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

[Christian Brauner]

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.

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.

Christian