Re: [PATCH 2/9] mnt: expose pointer to init_mnt_ns

[Christian Brauner <brauner@kernel.org>]

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On Wed, Sep 17, 2025 at 06:28:37PM +0200, Jan Kara wrote:
> On Wed 17-09-25 12:28:01, Christian Brauner wrote:
> > There's various scenarios where we need to know whether we are in the
> > initial set of namespaces or not to e.g., shortcut permission checking.
> > All namespaces expose that information. Let's do that too.
> > 
> > Signed-off-by: Christian Brauner <brauner@kernel.org>

I've changed this so it behaves exactly like all the other init
namespaces. See appended.

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From 1bf2ddb7bdd1f686d4e083380412e826a211c57d Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Wed, 17 Sep 2025 12:28:01 +0200
Subject: [PATCH v2] mnt: expose pointer to init_mnt_ns

There's various scenarios where we need to know whether we are in the
initial set of namespaces or not to e.g., shortcut permission checking.
All namespaces expose that information. Let's do that too.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/namespace.c                | 27 ++++++++++++++++-----------
 include/linux/mnt_namespace.h |  2 ++
 2 files changed, 18 insertions(+), 11 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index a68998449698..f0bddc9cf2a6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -6008,27 +6008,32 @@ SYSCALL_DEFINE4(listmount, const struct mnt_id_req __user *, req,
 	return ret;
 }
 
+struct mnt_namespace init_mnt_ns = {
+	.ns.inum	= PROC_MNT_INIT_INO,
+	.ns.ops		= &mntns_operations,
+	.user_ns	= &init_user_ns,
+	.ns.count	= REFCOUNT_INIT(1),
+	.passive	= REFCOUNT_INIT(1),
+	.mounts		= RB_ROOT,
+	.poll		= __WAIT_QUEUE_HEAD_INITIALIZER(init_mnt_ns.poll),
+};
+
 static void __init init_mount_tree(void)
 {
 	struct vfsmount *mnt;
 	struct mount *m;
-	struct mnt_namespace *ns;
 	struct path root;
 
 	mnt = vfs_kern_mount(&rootfs_fs_type, 0, "rootfs", NULL);
 	if (IS_ERR(mnt))
 		panic("Can't create rootfs");
 
-	ns = alloc_mnt_ns(&init_user_ns, true);
-	if (IS_ERR(ns))
-		panic("Can't allocate initial namespace");
-	ns->ns.inum = PROC_MNT_INIT_INO;
 	m = real_mount(mnt);
-	ns->root = m;
-	ns->nr_mounts = 1;
-	mnt_add_to_ns(ns, m);
-	init_task.nsproxy->mnt_ns = ns;
-	get_mnt_ns(ns);
+	init_mnt_ns.root = m;
+	init_mnt_ns.nr_mounts = 1;
+	mnt_add_to_ns(&init_mnt_ns, m);
+	init_task.nsproxy->mnt_ns = &init_mnt_ns;
+	get_mnt_ns(&init_mnt_ns);
 
 	root.mnt = mnt;
 	root.dentry = mnt->mnt_root;
@@ -6036,7 +6041,7 @@ static void __init init_mount_tree(void)
 	set_fs_pwd(current->fs, &root);
 	set_fs_root(current->fs, &root);
 
-	ns_tree_add(ns);
+	ns_tree_add(&init_mnt_ns);
 }
 
 void __init mnt_init(void)
diff --git a/include/linux/mnt_namespace.h b/include/linux/mnt_namespace.h
index 70b366b64816..6d1c4c218c14 100644
--- a/include/linux/mnt_namespace.h
+++ b/include/linux/mnt_namespace.h
@@ -11,6 +11,8 @@ struct fs_struct;
 struct user_namespace;
 struct ns_common;
 
+extern struct mnt_namespace init_mnt_ns;
+
 extern struct mnt_namespace *copy_mnt_ns(unsigned long, struct mnt_namespace *,
 		struct user_namespace *, struct fs_struct *);
 extern void put_mnt_ns(struct mnt_namespace *ns);
-- 
2.47.3