FAILED: patch "[PATCH] ns: don't add init_ipc_ns and init_net without namespace support" failed to apply to 6.18-stable tree

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 82f1c815aaef1905c5c6c386c8a0d74f6ff70a54
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '[email protected]' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 82f1c815aaef1905c5c6c386c8a0d74f6ff70a54 Mon Sep 17 00:00:00 2001
From: Christian Brauner <[email protected]>
Date: Fri, 30 Oct 2026 16:50:07 +0100
Subject: [PATCH] ns: don't add init_ipc_ns and init_net without namespace
support

When CONFIG_IPC_NS or CONFIG_NET_NS are turned off we still add the initial ipc and network namespace to the namespace trees. ipc_ns_init() calls ns_tree_add(&init_ipc_ns) whenever CONFIG_SYSVIPC is set and setup_net() calls ns_tree_add_raw() on init_net whenever CONFIG_NET is set.

But without CONFIG_IPC_NS and CONFIG_NET_NS these aren't namespaces userspace can do anything with. They don't have proc_ns_operations so there's no /proc/<pid>/ns/{ipc,net} for them, nsfs_fh_to_dentry() refuses them with -EOPNOTSUPP, and since ns->ops is NULL __ns_tree_add_raw() doesn't add them to the owner list of init_user_ns either. So they end up half registered. That's also not how we treat any of the other namespace types. Without CONFIG_UTS_NS, CONFIG_PID_NS, CONFIG_TIME_NS or CONFIG_USER_NS the initial namespace is never added to the tree.

listns() walks the unified tree and calls is_current_namespace() for every namespace it finds via may_list_ns(). But is_current_namespace() only knows about CLONE_NEWIPC and CLONE_NEWNET if CONFIG_IPC_NS and CONFIG_NET_NS are set. Otherwise we end up in the default case and its VFS_WARN_ON_ONCE(). Any unprivileged caller can trigger that with a plain listns() call:

  WARNING: CPU: 0 PID: 526 at fs/nsfs.c:508 is_current_namespace+0x12f/0x440
  Call Trace:
   legitimize_ns+0x25f/0x4c0
   do_listns+0x185/0x5c0
   __do_sys_listns+0x13b/0x300

Instead of teaching is_current_namespace() about namespaces that can't be opened, only add the initial namespaces to the tree if their namespace type is enabled. Move the registration of init_ipc_ns into ipc/namespace.c which is only built with CONFIG_IPC_NS. That also fixes CONFIG_SYSVIPC=n, CONFIG_POSIX_MQUEUE=y, CONFIG_IPC_NS=y where ipc/shm.c isn't built and init_ipc_ns was never added at all. For the network namespace only call ns_tree_add_raw() if CONFIG_NET_NS is set. Without CONFIG_NET_NS cleanup_net() isn't built and init_net is never removed so there's nothing to balance.

---
 ipc/namespace.c          | 7 +++++++
 ipc/shm.c                | 2 --
 net/core/net_namespace.c | 4 +++-
 3 files changed, 10 insertions(+), 3 deletions(-)

index 75aa7b4693484..b54d23fd83d70 100644
@@ -255,3 +255,10 @@ const struct proc_ns_operations ipcns_operations = {
 	.install	= ipcns_install,
 	.owner		= ipcns_owner,
 };
+
+static int __init ipc_ns_tree_init(void)
+{
+	ns_tree_add(&init_ipc_ns);
+	return 0;
+}
+pure_initcall(ipc_ns_tree_init);

index bb1a721a3e746..cbab3d16cf70b 100644
@@ -45,7 +45,6 @@
 #include <linux/mount.h>
 #include <linux/ipc_namespace.h>
 #include <linux/rhashtable.h>
-#include <linux/nstree.h>
 
 #include <linux/uaccess.h>
 

@@ -149,7 +148,6 @@ void shm_exit_ns(struct ipc_namespace *ns)
 static int __init ipc_ns_init(void)
 {
 	shm_init_ns(&init_ipc_ns);
-	ns_tree_add(&init_ipc_ns);
 	return 0;
 }
 

index da5f881fbd3b9..52c2bd44778a8 100644
@@ -447,7 +447,9 @@ static __net_init int setup_net(struct net *net)
 	down_write(&net_rwsem);
 	list_add_tail_rcu(&net->list, &net_namespace_list);
 	up_write(&net_rwsem);
-	ns_tree_add_raw(net);
+	/* Without CONFIG_NET_NS init_net isn't exposed as a namespace. */
+	if (IS_ENABLED(CONFIG_NET_NS))
+		ns_tree_add_raw(net);
 out:
 	return error;
 

2.53.0