Re: [PATCH 00/23] proc: Introduce /proc/namespaces/ directory to expose namespaces lineary

From: Kirill Tkhai
Date: Thu Aug 13 2020 - 04:12:58 EST

On 12.08.2020 20:53, Andrei Vagin wrote:
> On Tue, Aug 11, 2020 at 01:23:35PM +0300, Kirill Tkhai wrote:
>> On 10.08.2020 20:34, Andrei Vagin wrote:
>>> On Fri, Aug 07, 2020 at 11:47:57AM +0300, Kirill Tkhai wrote:
>>>> On 06.08.2020 11:05, Andrei Vagin wrote:
>>>>> On Mon, Aug 03, 2020 at 01:03:17PM +0300, Kirill Tkhai wrote:
>>>>>> On 31.07.2020 01:13, Eric W. Biederman wrote:
>>>>>>> Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> writes:
>>>>>>>> On 30.07.2020 17:34, Eric W. Biederman wrote:
>>>>>>>>> Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> writes:
>>>>>>>>>> Currently, there is no a way to list or iterate all or subset of namespaces
>>>>>>>>>> in the system. Some namespaces are exposed in /proc/[pid]/ns/ directories,
>>>>>>>>>> but some also may be as open files, which are not attached to a process.
>>>>>>>>>> When a namespace open fd is sent over unix socket and then closed, it is
>>>>>>>>>> impossible to know whether the namespace exists or not.
>>>>>>>>>> Also, even if namespace is exposed as attached to a process or as open file,
>>>>>>>>>> iteration over /proc/*/ns/* or /proc/*/fd/* namespaces is not fast, because
>>>>>>>>>> this multiplies at tasks and fds number.
>>>>>>>>> I am very dubious about this.
>>>>>>>>> I have been avoiding exactly this kind of interface because it can
>>>>>>>>> create rather fundamental problems with checkpoint restart.
>>>>>>>> restart/restore :)
>>>>>>>>> You do have some filtering and the filtering is not based on current.
>>>>>>>>> Which is good.
>>>>>>>>> A view that is relative to a user namespace might be ok. It almost
>>>>>>>>> certainly does better as it's own little filesystem than as an extension
>>>>>>>>> to proc though.
>>>>>>>>> The big thing we want to ensure is that if you migrate you can restore
>>>>>>>>> everything. I don't see how you will be able to restore these files
>>>>>>>>> after migration. Anything like this without having a complete
>>>>>>>>> checkpoint/restore story is a non-starter.
>>>>>>>> There is no difference between files in /proc/namespaces/ directory and /proc/[pid]/ns/.
>>>>>>>> CRIU can restore open files in /proc/[pid]/ns, the same will be with /proc/namespaces/ files.
>>>>>>>> As a person who worked deeply for pid_ns and user_ns support in CRIU, I don't see any
>>>>>>>> problem here.
>>>>>>> An obvious diffference is that you are adding the inode to the inode to
>>>>>>> the file name. Which means that now you really do have to preserve the
>>>>>>> inode numbers during process migration.
>>>>>>> Which means now we have to do all of the work to make inode number
>>>>>>> restoration possible. Which means now we need to have multiple
>>>>>>> instances of nsfs so that we can restore inode numbers.
>>>>>>> I think this is still possible but we have been delaying figuring out
>>>>>>> how to restore inode numbers long enough that may be actual technical
>>>>>>> problems making it happen.
>>>>>> Yeah, this matters. But it looks like here is not a dead end. We just need
>>>>>> change the names the namespaces are exported to particular fs and to support
>>>>>> rename().
>>>>>> Before introduction a principally new filesystem type for this, can't
>>>>>> this be solved in current /proc?
>>>>> do you mean to introduce names for namespaces which users will be able
>>>>> to change? By default, this can be uuid.
>>>> Yes, I mean this.
>>>> Currently I won't give a final answer about UUID, but I planned to show some
>>>> default names, which based on namespace type and inode num. Completely custom
>>>> names for any /proc by default will waste too much memory.
>>>> So, I think the good way will be:
>>>> 1)Introduce a function, which returns a hash/uuid based on ino, ns type and some static
>>>> random seed, which is generated on boot;
>>>> 2)Use the hash/uuid as default names in newly create /proc/namespaces: pid-{hash/uuid(ino, "pid")}
>>>> 3)Allow rename, and allocate space only for renamed names.
>>>> Maybe 2 and 3 will be implemented as shrinkable dentries and non-shrinkable.
>>>>> And I have a suggestion about the structure of /proc/namespaces/.
>>>>> Each namespace is owned by one of user namespaces. Maybe it makes sense
>>>>> to group namespaces by their user-namespaces?
>>>>> /proc/namespaces/
>>>>> user
>>>>> mnt-X
>>>>> mnt-Y
>>>>> pid-X
>>>>> uts-Z
>>>>> user-X/
>>>>> user
>>>>> mnt-A
>>>>> mnt-B
>>>>> user-C
>>>>> user-C/
>>>>> user
>>>>> user-Y/
>>>>> user
>>>> Hm, I don't think that user namespace is a generic key value for everybody.
>>>> For generic people tasks a user namespace is just a namespace among another
>>>> namespace types. For me it will look a bit strage to iterate some user namespaces
>>>> to build container net topology.
>>> I can’t agree with you that the user namespace is one of others. It is
>>> the namespace for namespaces. It sets security boundaries in the system
>>> and we need to know them to understand the whole system.
>>> If user namespaces are not used in the system or on a container, you
>>> will see all namespaces in one directory. But if the system has a more
>>> complicated structure, you will be able to build a full picture of it.
>>> You said that one of the users of this feature is CRIU (the tool to
>>> checkpoint/restore containers) and you said that it would be good if
>>> CRIU will be able to collect all container namespaces before dumping
>>> processes, sockets, files etc. But how will we be able to do this if we
>>> will list all namespaces in one directory?
>> There is no a problem, this looks rather simple. Two cases are possible:
>> 1)a container has dedicated namespaces set, and CRIU just has to iterate
>> files in /proc/namespaces of root pid namespace of the container.
>> The relationships between parents and childs of pid and user namespaces
>> are founded via ioctl(NS_GET_PARENT).
>> 2)container has no dedicated namespaces set. Then CRIU just has to iterate
>> all host namespaces. There is no another way to do that, because container
>> may have any host namespaces, and hierarchy in /proc/namespaces won't
>> help you.
>>> Here are my thoughts why we need to the suggested structure is better
>>> than just a list of namespaces:
>>> * Users will be able to understand securies bondaries in the system.
>>> Each namespace in the system is owned by one of user namespace and we
>>> need to know these relationshipts to understand the whole system.
>> Here are already NS_GET_PARENT and NS_GET_USERNS. What is the problem to use
>> this interfaces?
> We can use these ioctl-s, but we will need to enumerate all namespaces in
> the system to build a view of the namespace hierarchy. This will be very
> expensive. The kernel can show this hierarchy without additional cost.

No. We will have to iterate /proc/namespaces of a specific container to get
its namespaces. It's a subset of all namespaces in system, and these all the
namespaces, which are potentially allowed for the container.

>>> * This is simplify collecting namespaces which belong to one container.
>>> For example, CRIU collects all namespaces before dumping file
>>> descriptors. Then it collects all sockets with socket-diag in network
>>> namespaces and collects mount points via /proc/pid/mountinfo in mount
>>> namesapces. Then these information is used to dump socket file
>>> descriptors and opened files.
>> This is just the thing I say. This allows to avoid writing recursive dump.
> I don't understand this. How are you going to collect namespaces in CRIU
> without knowing which are used by a dumped container?

My patchset exports only the namespaces, which are allowed for a specific
container, and no more above this. All exported namespaces are alive,
so someone holds a reference on every of it. So they are used.

It seems you haven't understood the way I suggested here. See patch [11/23]
for the details. It's about permissions, and the subset of exported namespaces
is formalized there.

>> But this has nothing about advantages of hierarchy in /proc/namespaces.
> Really? You said that you implemented this series to help CRIU dumping
> namespaces. I think we need to implement the CRIU part to prove that
> this interface is usable for this case. Right now, I have doubts about
> this.

Yes, really. See my comment above and patch [11/23].

>>> * We are going to assign names to namespaces. But this means that we
>>> need to guarantee that all names in one directory are unique. The
>>> initial proposal was to enumerate all namespaces in one proc directory,
>>> that means names of all namespaces have to be unique. This can be
>>> problematic in some cases. For example, we may want to dump a container
>>> and then restore it more than once on the same host. How are we going to
>>> avoid namespace name conficts in such cases?
>> Previous message I wrote about .rename of proc files, Alexey Dobriyan
>> said this is not a taboo. Are there problem which doesn't cover the case
>> you point?
> Yes, there is. Namespace names will be visible from a container, so they
> have to be restored. But this means that two containers can't be
> restored from the same snapshot due to namespace name conflicts.
> But if we will show namespaces how I suggest, each container will see
> only its sub-tree of namespaces and we will be able to specify any name
> for the container root user namespace.

Now I'm sure you missed my idea. See proc_namespaces_readdir() in [11/23].

I do export sub-tree.

>>> If we will have per-user-namespace directories, we will need to
>>> guarantee that names are unique only inside one user namespace.
>> Unique names inside one user namespace won't introduce a new /proc
>> mount. You can't pass a sub-directory of /proc/namespaces/ to a specific
>> container. To give a virtualized name you have to have a dedicated pid ns.
>> Let we have in one /proc mount:
>> /mnt1/proc/namespaces/userns1/.../[namespaceX_name1 -- inode XXX]
>> In another another /proc mount we have:
>> /mnt2/proc/namespaces/userns1/.../[namespaceX_name1_synonym -- inode XXX]
>> The virtualization is made per /proc (i.e., per pid ns). Container should
>> receive either /mnt1/proc or /mnt2/proc on restore as it's /proc.
>> There is no a sense of directory hierarchy for virtualization, since
>> you can't use specific sub-directory as a root directory of /proc/namespaces
>> to a container. You still have to introduce a new pid ns to have virtualized
>> /proc.
> I think we can figure out how to implement this. As the first idea, we
> can use the same way how /proc/net is implemented.
>>> * With the suggested structure, for each user namepsace, we will show
>>> only its subtree of namespaces. This looks more natural than
>>> filltering content of one directory.
>> It's rather subjectively I think. /proc is related to pid ns, and user ns
>> hierarchy does not look more natural for me.
> or /proc is wrong place for this