Message-ID: <87eeo59k8v.fsf@x220.int.ebiederm.org>
Date:   Mon, 17 Aug 2020 13:53:52 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Kirill Tkhai <ktkhai@...tuozzo.com>,
        Andrei Vagin <avagin@...il.com>, adobriyan@...il.com,
        viro@...iv.linux.org.uk, davem@...emloft.net,
        akpm@...ux-foundation.org, areber@...hat.com, serge@...lyn.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        <linux-api@...r.kernel.org>
Subject: Re: [PATCH 00/23] proc: Introduce /proc/namespaces/ directory to expose namespaces lineary

Christian Brauner <christian.brauner@...ntu.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

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