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

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

[Christian Brauner]

[Cc: linux-api]

On Thu, Jul 30, 2020 at 03:08:53PM +0200, Christian Brauner wrote:
> On Thu, Jul 30, 2020 at 02:59:20PM +0300, Kirill Tkhai wrote:
> > 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.
> > 
> > This patchset introduces a new /proc/namespaces/ directory, which exposes
> > subset of permitted namespaces in linear view:
> > 
> > # ls /proc/namespaces/ -l
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'cgroup:[4026531835]' -> 'cgroup:[4026531835]'
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'ipc:[4026531839]' -> 'ipc:[4026531839]'
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026531840]' -> 'mnt:[4026531840]'
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026531861]' -> 'mnt:[4026531861]'
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026532133]' -> 'mnt:[4026532133]'
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026532134]' -> 'mnt:[4026532134]'
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026532135]' -> 'mnt:[4026532135]'
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'mnt:[4026532136]' -> 'mnt:[4026532136]'
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'net:[4026531993]' -> 'net:[4026531993]'
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'pid:[4026531836]' -> 'pid:[4026531836]'
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'time:[4026531834]' -> 'time:[4026531834]'
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'user:[4026531837]' -> 'user:[4026531837]'
> > lrwxrwxrwx 1 root root 0 Jul 29 16:50 'uts:[4026531838]' -> 'uts:[4026531838]'
> > 
> > Namespace ns is exposed, in case of its user_ns is permitted from /proc's pid_ns.
> > I.e., /proc is related to pid_ns, so in /proc/namespace we show only a ns, which is
> > 
> > 	in_userns(pid_ns->user_ns, ns->user_ns).
> > 
> > In case of ns is a user_ns:
> > 
> > 	in_userns(pid_ns->user_ns, ns).
> > 
> > The patchset follows this steps:
> > 
> > 1)A generic counter in ns_common is introduced instead of separate
> >   counters for every ns type (net::count, uts_namespace::kref,
> >   user_namespace::count, etc). Patches [1-8];
> > 2)Patch [9] introduces IDR to link and iterate alive namespaces;
> > 3)Patch [10] is refactoring;
> > 4)Patch [11] actually adds /proc/namespace directory and fs methods;
> > 5)Patches [12-23] make every namespace to use the added methods
> >   and to appear in /proc/namespace directory.
> > 
> > This may be usefull to write effective debug utils (say, fast build
> > of networks topology) and checkpoint/restore software.
> 
> Kirill,
> 
> Thanks for working on this!
> We have a need for this functionality too for namespace introspection.
> I actually had a prototype of this as well but mine was based on debugfs
> but /proc/namespaces seems like a good place.

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

[Eric W. Biederman]

Christian Brauner <christian.brauner@ubuntu.com> writes:

> On Mon, Aug 17, 2020 at 10:48:01AM -0500, Eric W. Biederman wrote:
>> 
>> Creating names in the kernel for namespaces is very difficult and
>> problematic.  I have not seen anything that looks like  all of the
>> problems have been solved with restoring these new names.
>> 
>> When your filter for your list of namespaces is user namespace creating
>> a new directory in proc is highly questionable.
>> 
>> As everyone uses proc placing this functionality in proc also amplifies
>> the problem of creating names.
>> 
>> 
>> Rather than proc having a way to mount a namespace filesystem filter by
>> the user namespace of the mounter likely to have many many fewer
>> problems.  Especially as we are limiting/not allow new non-process
>> things and ideally finding a way to remove the non-process things.
>> 
>> 
>> Kirill you have a good point that taking the case where a pid namespace
>> does not exist in a user namespace is likely quite unrealistic.
>> 
>> Kirill mentioned upthread that the list of namespaces are the list that
>> can appear in a container.  Except by discipline in creating containers
>> it is not possible to know which namespaces may appear in attached to a
>> process.  It is possible to be very creative with setns, and violate any
>> constraint you may have.  Which means your filtered list of namespaces
>> may not contain all of the namespaces used by a set of processes.  This
>
> Indeed. We use setns() quite creatively when intercepting syscalls and
> when attaching to a container.
>
>> further argues that attaching the list of namespaces to proc does not
>> make sense.
>> 
>> Andrei has a good point that placing the names in a hierarchy by
>> user namespace has the potential to create more freedom when
>> assigning names to namespaces, as it means the names for namespaces
>> do not need to be globally unique, and while still allowing the names
>> to stay the same.
>> 
>> 
>> To recap the possibilities for names for namespaces that I have seen
>> mentioned in this thread are:
>>   - Names per mount
>>   - Names per user namespace
>> 
>> I personally suspect that names per mount are likely to be so flexibly
>> they are confusing, while names per user namespace are likely to be
>> rigid, possibly too rigid to use.
>> 
>> It all depends upon how everything is used.  I have yet to see a
>> complete story of how these names will be generated and used.  So I can
>> not really judge.
>
> So I haven't fully understood either what the motivation for this
> patchset is.
> I can just speak to the use-case I had when I started prototyping
> something similar: We needed a way to get a view on all namespaces
> that exist on the system because we wanted a way to do namespace
> debugging on a live system. This interface could've easily lived in
> debugfs. The main point was that it should contain all namespaces.
> Note, that it wasn't supposed to be a hierarchical format it was only
> mean to list all namespaces and accessible to real root.
> The interface here is way more flexible/complex and I haven't yet
> figured out what exactly it is supposed to be used for.
>
>> 
>> 
>> Let me add another take on this idea that might give this work a path
>> forward. If I were solving this I would explore giving nsfs directories
>> per user namespace, and a way to mount it that exposed the directory of
>> the mounters current user namespace (something like btrfs snapshots).
>> 
>> Hmm.  For the user namespace directory I think I would give it a file
>> "ns" that can be opened to get a file handle on the user namespace.
>> Plus a set of subdirectories "cgroup", "ipc", "mnt", "net", "pid",
>> "user", "uts") for each type of namespace.  In each directory I think
>> I would just have a 64bit counter and each new entry I would assign the
>> next number from that counter.
>> 
>> The restore could either have the ability to rename files or simply the
>> ability to bump the counter (like we do with pids) so the names of the
>> namespaces can be restored.
>> 
>> That winds up making a user namespace the namespace of namespaces, so
>> I am not 100% about the idea. 
>
> I think you're right that we need to understand better what the use-case
> is. If I understand your suggestion correctly it wouldn't allow to show
> nested user namespaces if the nsfs mount is per-user namespace.

So what I was thinking is that we have the user namespace directories
and that the mount code would perform a bind mount such that the
directory that matches the mounters user namespace is the root
directory.

> Let me throw in a crazy idea: couldn't we just make the ioctl_ns() walk
> a namespace hierarchy? For example, you could pass in a user namespace
> fd and then you'd get back a struct with handles for fds for the
> namespaces owned by that user namespace and then you could use
> NS_GET_USERNS/NS_GET_PARENT to walk upwards from the user namespace fd
> passed in initially and so on? Or something similar/simpler. This would
> also decouple this from procfs somewhat.

Hmm.

That would remove the need to have names.  We could just keep a list
of the namespaces in creation order.  Hopefully the CRIU folks could
preserve that create order without too much trouble.

Say with an ioctl NS_NEXT_CREATION which takes two fds, and returns
a new file descriptor.  The arguments would be the user namespace
and -1 or the file descriptor last returned fro NS_NEXT_CREATION.


Assuming that is not difficult for CRIU to restore that would be a very
simple patch.

Eric