
> forks..

I see,

even if we don't see right away why the limitation was imposed and who
depends on this 'UP interfaces keep their names' semantics it may not be
worth it to open this pandora box just because of the netvsc driver
change...

-- 
  Vitaly

Re: [PATCH RFC net-next] net: Allow name change of IFF_UP interfaces

[Andrew Lunn]

> >> Can you think of any particular real world scenarios which are broken by
> >> the change?
> >
> > How about:
> >
> > man 8 dhclient-script
> >
> > The interface name is passed in $interface to the scripts. Do we get
> > the old name or the new name? I suspect scripts are going to break if
> > they are given the old name, which no longer exists.
> 
> Yes but why would anyone change interface name while dhclient-script is
> running? Things will also go wrong if you try bringing interface down
> during the run or do some other configuration, right?

dhclient already handles the interface going down. sendto/recvfrom
fails and returns an error code. As far as i remember, dhclient then
exits.

> Running multiple configuration tools at the same moment is a bad
> idea, you never know what you're gonna end up with.

It could be argued that configuring an interface vs renaming an
interface are at different levels.

	  Andrew