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Message-ID: <97bb1f9baba905e0e8bde62cce858b0def091d5c.camel@kernel.org>
Date: Wed, 22 Oct 2025 07:28:26 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Christian Brauner <brauner@...nel.org>, linux-fsdevel@...r.kernel.org,
Josef Bacik <josef@...icpanda.com>
Cc: Jann Horn <jannh@...gle.com>, Mike Yuan <me@...dnzj.com>, Zbigniew
Jędrzejewski-Szmek <zbyszek@...waw.pl>, Lennart
Poettering <mzxreary@...inter.de>, Daan De Meyer
<daan.j.demeyer@...il.com>, Aleksa Sarai <cyphar@...har.com>, Amir
Goldstein <amir73il@...il.com>, Tejun Heo <tj@...nel.org>, Johannes Weiner
<hannes@...xchg.org>, Thomas Gleixner <tglx@...utronix.de>, Alexander Viro
<viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, bpf@...r.kernel.org,
Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
netdev@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH RFC DRAFT 00/50] nstree: listns()
On Tue, 2025-10-21 at 13:43 +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.
>
Is this capability something we need to preserve? It seems like the
fact that SIOCGSKNS works when there are no active references left
might have been an accident. Is there a legit use-case for allowing
that?
I don't see a problem with active+passive refcounts. They're more
complicated to deal with, but we've used them elsewhere so it's a
pattern we all know (even if we don't necessarily love them).
I'll also point out that net namespaces already have two refcounts for
this exact reason. Do you plan to replace the passive refcount in
struct net with the new passive refcount you're implementing here?
> 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.
>
Is listns() the only reason we'd need a active/passive refcounts? It
seems like we might need them for other reasons (e.g. struct net).
In any case, given that this is a privileged syscall, I don't
necessarily see a problem with #2 here. Leaked namespaces can be a
problem and we don't have good visibility into them at the moment.
IMO, even if you keep the active+passive refcounts, it would be good to
be able to tell listns() to return all the namespaces, and not just the
ones that are still active. Maybe that can be the first flag for this
new syscall?
> =====================================================================
>
> Add a new listns() system call that allows userspace to iterate through
> namespaces in the system. This provides a programmatic interface to
> discover and inspect namespaces, enhancing existing namespace apis.
>
> Currently, there is no direct way for userspace to enumerate namespaces
> in the system. Applications must resort to scanning /proc/<pid>/ns/
> across all processes, which is:
>
> 1. Inefficient - requires iterating over all processes
> 2. Incomplete - misses inactive namespaces that aren't attached to any
> running process but are kept alive by file descriptors, bind mounts,
> or parent namespace references
> 3. Permission-heavy - requires access to /proc for many processes
> 4. No ordering or ownership.
> 5. No filtering per namespace type: Must always iterate and check all
> namespaces.
>
> The list goes on. The listns() system call solves these problems by
> providing direct kernel-level enumeration of namespaces. It is similar
> to listmount() but obviously tailored to namespaces.
>
> /*
> * @req: Pointer to struct ns_id_req specifying search parameters
> * @ns_ids: User buffer to receive namespace IDs
> * @nr_ns_ids: Size of ns_ids buffer (maximum number of IDs to return)
> * @flags: Reserved for future use (must be 0)
> */
> ssize_t listns(const struct ns_id_req *req, u64 *ns_ids,
> size_t nr_ns_ids, unsigned int flags);
>
> Returns:
> - On success: Number of namespace IDs written to ns_ids
> - On error: Negative error code
>
> /*
> * @size: Structure size
> * @ns_id: Starting point for iteration; use 0 for first call, then
> * use the last returned ID for subsequent calls to paginate
> * @ns_type: Bitmask of namespace types to include (from enum ns_type):
> * 0: Return all namespace types
> * MNT_NS: Mount namespaces
> * NET_NS: Network namespaces
> * USER_NS: User namespaces
> * etc. Can be OR'd together
> * @user_ns_id: Filter results to namespaces owned by this user namespace:
> * 0: Return all namespaces (subject to permission checks)
> * LISTNS_CURRENT_USER: Namespaces owned by caller's user namespace
> * Other value: Namespaces owned by the specified user namespace ID
> */
> struct ns_id_req {
> __u32 size; /* sizeof(struct ns_id_req) */
> __u32 spare; /* Reserved, must be 0 */
> __u64 ns_id; /* Last seen namespace ID (for pagination) */
> __u32 ns_type; /* Filter by namespace type(s) */
> __u32 spare2; /* Reserved, must be 0 */
> __u64 user_ns_id; /* Filter by owning user namespace */
> };
>
> Example 1: List all namespaces
>
> void list_all_namespaces(void)
> {
> struct ns_id_req req = {
> .size = sizeof(req),
> .ns_id = 0, /* Start from beginning */
> .ns_type = 0, /* All types */
> .user_ns_id = 0, /* All user namespaces */
> };
> uint64_t ids[100];
> ssize_t ret;
>
> printf("All namespaces in the system:\n");
> do {
> ret = listns(&req, ids, 100, 0);
> if (ret < 0) {
> perror("listns");
> break;
> }
>
> for (ssize_t i = 0; i < ret; i++)
> printf(" Namespace ID: %llu\n", (unsigned long long)ids[i]);
>
> /* Continue from last seen ID */
> if (ret > 0)
> req.ns_id = ids[ret - 1];
> } while (ret == 100); /* Buffer was full, more may exist */
> }
>
> Example 2 : List network namespaces only
>
> void list_network_namespaces(void)
> {
> struct ns_id_req req = {
> .size = sizeof(req),
> .ns_id = 0,
> .ns_type = NET_NS, /* Only network namespaces */
> .user_ns_id = 0,
> };
> uint64_t ids[100];
> ssize_t ret;
>
> ret = listns(&req, ids, 100, 0);
> if (ret < 0) {
> perror("listns");
> return;
> }
>
> printf("Network namespaces: %zd found\n", ret);
> for (ssize_t i = 0; i < ret; i++)
> printf(" netns ID: %llu\n", (unsigned long long)ids[i]);
> }
>
> Example 3 : List namespaces owned by current user namespace
>
> void list_owned_namespaces(void)
> {
> struct ns_id_req req = {
> .size = sizeof(req),
> .ns_id = 0,
> .ns_type = 0, /* All types */
> .user_ns_id = LISTNS_CURRENT_USER, /* Current userns */
> };
> uint64_t ids[100];
> ssize_t ret;
>
> ret = listns(&req, ids, 100, 0);
> if (ret < 0) {
> perror("listns");
> return;
> }
>
> printf("Namespaces owned by my user namespace: %zd\n", ret);
> for (ssize_t i = 0; i < ret; i++)
> printf(" ns ID: %llu\n", (unsigned long long)ids[i]);
> }
>
> Example 4 : List multiple namespace types
>
> void list_network_and_mount_namespaces(void)
> {
> struct ns_id_req req = {
> .size = sizeof(req),
> .ns_id = 0,
> .ns_type = NET_NS | MNT_NS, /* Network and mount */
> .user_ns_id = 0,
> };
> uint64_t ids[100];
> ssize_t ret;
>
> ret = listns(&req, ids, 100, 0);
> printf("Network and mount namespaces: %zd found\n", ret);
> }
>
> Example 5 : Pagination through large namespace sets
>
> void list_all_with_pagination(void)
> {
> struct ns_id_req req = {
> .size = sizeof(req),
> .ns_id = 0,
> .ns_type = 0,
> .user_ns_id = 0,
> };
> uint64_t ids[50];
> size_t total = 0;
> ssize_t ret;
>
> printf("Enumerating all namespaces with pagination:\n");
>
> while (1) {
> ret = listns(&req, ids, 50, 0);
> if (ret < 0) {
> perror("listns");
> break;
> }
> if (ret == 0)
> break; /* No more namespaces */
>
> total += ret;
> printf(" Batch: %zd namespaces\n", ret);
>
> /* Last ID in this batch becomes start of next batch */
> req.ns_id = ids[ret - 1];
>
> if (ret < 50)
> break; /* Partial batch = end of results */
> }
>
> printf("Total: %zu namespaces\n", total);
> }
>
> listns() respects namespace isolation and capabilities:
>
> (1) Global listing (user_ns_id = 0):
> - Requires CAP_SYS_ADMIN in the namespace's owning user namespace
> - OR the namespace must be in the caller's namespace context (e.g.,
> a namespace the caller is currently using)
> - User namespaces additionally allow listing if the caller has
> CAP_SYS_ADMIN in that user namespace itself
> (2) Owner-filtered listing (user_ns_id != 0):
> - Requires CAP_SYS_ADMIN in the specified owner user namespace
> - OR the namespace must be in the caller's namespace context
> - This allows unprivileged processes to enumerate namespaces they own
> (3) Visibility:
> - Only "active" namespaces are listed
> - A namespace is active if it has a non-zero __ns_ref_active count
> - This includes namespaces used by running processes, held by open
> file descriptors, or kept active by bind mounts
> - Inactive namespaces (kept alive only by internal kernel
> references) are not visible via listns()
>
> Signed-off-by: Christian Brauner <brauner@...nel.org>
> ---
> Christian Brauner (50):
> libfs: allow to specify s_d_flags
> nsfs: use inode_just_drop()
> nsfs: raise DCACHE_DONTCACHE explicitly
> pidfs: raise DCACHE_DONTCACHE explicitly
> nsfs: raise SB_I_NODEV and SB_I_NOEXEC
> nstree: simplify return
> ns: initialize ns_list_node for initial namespaces
> ns: add __ns_ref_read()
> ns: add active reference count
> ns: use anonymous struct to group list member
> nstree: introduce a unified tree
> nstree: allow lookup solely based on inode
> nstree: assign fixed ids to the initial namespaces
> ns: maintain list of owned namespaces
> nstree: add listns()
> arch: hookup listns() system call
> nsfs: update tools header
> selftests/filesystems: remove CLONE_NEWPIDNS from setup_userns() helper
> selftests/namespaces: first active reference count tests
> selftests/namespaces: second active reference count tests
> selftests/namespaces: third active reference count tests
> selftests/namespaces: fourth active reference count tests
> selftests/namespaces: fifth active reference count tests
> selftests/namespaces: sixth active reference count tests
> selftests/namespaces: seventh active reference count tests
> selftests/namespaces: eigth active reference count tests
> selftests/namespaces: ninth active reference count tests
> selftests/namespaces: tenth active reference count tests
> selftests/namespaces: eleventh active reference count tests
> selftests/namespaces: twelth active reference count tests
> selftests/namespaces: thirteenth active reference count tests
> selftests/namespaces: fourteenth active reference count tests
> selftests/namespaces: fifteenth active reference count tests
> selftests/namespaces: add listns() wrapper
> selftests/namespaces: first listns() test
> selftests/namespaces: second listns() test
> selftests/namespaces: third listns() test
> selftests/namespaces: fourth listns() test
> selftests/namespaces: fifth listns() test
> selftests/namespaces: sixth listns() test
> selftests/namespaces: seventh listns() test
> selftests/namespaces: ninth listns() test
> selftests/namespaces: ninth listns() test
> selftests/namespaces: first listns() permission test
> selftests/namespaces: second listns() permission test
> selftests/namespaces: third listns() permission test
> selftests/namespaces: fourth listns() permission test
> selftests/namespaces: fifth listns() permission test
> selftests/namespaces: sixth listns() permission test
> selftests/namespaces: seventh listns() permission test
>
> arch/alpha/kernel/syscalls/syscall.tbl | 1 +
> arch/arm/tools/syscall.tbl | 1 +
> arch/arm64/tools/syscall_32.tbl | 1 +
> arch/m68k/kernel/syscalls/syscall.tbl | 1 +
> arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
> arch/mips/kernel/syscalls/syscall_n32.tbl | 1 +
> arch/mips/kernel/syscalls/syscall_n64.tbl | 1 +
> arch/mips/kernel/syscalls/syscall_o32.tbl | 1 +
> arch/parisc/kernel/syscalls/syscall.tbl | 1 +
> arch/powerpc/kernel/syscalls/syscall.tbl | 1 +
> arch/s390/kernel/syscalls/syscall.tbl | 1 +
> arch/sh/kernel/syscalls/syscall.tbl | 1 +
> arch/sparc/kernel/syscalls/syscall.tbl | 1 +
> arch/x86/entry/syscalls/syscall_32.tbl | 1 +
> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> arch/xtensa/kernel/syscalls/syscall.tbl | 1 +
> fs/libfs.c | 1 +
> fs/namespace.c | 8 +-
> fs/nsfs.c | 79 +-
> fs/pidfs.c | 1 +
> include/linux/ns_common.h | 147 +-
> include/linux/nsfs.h | 3 +
> include/linux/nstree.h | 26 +-
> include/linux/pseudo_fs.h | 1 +
> include/linux/syscalls.h | 4 +
> include/uapi/asm-generic/unistd.h | 4 +-
> include/uapi/linux/nsfs.h | 58 +
> init/version-timestamp.c | 5 +
> ipc/msgutil.c | 5 +
> ipc/namespace.c | 1 +
> kernel/cgroup/cgroup.c | 5 +
> kernel/cgroup/namespace.c | 1 +
> kernel/cred.c | 17 +
> kernel/exit.c | 1 +
> kernel/nscommon.c | 59 +-
> kernel/nsproxy.c | 7 +
> kernel/nstree.c | 527 ++++-
> kernel/pid.c | 15 +
> kernel/pid_namespace.c | 1 +
> kernel/time/namespace.c | 6 +
> kernel/user.c | 5 +
> kernel/user_namespace.c | 1 +
> kernel/utsname.c | 1 +
> net/core/net_namespace.c | 3 +-
> scripts/syscall.tbl | 1 +
> tools/include/uapi/linux/nsfs.h | 70 +
> tools/testing/selftests/filesystems/utils.c | 2 +-
> tools/testing/selftests/namespaces/.gitignore | 3 +
> tools/testing/selftests/namespaces/Makefile | 7 +-
> .../selftests/namespaces/listns_permissions_test.c | 777 +++++++
> tools/testing/selftests/namespaces/listns_test.c | 656 ++++++
> .../selftests/namespaces/ns_active_ref_test.c | 2226 ++++++++++++++++++++
> tools/testing/selftests/namespaces/wrappers.h | 35 +
> 53 files changed, 4737 insertions(+), 48 deletions(-)
> ---
> base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
> change-id: 20251020-work-namespace-nstree-listns-9fd71518515c
--
Jeff Layton <jlayton@...nel.org>
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