[PATCH RFC v2 4/6] proc: support mounting private procfs instances inside same pid namespace

[Andy Lutomirski]

On Tue, May 2, 2017 at 7:29 AM, Djalal Harouni <tixxdz at gmail.com> wrote:
> On Thu, Apr 27, 2017 at 12:13 AM, Andy Lutomirski <luto at kernel.org> wrote:
>> On Tue, Apr 25, 2017 at 5:23 AM, Djalal Harouni <tixxdz at gmail.com> wrote:
> [...]
>>> We have to align procfs and modernize it to have a per mount context
>>> where at least the mount option do not propagate to all other mounts,
>>> then maybe we can continue to implement new features. One example is to
>>> require CAP_SYS_ADMIN in the init user namespace on some /proc/* which are
>>> not pids and which are are not virtualized by design, or CAP_NET_ADMIN
>>> inside userns on the net bits that are virtualized, etc.
>>> These mount options won't propagate to previous mounts, and the system
>>> will continue to be usable.
>>>
>>> Ths patch introduces the new 'limit_pids' mount option as it was also
>>> suggesed by Andy Lutomirski [1]. When this option is passed we
>>> automatically create a private procfs instance. This is not the default
>>> behaviour since we do not want to break userspace and we do not want to
>>> provide different devices IDs by default, please see [1] for why.
>>
>> I think that calling the option to make a separate instance
>> "limit_pids" is extremely counterintuitive.
>
> Ok.
>
>> My strong preference would be to make proc *always* make a separate
>> instance (unless it's a bind mount) and to make it work.  If that
>> means fudging stat() output, so be it.
>
> I also agree, but as said if we change stat(), userspace won't be able
> to notice if these two proc instances are really separated, the device
> ID is the only indication here.

I re-read all the threads and I'm still not convinced I see why we
need new_instance to be non-default.  It's true that the device
numbers of /proc/ns/* matter, but if you look (with stat -L, for
example), they're *already* not tied to the procfs instance.

I'm okay with adding new_instance to be on the safe side, but I'd like
it to be done in a way that we could make it become the default some
day without breaking anything.  This means that we need to be rather
careful about how new_instance and hidepid interact.
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