On Tue, Oct 21, 2025 at 01:43:06PM +0200, Christian Brauner wrote:

Hey,

As announced a while ago this is the next step building on the nstree
work from prior cycles. There's a bunch of fixes and semantic cleanups
in here and a ton of tests.

I need helper here!: Consider the following current design:

Currently listns() is relying on active namespace reference counts which
are introduced alongside this series.

The active reference count of a namespace consists of the live tasks
that make use of this namespace and any namespace file descriptors that
explicitly pin the namespace.

Once all tasks making use of this namespace have exited or reaped, all
namespace file descriptors for that namespace have been closed and all
bind-mounts for that namespace unmounted it ceases to appear in the
listns() output.

My reason for introducing the active reference count was that namespaces
might obviously still be pinned internally for various reasons. For
example the user namespace might still be pinned because there are still
open files that have stashed the openers credentials in file->f_cred, or
the last reference might be put with an rcu delay keeping that namespace
active on the namespace lists.

But one particularly strange example is CONFIG_MMU_LAZY_TLB_REFCOUNT=y.
Various architectures support the CONFIG_MMU_LAZY_TLB_REFCOUNT option
which uses lazy TLB destruction.

When this option is set a userspace task's struct mm_struct may be used
for kernel threads such as the idle task and will only be destroyed once
the cpu's runqueue switches back to another task. So the kernel thread
will take a reference on the struct mm_struct pinning it.

And for ptrace() based access checks struct mm_struct stashes the user
namespace of the task that struct mm_struct belonged to originally and
thus takes a reference to the users namespace and pins it.

So on an idle system such user namespaces can be persisted for pretty
arbitrary amounts of time via struct mm_struct.

Now, without the active reference count regulating visibility all
namespace that still are pinned in some way on the system will appear in
the listns() output and can be reopened using namespace file handles.

Of course that requires suitable privileges and it's not really a
concern per se because a task could've also persist the namespace
recorded in struct mm_struct explicitly and then the idle task would
still reuse that struct mm_struct and another task could still happily
setns() to it afaict and reuse it for something else.

The active reference count though has drawbacks itself. Namely that
socket files break the assumption that namespaces can only be opened if
there's either live processes pinning the namespace or there are file
descriptors open that pin the namespace itself as the socket SIOCGSKNS
ioctl() can be used to open a network namespace based on a socket which
only indirectly pins a network namespace.

So that punches a whole in the active reference count tracking. So this
will have to be handled as right now socket file descriptors that pin a
network namespace that don't have an active reference anymore (no live
processes, not explicit persistence via namespace fds) can't be used to
issue a SIOCGSKNS ioctl() to open the associated network namespace.

So two options I see if the api is based on ids:

(1) We use the active reference count and somehow also make it work with
    sockets.
(2) The active reference count is not needed and we say that listns() is
    an introspection system call anyway so we just always list
    namespaces regardless of why they are still pinned: files,
    mm_struct, network devices, everything is fair game.
(3) Throw hands up in the air and just not do it.

I think the active reference counts are just nice to have, if I'm not missing
something we still have to figure out which pid is using the namespace we may
want to enter, so there's already a "time of check, time of use" issue. I think
if we want to have the active count we can do it just as an advisory thing, have
a flag that says "this ns is dying and you can't do anything with it", and then
for network namespaces we can just never set the flag and let the existing
SIOCKGSNS ioctl work as is.

The bigger question (and sorry I didn't think about this before now), is how are
we going to integrate this into the rest of the NS related syscalls? Having
progromatic introspection is excellent from a usabiility point of view, but we
also want to be able to have an easy way to get a PID from these namespaces, and
even eventually do things like setns() based on these IDs. Those are followup
series of course, but we should at least have a plan for them. Thanks,

Josef