On Fri, 2016-07-08 at 02:44 -0500, Eric W. Biederman wrote:
Andrew Vagin [off-list ref] writes:
quoted
On Wed, Jul 06, 2016 at 10:46:33AM -0500, Eric W. Biederman wrote:
quoted
"Serge E. Hallyn" [off-list ref] writes:
quoted
On Wed, Jul 06, 2016 at 10:41:48AM +0200, Michael Kerrisk (man
-pages) wrote:
quoted
[Rats! Doing now what I should have down to start with.
Looping some
lists and CRIU and other possibly relevant people into this
conversation]
Hi Eric,
On 5 July 2016 at 23:47, Eric W. Biederman <
ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org> wrote:
quoted
"Michael Kerrisk (man-pages)" [off-list ref]
writes:
quoted
Hi Eric,
I have a question. Is there any way currently to discover
which user namespace a particular nonuser namespace is
governed by? Maybe I am missing something, but there does
not seem to be a way to do this. Also, can one discover
which userns is the parent of a given userns? Again, I
can't see a way to do this.
The point here is introspecting so that a process might
determine what its capabilities are when operating on
some resource governed by a (nonuser) namespace.
To the best of my knowledge that there is not an interface
to get that information. It would be good to have such an
interface for no other reason than the CRIU folks are going
to need it at some point. I am a bit surprised they have
not complained yet.
I don't think they need it. They do in fact have what they
need. Assume you have tasks T1, T2, T1_1 and T2_1; T1 and T2
are in init_user_ns; T1 spawned T1_1 in a new userns; T2
spawned T2_1 which setns()d to T1_1's ns. There's some
{handwave} uid mapping, does not matter.
At restart, it doesn't matter which task originally created the
new userns. criu knows T1_1 and T2_1 are in the same userns;
it creates the userns, sets up the mapping, and T1_1 and T2_1
setns() to it.
Given that the simple cases are so easy it probably doesn't
matter in that sense.
However we now have the case where user namespaces own pid
namespaces, and uts namespaces, and network namespaces, and ipc
namespaces, and filesystems. Throw in some mount propagation and
use of setns and things could get confusing. It is something
that will need to be figured out if CRIU is going to properly
checkpoint containers containing containers containing containers
containing containers.
It isn't a joke:). We have a few requests to support CR of
containers with Docker containers inside. And we are going to start
this task in a near future, so we would like to have interface to
get dependencies between namespaces too.
BTW: CRIU already supports nested mount namespaces, because systemd
creates them for services.
The tricky part about this and what messes up James proposed plan is
that the interface needs to be something that returns a namespace
file descriptor. So we can't print something out in a simple text
file.
I actually described two problems: the first was how we get the
information in the first place. Currently the owning or parent user_ns
is tucked inside an opaque structure. I think we need to move that to
ns_common where it would be the owning userns for all non-user
namespaces and the parent for the userns.
Once we actually have the information, we can also add a set of proc
links, say either
/proc/<pid>/ns/X-userns
Which might be a bit messy since it doubles the number of files, or
perhaps in a simple directory.
Well I suppose we could print an device number and inode number pair.
But then someone would still have to scour processes looking for a
user namespace so that is likely less than ideal.
There's no reason any of the proposed methods so far have to be
exclusive: nsfs.c has a lot of flexibility.
Starting with 4.8 we are also going to need to be able to retrieve
the user namespace owner of filesystems. That will be an interesting
mix.
This is per mount point, isn't it? so it can't be in /proc/fs/ and it
would have to be per local mount tree. Yes, that is a bit nasty.
Sounds like we might need to unfold mount or mountinfo into something
that has one directory per entry?
James
Eric
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